could not read symbols: File in wrong format

hubin1225 2009-12-25 10:54:15
我在使用arm-linux-gcc编译程序的时候出现以下错误提示:
arm-linux-gcc -Wall -I../../../include -I../../../../include -I../../../../include/linux -c ../../readwrite.c
arm-linux-gcc -o readwrite -L../../../../lib/linux/ixp4x5 \
readwrite.o -lrfid -lrfidtx -lcpl -lpthread -lrt -lstdc++
../../../../lib/linux/ixp4x5/librfid.so: could not read symbols: File in wrong format
collect2: ld returned 1 exit status
make: *** [readwrite] Error 1

librfid.so文件是厂商提供的,在ixp425文件见下存在,我也拷贝此文件到/usr/lib中,并运行了ldconfig命令,问什么会出现这错误,请高手帮帮忙,谢谢~~
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tianziczj 2010-11-17
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[Quote=引用 9 楼 armed 的回复:]

gcc-4.0以上可以支持ARM的EABI,你也可以这样:
arm-linux-gcc -o hello hello.c
arm-linux-readelf -h hello
这一行会显示是否使用了EABI,
Flags: 0x202, has entry point, GNU EABI, software FP
上面这一行表示不……
[/Quote]请问我原来的arm板子是不支持eabi的,现在想升级到支持eabi的,应该怎样做?是不是应该重新烧个支持eabi的内核?那我原来板子上跑的应用程序是不是都要重新编译呢?非常感谢!
armed 2010-01-06
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gcc-4.0以上可以支持ARM的EABI,你也可以这样:
arm-linux-gcc -o hello hello.c
arm-linux-readelf -h hello
这一行会显示是否使用了EABI,
Flags: 0x202, has entry point, GNU EABI, software FP
上面这一行表示不使用EABI(能和其它编译器链接的EABI表示为Version5 EABI),软浮点

你也可以使用上面的命令测试一下那个librfid.so的输出,只有当格式完全一致时才能连接。
hubin1225 2009-12-30
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[Quote=引用 5 楼 deep_pro 的回复:]
跟eabi有关吗?你的工具链支持eabi吗?
不行就要厂商用你的工具链编译库
要么你就用厂商的工具链试试

[/Quote]

您所说的“工具链支持eabi”,我不知道我的arm-linux-gcc支不支持eabi,请问怎么判断呀?
Wenxy1 2009-12-28
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linux command:
which gcc
gcc -v
deep_pro 2009-12-25
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跟eabi有关吗?你的工具链支持eabi吗?
不行就要厂商用你的工具链编译库
要么你就用厂商的工具链试试
hubin1225 2009-12-25
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用file命令查看此文件为:librfid.so: ELF 32-bit MSB shared object, ARM, version 1 (SYSV), not stripped
这种类型应该是arm板子能支持的,但是就编译不过~~
hubin1225 2009-12-25
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厂商提供的.so文件是用scons工具编译出来的,如果用GCC编译的话不会提示错误,但不能运行,用arm-linux-gcc则提示错误
rzsheng 2009-12-25
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文件格式可能跟你不是 arm 下编译的。
用file 命令查看一下。

如果是厂商提供,那需要他们提供你所需要的平台下的编译文件。
Wenxy1 2009-12-25
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file librfid.so,看看此文件的格式。
你的工具链跟librfid.so的格式不区配,重新用你的工具编译下它吧。
deep_pro 2009-12-25
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librfid.so: ELF 32-bit MSB shared object, ARM, version 1 (SYSV), not stripped
-------------
lz把你的工具链编译出的库信息也拿来比较一下
安装步骤: 0、交叉编译环境配置 在/etc/profiel 和 .bashrc 文件中增加以下内容 export PTAH=$PATH://root/compile/arm-none-linux-gnueabi/bin source .bashrc 1、解压 tar -zxvf vsftpd-3.0.3.tar.gz.tar 2、修改Makefile文件 vim Makefile CC=arm-none-linux-gnueabi-gcc 3、执行make,已修复以下错误 make (1)错误提示:/bin/sh:./vsf_findlibs.sh: Permission denied 解决措施:chmod 775 ./vsf_findlibs.sh (2)错误提示:/lib/libcap.so.2:could not read symbols: File in wrong format 解决措施:修改vsffindlibs.sh,屏蔽掉50-60行 (3)错误提示:sysdeputil.c:(.text+0xdcc):undefined reference to `crypt' 解决措施:arm-none-linux-gnueabi/libc/lib/下的libcrypt.so.1复制到开发板根文件系统/lib目录下 4、拷贝文件到开发板中 make成功之后有在当前目录生成了vsftpd文件,通过以下命令查找对应的依赖性 arm-linux-readelf -d vsftpd   Tag        Type                        Name/Value 0x00000001 (NEEDED)                    Shared library: [libcrypt.so.1] 0x00000001(NEEDED)                     Shared library: [libc.so.6] 把vsftpd、libcrypt.so.1及这两个连接所指向的源文件拷贝开发板系统根目录/lib中。 把生成的vsftpd拷贝到根文件系统的sbin目录,vsftpd.conf文件拷贝到开发板系统根文件系统的etc目录下。 5、配置vsftpd.conf #使vsftpd处于独立启动模式 listen=YES listen_port=21 anon_other_write_enable=YES 6、开发板根文件系统创建必要的目录: mkdir-p /var/ftp/pub mkdir -p /usr/share/empty chown root:root/var/ftp chmod 755 /var/ftp cd /var/ftp chmod 777 pub pub这个目录是用来供匿名用户上传文件的,上传的文件存放在此处,故其属性为777,而其他目录无此属性,不能用来作为上传目录。 7、开发板执行以下命令启动 vsftpd/etc/vsftpd.conf& 8、启动ftp登录 用户名root密码空,可访问开发板任意目录; 用户名anonymous密码空,可访问/var/ftp/pub目录 用户名ftp密码空,可访问/var/ftp/pub目录
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threads" option is changed from Boolean flag into enum. You need to re-setup this option 47)..Changed: Disable EurekaLog for thread when creating call stack or handle exception - this increases stability and performance 48)..Changed: LastException property is remove from exception manager as not thread safe. Use LastThreadException property instead 49)..Changed: Lock/Unlock from thread manager and exception manager are removed to avoid deadlocks 50)..Changed: ThreadsSnapshot tool now tries to capture call stack without injecting DLL 51)..Changed: Build events now runs with CREATE_NO_WINDOW flag (console window is hidden) 52)..Improved: More articles in help EurekaLog 7.0.06 (7.0.6.0), 1-June-2013 1)....Added: Experimental 64 bit C++ Builder support 2)....Added: New tab in EurekaLog project options: "External tools" 3)....Added: Option to catch all IDE errors (to debug your own IDE packages) 4)....Added: Option to catch only exceptions from current module 5)....Added: Option to defer building call stack 6)....Added: RAD Studio XE4 support 7)....Added: Support for AppWave 8)....Fixed: Fixed event handlers declarations for the EurekaLog component 9)....Fixed: Infinite recursive calls when using ToString from EndReport event handler 10)..Fixed: UPX compatibility issue 11)..Fixed: Range check errors for system error codes 12)..Fixed: Rare IDE stack overflow 13)..Fixed: JIRA unit was not added automatically 14)..Fixed: EurekaLog no longer tries to check for leaks when memory manager filter is disabled 15)..Fixed: Possible deadlock on shutdown with freeze checks active 16)..Fixed: Issues with settings dialog and Win32 Service application type 17)..Fixed: ThreadSnapshot tool was not able to take snapshots of Win64 processes 18)..Fixed: WCT is disabled for leaks 19)..Fixed: TContext declarations for Win64 20)..Fixed: Check for updates now correctly sets time of last check 21)..Fixed: (Win64) Several Pointer Integer convertion errors 22)..Fixed: Internal error when exception info object was deleted while it was still used by SysUtils exception object 23)..Fixed: Semeral problems with "EurekaLog look & feel" style for EurekaLog error dialog 24)..Fixed: Using text collection resets exception filters 25)..Fixed: Rare access violation if registering event handlers is placed too early 26)..Fixed: SMTP RFC date formatting 27)..Fixed: Rare empty call stack bug 28)..Fixed: Hang detection was not working if EurekaLog was disabled in threads 29)..Fixed: AV for double-free TEncoding 30)..Changed: ecc32/emake no longer alters arguments for dcc32/make unless new options --el_add_default_options is specified 31)..Changed: Save/load options methods was moved to TEurekaModuleOptions class 32)..Changed: Saving options to EOF file now adds hidden options and removes obsolete options (only when compatibility mode is off) 33)..Changed: Compiling installed packages now silently ignores EurekaLog instead of showing "File is in use" error message 34)..Improved: More readable disk/memory sizes in bug reports 35)..Improved: More descriptive settings dialog when using external configuration 36)..Improved: ThreadSnapshot tool now aquired DEBUG priviledge for taking snapshot. This allows it to bypass security access checks when opening target process. 37)..Improved: Changed BugID default generation to include error code for OS errors and error message for DB errors 38)..Improved: Mantis API (WSDL) was updated to the latest version (1.2.14) 39)..Improved: IntraWeb compatibility (old and new versions) 40)..Improved: COM applications compatibility 41)..Improved: Build events now accept shell commands 42)..Improved: More articles in help EurekaLog 7.0.05 (7.0.5.0), 7-February-2013 1)....Added: JIRA support 2)....Added: Virtual machine detection (new field in bug reports) 3)....Fixed: "Use Main Module options" option was loading empty options for some cases 4)....Fixed: Wrong record declarations for Simple MAPI on Win64 5)....Fixed: Performance issues with batch module options updating 6)....Fixed: Wrong leaks report with both MemLeaks/ResLeaks options active 7)....Fixed: Wrong info for nested exceptions in some cases 8)....Fixed: AV under debugger for Win64 (added support for _TExitDllException) 9)....Fixed: Wrong record declarations for process/thread info on Win64 10)..Fixed: Support for FinalBuilder on XE2/XE3 with spaces in file paths 11)..Fixed: Rare double-free of module information (ModuleInfoList) 12)..Fixed: Rare External Exception C000071C on shutdown (only under debuggger) 13)..Fixed: Added large addresses support in Viewer 14)..Fixed: Counter options in memory leaks category is now working properly 15)..Fixed: Rare range-check error in TEurekaModulesList.AddModuleFromFileName 16)..Fixed: FTP force directories dead lock 17)..Fixed: Fixed wrong index being used when clearing compatibility mode (EurekaLog project options dialog) 18)..Fixed: Default thread state do not affect main thread now 19)..Fixed: Sometimes wrong thread may be used when altering EurekaLog active state for external thread 20)..Fixed: Wrong DNS lookup on ANSI 21)..Fixed: Problems with IDE expert and projects on network paths 22)..Fixed: Added support for arguments in URLs (HTTP sending) 23)..Fixed: Possible deadlock in multithreaded applications 24)..Fixed: Problems with unicode characters in project files on non-Unicode IDEs 25)..Fixed: Infinite recursive calls when using ToString from EndReport event handler 26)..Fixed: Win64 GetCaller now returns pointer to call instruction, not return address 27)..Improved: Standalone Editor do not force save/load folder by default 28)..Improved: DLL profile now can use additional application type hooks automatically 29)..Improved: EurekaLog now able to work with read-only projects (see help for more info) EurekaLog 7.0.04 (7.0.4.0), 2-December-2012 1)....Added: Support for nested exceptions in DLLs 2)....Fixed: Options bug in EurekaLogSendEmail function 3)....Fixed: Weird behaviour for steps to reproduce and custom fields 4)....Fixed: Installation for single personality (BDS) 5)....Fixed: Range check error in EModules 6)....Fixed: Bug in exception destroy hook 7)....Fixed: OnExceptionNotify event is no longer called for handled exceptions without option checked 8)....Fixed: DEP checks on startup no longer cause exception 9)....Fixed: Invalid declaration for MS Debug API 10)..Fixed: OLE mode change error for "Test" send button 11)..Fixed: Fixes for multiply loading of the same DLL 12)..Fixed: Removed PNG compression from icons (tools) 13)..Fixed: Range-check error in dialogs with EurekaLog style enabled 14)..Fixed: Send progress dialog may keep busy forever processing window messages (message flood from rapid application GUI updates) 15)..Fixed: Thread pausing options now work correctly 16)..Improved: New features in exception filters - marking exceptions as "expected", filtering by properties (RTTI) 17)..Improved: Recovery from memory errors without debugging memory manager 18)..Improved: Viewer's password edit now hides password with asterisks 19)..Updated: Changed names of .inc files to avoid name conflicts with other libraries 20)..Updated: Help EurekaLog 7.0.03 (7.0.3.0), 6-October-2012 1)....Fixed: Removed some consts keywords for event handlers, so now C++ Builder can alter arguments (this change may require you to adjust your custom code) 2)....Fixed: Fallback code for false-positive results on memory probing 3)....Fixed: Range check errors in SSL/TLS implementation 4)....Fixed: "EurekaLog is not active" error message during send testing 5)....Fixed: Incorrect memory probing when DEP is off (old systems) 6)....Fixed: Installation of 64-bit BPLs 7)....Fixed: Dialog preview 8)....Fixed: Win64 fixes for XE3 9)....Fixed: Support for project groups (mixed project types) 10)..Fixed: Windows 2000 hooks compatibility 11)..Fixed: mailto double quotes escaping 12)..Fixed: Simple MAPI WOW compatibility 13)..Fixed: Simple MAPI modal issues 14)..Fixed: Various range check errors 15)..Changed: Removed minor version number from program group name 16)..Updated: Help EurekaLog 7.0.02 hot-fix 1 (7.0.2.1), 12-September-2012 1)....Fixed: Range check error in Viewer 2)....Fixed: Bug in hooking code EurekaLog 7.0.02 (7.0.2.0), 11-September-2012 1)....Added: Improved memory problems detection 2)....Added: Minor IDE Expert usability improvements 3)....Added: Auto-size feature for detailed error dialog 4)....Added: Workaround for QC #106935 5)....Added: Workaround for bug in InvokeRegistry (SOAP/Mantis) 6)....Fixed: Nested OS exceptions 7)....Fixed: Multiply Win64 fixes 8)....Fixed: Compatibility mode fixes 9)....Fixed: Altered behaviour of "Add BugID/Date/ComputerName" options 10)..Fixed: Blank screenshots 11)..Fixed: Check file for corruptions 12)..Fixed: Viewer is unable to decrypt certain bug reports 13)..Fixed: Internal DoNoTouch option now works for post-processing and condtionals 14)..Fixed: Possible out of memory error for "Do not store class/procedure names" option 15)..Fixed: EurekaLog did not properly install itself when there is only Delphi installed, but no C++ Builder of the same version (or visa versa) 16)..Fixed: Wrong argument for OnRaise event 17)..Fixed: Handling memory errors in initialization/finalization sections 18)..Fixed: Updating steps to reproduce and user e-mail in bug report 19)..Fixed: Proper Success/Failure for some errors during SMTP send 20)..Added: Workaround for wrong GUI fonts 21)..Added: Delphi XE3 support 22)..Added: Individual options for each exception EurekaLog 7.0.01 (7.0.1.0), 28-June-2012 1)....Added: New "Modal window" option (MS Classic and EurekaLog dialogs) 2)....Added: New "Owned window" option (MS Classic and EurekaLog dialogs) 3)....Added: New "Catch EurekaLog IDE Expert errors" option 4)....Added: Backup memory manager to recover from critical errors 5)....Added: Alternative methods to provide additional features when memory filter is not set 6)....Fixed: Contains fixes from hotfixes 1-3 7)....Fixed: Performance improvements 8)....Fixed: Improved IDE Expert's speed, stability and compatibility with other 3rd party extensions 9)....Fixed: MS Classic dialog size adjustments for large "click here" translations 10)..Fixed: Fixed resetting few EurekaLog project options to defaults 11)..Fixed: Multiplying exception filters when options are assigned (for example: when switching to/from "Custom" page in project options) 12)..Fixed: (Compatibility mode) Fixed send options merging 13)..Fixed: Updated help EurekaLog 7.0 hot-fix 3 (7.0.0.273), 20-June-2012 --------------------------- 1)....Fixed: ERangeError in EResLeaks (THandle Integer) 2)....Fixed: C++ Builder breakpoints for large projects 3)....Fixed: Help (updates policy changed) 4)....Fixed: Text collections applying 5)....Fixed: Build events are now called for unlocked file 6)....Fixed: Proper handling of C++ Builder project options files from Delphi code (settings editor and IDE expert) 7)....Fixed: Terminate/Checked sub-option for MS Classic dialog 8)....Fixed: Confusing message for already post-processed executables 9)....Fixed: Access violation for some EurekaLog IDE menu items when no project was loaded 10)..Fixed: Invoking help for "Variables" window 11)..Fixed: EurekaLog Viewer version info 12)..Fixed: Events in components 13)..Added: Retry option for "Sorry, you must close all running IDE instances before installation" 14)..Added: Italian translation 15)..Added: Actual change log is now included into installer 16)..Added: Even more setup logging 17)..Added: New help articles (recompilation and manual installation) EurekaLog 7.0 hot-fix 2 (7.0.0.261), 10-June-2012 --------------------------- 1)....Fixed: Wrong version info reporting to IDE 2)....Added: Workaround for Delphi 2005 TListView bug 3)....Added: Workaround for possible invalid FPU state in exception handlers 4)....Added: Missed declarations for ExceptionLog (compatibility mode) 5)....Fixed: Work for unsaved projects 6)....Added: Escaping for '--' in options (confuses IDE's XML parsing) 7)....Added: Storing thread's class/name in call stack for terminated threads 8)....Added: More setup logging 9)....Fixed: Help (broken links) 10)..Added: "Upgrade to EurekaLog 7" help topic 11)..Fixed: Clean up installed files EurekaLog 7.0 hot-fix 1 (7.0.0.256), 6-June-2012 --------------------------- 1)....Fixed: Invalid Format() arguments in ELogBuilder. EurekaLog 7.0, 1-June-2012 --------------------------- 1)....Improved: Main change - EurekaLog's core was rewritten (refactored) to allow more easy modification and remove hacks. 2)....Improved: New plugin-like architecture now allows you to exclude unused code. 3)....Improved: New plugin-like architecture now allows you to easily extends EurekaLog. 4)....Improved: Greatly extended documentation. 5)....Improved: Installer is now localized. 6)....Improved: Greatly speed ups creation of minimal bug report (with most information disabled). 7)....Changed: EurekaLog's root IDE menu was relocated to under Tools and extended with new items. 8)....Added: New examples. 9)....Added: New tools (address lookup, error lookup, threads snapshot, standalone settings editor). 10)..Added: Support for DBG/PDB formats of debug information (including symbol server support and auto-downloading). 11)..Added: Support for madExcept debug information (experimental). 12)..Added: WER (Windows Error Reporting) support. 13)..Added: Full unicode support. 14)..Added: Professional and Trial editions: added source code (interface sections only) 15)..Improved: Dialogs - new options and new customization possibilities: 16)..Added: All GUI dialogs: ability to test dialog directly from configuration dialog by displaying a sample window with currently specified settings. 17)..Improved: All GUI dialogs: dialogs are DPI-awared now (auto-scale for different DPI). 18)..Added: MessageBox dialog: added detailed mode (shows a compact call stack). 19)..Added: MessageBox dialog: added ability for asking a send consent. 20)..Added: MessageBox dialog: added support to switch to "native" message box for application. 21)..Added: MS Classic dialog: added control over "user e-mail" edit's visibility. 22)..Added: MS Classic dialog: added ability to personalize dialog view with application's name and icon. 23)..Added: MS Classic dialog: added ability to show terminate/restart checkbox initially checked. 24)..Added: EurekaLog dialog: added ability to personalize dialog view with application's name and icon. 25)..Added: EurekaLog dialog: added ability to show terminate/restart checkbox initially checked. 26)..Added: EurekaLog dialog: added ability to switch back to non-detailed view. 27)..Added: WEB dialog: added new tags to customize bug report page. 28)..Improved: WEB dialog: improved support for unicode and charset. 29)..Added: New dialog type: RTL dialog. 30)..Added: New dialog type: console output. 31)..Added: New dialog type: system logging. 32)..Added: New dialog type: Windows Error Reporting. 33)..Improved: Sending - new options and new customization possibilities: 34)..Added: All send methods: added ability to setup multiply send methods. 35)..Added: All send methods: added ability to change send method order. 36)..Added: All send methods: added separate settings for each send method. 37)..Added: All send methods: ability to test send method directly from configuration dialog by sending a demo bug report. 38)..Added: SMTP client send method: added SSL support. 39)..Added: SMTP client send method: added TLS support. 40)..Added: SMTP client send method: added option for using real e-mail address. 41)..Added: SMTP server send method: added option for using real e-mail address. 42)..Added: HTTP upload send method: added support for custom backward feedback messages. 43)..Added: FTP upload send method: added creating folders on FTP (like remote ForceDirectories). 44)..Added: Mantis send method: added API support (MantisConnect, out-of-the-box since Mantis 1.1.0, available as add-on for previous versions). 45)..Added: Mantis send method: added support for custom "Count" field. 46)..Added: Mantis send method: added options for controlling duplicates. 47)..Added: Mantis send method: added support for SSL/TLS. 48)..Added: FogBugz send method: added API support (out-of-the-box since ForBugz 7, available as add-on for FogBugz 6). 49)..Added: FogBugz send method: EurekaLog will update "Occurrences" field (count of bugs). 50)..Added: FogBugz send method: EurekaLog will respect "Stop reporting" option (BugzScout's setting). 51)..Added: FogBugz send method: EurekaLog will respect "Scout message" option (BugzScout's setting). 52)..Added: FogBugz send method: EurekaLog will store client's e-mail as issue's correspondent. 53)..Added: FogBugz send method: added options for controlling duplicates. 54)..Added: FogBugz send method: added support for "Area" field. 55)..Added: FogBugz send method: added support for SSL/TLS. 56)..Added: BugZilla send method: added API support. 57)..Added: BugZilla send method: added support for custom "Count" field. 58)..Added: BugZilla send method: added options for controlling duplicates. 59)..Added: BugZilla send method: added support for SSL/TLS. 60)..Added: New send method: Shell (mailto protocol). 61)..Added: New send method: extended MAPI. 62)..Added: Support for separate code and debug info injection. 63)..Added: Ability to use custom units before EurekaLog's units. 64)..Added: Support for external configuration file in IDE expert. 65)..Added: Now EurekaLog stores only those project options which are different from defaults (to save disk space and reduce noise in project file). 66)..Added: Now EurekaLog stores project options sorted (alphabet order). 67)..Added: Separate settings for saving modules and processes lists to bug report. 68)..Added: Support for taking screenshots of multiply monitors. 69)..Added: More screenshot customization options. 70)..Added: More control over bug report's file names. 71)..Added: New environment variables. 72)..Added: Deleting .map file after compilation. 73)..Added: Support for different .dpr and .dproj file names. 74)..Improved: memory leaks detection feature - new options and new customization possibilities: 75)..Added: Ability to track memory problems without activation of leaks checking. 76)..Added: Support for sharing memory manager. 77)..Added: Support for tracking leaks in applications built with run-time packages. 78)..Added: Option to zero-fill freed memory. 79)..Added: Option to enable leaks detection only when running under debugger. 80)..Added: Option for manual activation control for leaks detection (via command-line switches). 81)..Added: Option to select stack tracing method for memory problems. 82)..Added: Option to trigger memory leak reporting only for large leaked memory's size. 83)..Added: Option to control limit of number of reported leak. 84)..Added: CheckHeap function to force check of heap's consistency. 85)..Added: DumpAllocationsToFile function to save information about allocated memory to log file. 86)..Added: Registered leaks feature. 87)..Added: Run-time control over memory leak registering. 88)..Added: New recognized leak type: String (both ANSI and Unicode are supported). 89)..Added: Memory features support for C++ Builder. 90)..Added: Resource leaks detection feature. 91)..Improved: Compilation speed increased. 92)..Added: Support for generics in debug information. 93)..Added: Chained/nested exceptions support. 94)..Added: Wait Chain Traversal support. 95)..Added: Support for named threads. 96)..Added: Additional information for threads in call stack. 97)..Improved: EurekaLog Viewer Tool: 98)..Added: Now Viewer has its own help file 99)..Added: Viewer now supports a FireBird based database on local file or remote server. 100).Added: You can have more that one user account for FireBird based database. 101).Added: Viewer now can be launched in View mode (Viewer can be configured to any DB or View mode). 102).Added: Viewer's database now supports storing files, associated with the report (you can also add and remove files manually). 103).Added: Viewer supports "Import" and "View" commands for report files. 104).Improved: Extended support for more log formats (XML, packed ELF, etc). 105).Added: Columns in report's list now can be configured (you can hide and show them). 106).Added: There are a plenty of new columns added to report's list. 107).Added: Ability of auto-download reports from e-mail account. 108).Improved: printing - now you can print the entire report (including screenshots). Old behaviour of printing just one tab (call stack only, for example) also remains. 109).Added: Viewer can now have more that one run-time instance . 110).Added: File import status dialog is now configurable (you can disable it, if you want to). 111).Added: There is a preview area for screenshots, available in reports. 112).Improved: Now Viewer is more Vista-friendly (i.e. file associations are managed in HKCU, rather that in HKLM, storing configuration in user's Application Data, etc, etc). 113).Added: Report's list now supports multi-select, so operations can be performed on many reports at time. 114).Added: There are plenty of new command line abilities, like specifying several files and new switches. 115).Improved: Bunch of minor changes and improvements. WARNING: -------- There are many changes in this release. See the "Changed from the old 6.x version" help topic for further information! EurekaLog 7 also have "EurekaLog 6 backward compatibility mode". Please, refer to help file for more information. We also have the detailed "Upgrade guide" in our help system.
https://github.com/iBotPeaches/Apktool Introduction Basic First lets take a lesson into apk files. apks are nothing more than a zip file containing resources and compiled java. If you were to simply unzip an apk like so, you would be left with files such as classes.dex and resources.arsc. $ unzip testapp.apk Archive: testapp.apk inflating: AndroidManifest.xml inflating: classes.dex extracting: res/drawable-hdpi/ic_launcher.png inflating: res/xml/literals.xml inflating: res/xml/references.xml extracting: resources.arsc However, at this point you have simply inflated compiled sources. If you tried to view AndroidManifest.xml. You'd be left viewing this. P4F0\fnversionCodeversionNameandroid*http://schemas.android.com/apk/res/androidpackageplatformBuildVersionCodeplatformBuildVersionNamemanifestbrut.apktool.testapp1.021APKTOOL Obviously, editing or viewing a compiled file is next to impossible. That is where Apktool comes into play. $ apktool d testapp.apk I: Using Apktool 2.0.0 on testapp.apk I: Loading resource table... I: Decoding AndroidManifest.xml with resources... I: Loading resource table from file: 1.apk I: Regular manifest package... I: Decoding file-resources... I: Decoding values */* XMLs... I: Baksmaling classes.dex... I: Copying assets and libs... $ Viewing AndroidManifest.xml again results in something much more human readable In addition to XMLs, resources such as 9 patch images, layouts, strings and much more are correctly decoded to source form. Decoding The decode option on Apktool can be invoked either from d or decode like shown below. $ apktool d foo.jar // decodes foo.jar to foo.jar.out folder $ apktool decode foo.jar // decodes foo.jar to foo.jar.out folder $ apktool d bar.apk // decodes bar.apk to bar folder $ apktool decode bar.apk // decodes bar.apk to bar folder $ apktool d bar.apk -o baz // decodes bar.apk to baz folder Building The build option can be invoked either from b or build like shown below $ apktool b foo.jar.out // builds foo.jar.out folder into foo.jar.out/dist/foo.jar file $ apktool build foo.jar.out // builds foo.jar.out folder into foo.jar.out/dist/foo.jar file $ apktool b bar // builds bar folder into bar/dist/bar.apk file $ apktool b . // builds current directory into ./dist $ apktool b bar -o new_bar.apk // builds bar folder into new_bar.apk $ apktool b bar.apk // WRONG: brut.androlib.AndrolibException: brut.directory.PathNotExist: apktool.yml // Must use folder, not apk/jar file InfoIn order to run a rebuilt application. You must resign the application. Android documentation can help with this. Frameworks Frameworks can be installed either from if or install-framework, in addition two parameters -p, --frame-path - Store framework files into -t, --tag - Tag frameworks using Allow for a finer control over how the files are named and how they are stored. $ apktool if framework-res.apk I: Framework installed to: 1.apk // pkgId of framework-res.apk determines number (which is 0x01) $ apktool if com.htc.resources.apk I: Framework installed to: 2.apk // pkgId of com.htc.resources is 0x02 $ apktool if com.htc.resources.apk -t htc I: Framework installed to: 2-htc.apk // pkgId-tag.apk $ apktool if framework-res.apk -p foo/bar I: Framework installed to: foo/bar/1.apk $ apktool if framework-res.apk -t baz -p foo/bar I: Framework installed to: foo/bar/1-baz.apk Migration Instructions v2.1.1 -> v2.2.0 Run the following commands to migrate your framework directory Apktool will work fine without running these commands, this will just cleanup abandoned files unix - mkdir -p ~/.local/share; mv ~/apktool ~/.local/share windows - move %USERPROFILE%\apktool %USERPROFILE%\AppData\Local v2.0.1 -> v2.0.2 Update apktool to v2.0.2 Remove framework file $HOME/apktool/framework/1.apk due to internal API update (Android Marshmallow) v1.5.x -> v2.0.0 Java 1.7 is required Update apktool to v2.0.0 aapt is now included inside the apktool binary. It's not required to maintain your own aapt install under $PATH. (However, features like -a / --aapt are still used and can override the internal aapt) The addition of aapt replaces the need for separate aapt download packages. Helper Scripts may be found here Remove framework $HOME/apktool/framework/1.apk Eagle eyed users will notice resources are now decoded before sources now. This is because we need to know the API version via the manifest for decoding the sources Parameter Changes Smali/baksmali 2.0 are included. This is a big change from 1.4.2. Please read the smali updates here for more information -o / --output is now used for the output of apk/directory -t / --tag is required for tagging framework files -advance / --advanced will launch advance parameters and information on the usage output -m / --match-original is a new feature for apk analysis. This retains the apk is nearly original format, but will make rebuild more than likely not work due to ignoring the changes that newer aapt requires After [d]ecode, there will be new folders (original / unknown) in the decoded apk folder original = META-INF folder / AndroidManifest.xml, which are needed to retain the signature of apks to prevent needing to resign. Used with -c / --copy-original on [b]uild unknown = Files / folders that are not part of the standard AOSP build procedure. These files will be injected back into the rebuilt APK. apktool.yml collects more information than last version SdkInfo - Used to repopulate the sdk information in AndroidManifest.xml since newer aapt requires version information to be passed via parameter packageInfo - Used to help support Android 4.2 renamed manifest feature. Automatically detects differences between resource and manifest and performs automatic --rename-manifest-package on [b]uild versionInfo - Used to repopulate the version information in AndroidManifest.xml since newer aapt requires version information to be passed via parameter compressionType - Used to determine the compression that resources.arsc had on the original apk in order to replicate during [b]uild unknownFiles - Used to record name/location of non-standard files in an apk in order to place correctly on rebuilt apk sharedLibrary - Used to help support Android 5 shared library feature by automatically detecting shared libraries and using --shared-lib on [b]uild Examples of new usage in 2.0 vs 1.5.x Old (Apktool 1.5.x) New (Apktool 2.0.x) apktool if framework-res.apk tag apktool if framework-res.apk -t tag apktool d framework-res.apk output apktool d framework.res.apk -o output apktool b output new.apk apktool b output -o new.apk v1.4.x -> v1.5.1 Update apktool to v1.5.1 Update aapt manually or use package r05-ibot via downloading Mac, Windows or Linux Remove framework file $HOME/apktool/framework/1.apk Intermediate Framework Files As you probably know, Android apps utilize code and resources that are found on the Android OS itself. These are known as framework resources and Apktool relies on these to properly decode and build apks. Every Apktool release contains internally the most up to date AOSP framework at the time of the release. This allows you to decode and build most apks without a problem. However, manufacturers add their own framework files in addition to the regular AOSP ones. To use apktool against these manufacturer apks you must first install the manufacturer framework files. Example Lets say you want to decode HtcContacts.apk from an HTC device. If you try you will get an error message. $ apktool d HtcContacts.apk I: Loading resource table... I: Decoding resources... I: Loading resource table from file: 1.apk W: Could not decode attr value, using undecoded value instead: ns=android, name=drawable W: Could not decode attr value, using undecoded value instead: ns=android, name=icon Can't find framework resources for package of id: 2. You must install proper framework files, see project website for more info. We must get HTC framework resources before decoding this apk. We pull com.htc.resources.apk from our device and install it $ apktool if com.htc.resources.apk I: Framework installed to: 2.apk Now we will try this decode again. $ apktool d HtcContacts.apk I: Loading resource table... I: Decoding resources... I: Loading resource table from file: /home/brutall/apktool/framework/1.apk I: Loading resource table from file: /home/brutall/apktool/framework/2.apk I: Copying assets and libs... As you can see. Apktool leveraged both 1.apk and 2.apk framework files in order to properly decode this application. Finding Frameworks For the most part any apk in /system/framework on a device will be a framework file. On some devices they might reside in /data/system-framework and even cleverly hidden in /system/app or /system/priv-app. They are usually named with the naming of "resources", "res" or "framework". Example HTC has a framework called com.htc.resources.apk, LG has one called lge-res.apk After you find a framework file you could pull it via adb pull /path/to/file or use a file manager application. After you have the file locally, pay attention to how Apktool installs it. The number that the framework is named during install corresponds to the pkgId of the application. These values should range from 1 to 9. Any APK that installs itself as 127 is 0x7F which is an internal pkgId. Internal Frameworks Apktool comes with an internal framework like mentioned above. This file is copied to $HOME/apktool/framework/1.apk during use. Warning Apktool has no knowledge of what version of framework resides there. It will assume its up to date, so delete the file during Apktool upgrades Managing framework files Frameworks are stored in $HOME/apktool/framework for Windows and Unix systems. Mac OS X has a slightly different folder location of $HOME/Library/apktool/framework. If these directories are not available it will default to java.io.tmpdir which is usually /tmp. This is a volatile directory so it would make sense to take advantage of the parameter --frame-path to select an alternative folder for framework files. Note Apktool has no control over the frameworks once installed, but you are free to manage these files on your own. Tagging framework files Frameworks are stored in the naming convention of: -.apk. They are identified by pkgId and optionally custom tag. Usually tagging frameworks isn't necessary, but if you work on apps from many different devices and they have incompatible frameworks, you will need some way to easily switch between them. You could tag frameworks by: $ apktool if com.htc.resources.apk -t hero I: Framework installed to: /home/brutall/apktool/framework/2-hero.apk $ apktool if com.htc.resources.apk -t desire I: Framework installed to: /home/brutall/apktool/framework/2-desire.apk Then: $ apktool d HtcContacts.apk -t hero I: Loading resource table... I: Decoding resources... I: Loading resource table from file: /home/brutall/apktool/framework/1.apk I: Loading resource table from file: /home/brutall/apktool/framework/2-hero.apk I: Copying assets and libs... $ apktool d HtcContacts.apk -t desire I: Loading resource table... I: Decoding resources... I: Loading resource table from file: /home/brutall/apktool/framework/1.apk I: Loading resource table from file: /home/brutall/apktool/framework/2-desire.apk I: Copying assets and libs... You don't have to select a tag when building apk - apktool automatically uses the same tag, as when decoding. Smali Debugging Warning SmaliDebugging has been marked as deprecated in 2.0.3, and removed in 2.1. Please check SmaliIdea for a debugger. Apktool makes possible to debug smali code step by step, watch variables, set breakpoints, etc. General information Generally we need several things to run Java debugging session: debugger server (usually Java VM) debugger client (usually IDE like IntelliJ, Eclipse or Netbeans) client must have sources of debugged application server must have binaries compiled with debugging symbols referencing these sources sources must be java files with at least package and class definitions, to properly connect them with debugging symbols In our particular situation we have: server: Monitor (Previously DDMS), part of Android SDK, standard for debugging Android applications - explained here client: any JPDA client - most of decent IDEs have support for this protocol. sources: smali code modified by apktool to satisfy above requirements (".java" extension, class declaration, etc.). Apktool modifies them when decoding apk in debug mode. binaries: when building apk in debug mode, apktool removes original symbols and adds new, which are referencing smali code (line numbers, registers/variables, etc.) Info To successfully run debug sessions, the apk must be both decoded and built in debug mode. Decoding with debug decodes the application differently to allow the debug rebuild option to inject lines allowing the debugger to identify variables and types.-d / --debug General instructions Above information is enough to debug smali code using apktool, but if you aren't familiar with DDMS and Java debugging, then you probably still don't know how to do it. Below are simple instructions for doing it using IntelliJ or Netbeans. Decode apk in debug mode: $ apktool d -d -o out app.apk Build new apk in debug mode: $ apktool b -d out Sign, install and run new apk. Follow sub-instructions below depending on IDE. IntelliJ (Android Studio) instructions In IntelliJ add new Java Module Project selecting the "out" directory as project location and the "smali" subdirectory as content root dir. Run Monitor (Android SDK /tools folder), find your application on a list and click it. Note port information in last column - it should be something like "86xx / 8700". In IntelliJ: Debug -> Edit Configurations. Since this is a new project, you will have to create a Debugger. Create a Remote Debugger, with the settings on "Attach" and setting the Port to 8700 (Or whatever Monitor said). The rest of fields should be ok, click "Ok". Start the debugging session. You will see some info in a log and debugging buttons will show up in top panel. Set breakpoint. You must select line with some instruction, you can't set breakpoint on lines starting with ".", ":" or "#". Trigger some action in application. If you run at breakpoint, then thread should stop and you will be able to debug step by step, watch variables, etc. Netbeans instructions In Netbeans add new Java Project with Existing Sources, select "out" directory as project root and "smali" subdirectory as sources dir. Run DDMS, find your application on a list and click it. Note port information in last column - it should be something like "86xx / 8700". In Netbeans: Debug -> Attach Debugger -> select JPDA and set Port to 8700 (or whatever you saw in previous step). Rest of fields should be ok, click "Ok". Debugging session should start: you will see some info in a log and debugging buttons will show up in top panel. Set breakpoint. You must select line with some instruction, you can't set breakpoint on lines starting with ".", ":" or "#". Trigger some action in application. If you run at breakpoint, then thread should stop and you will be able to debug step by step, watch variables, etc. Limitations/Issues Because IDE doesn't have full sources, it doesn't know about class members and such. Variables watching works because most of data could be read from memory (objects in Java know about their types), but if for example, you watch an object and it has some nulled member, then you won't see, what type this member is. 9Patch Images Docs exist for the mysterious 9patch images here and there. (Read these first). These docs though are meant for developers and lack information for those who work with already compiled 3rd party applications. There you can find information how to create them, but no information about how they actually work. I will try and explain it here. The official docs miss one point that 9patch images come in two forms: source & compiled. source - You know this one. You find it in the source of an application or freely available online. These are images with a black border around them. compiled - The mysterious form found in apk files. There are no borders and the 9patch data is written into a binary chunk called npTc. You can't see or modify it easily, but Android OS can as its quicker to read. There are problems related to the above two points. You can't move 9patch images between both types without a conversion. If you try and unpack 9patch images from an apk and use it in the source of another, you will get errors during build. Also vice versa, you cannot take source 9patch images directly into an apk. 9patch binary chunk isn't recognized by modern image processing tools. So modifying the compiled image will more than likely break the npTc chunk, thus breaking the image on the device. The only solution to this problem is to easily convert between these two types. The encoder (which takes source to compiled) is built into the aapt tool and is automatically used during build. This means we only need to build a decoder which has been in apktool since v1.3.0 and is automatically ran on all 9patch images during decode. So if you want to modify 9patch images, don't do it directly. Use apktool to decode the application (including the 9patch images) and then modify the images. At that point when you build the application back, the source 9patch images will be compiled. Other FAQ What about the -j switch shown from the original YouTube videos? Read Issue 199. In short - it doesn't exist. Is it possible to run apktool on a device? Sadly not. There are some incompatibilities with SnakeYAML, java.nio and aapt Where can I download sources of apktool? From our Github or Bitbucket project. Resulting apk file is much smaller than original! Is there something missing? There are a couple of reasons that might cause this. Apktool builds unsigned apks. This means an entire directory META-INF is missing. New aapt binary. Newer versions of apktool contain a newer aapt which optimizes images differently. These points might have contributed to a smaller than normal apk There is no META-INF dir in resulting apk. Is this ok? Yes. META-INF contains apk signatures. After modifying the apk it is no longer signed. You can use -c / --copy-original to retain these signatures. However, using -c uses the original AndroidManifest.xml file, so changes to it will be lost. What do you call "magic apks"? For some reason there are apks that are built using modified build tools. These apks don't work on a regular AOSP Android build, but usually are accompanied by a modified system that can read these modified apks. Apktool cannot handle these apks, therefore they are "magic". Could I integrate apktool into my own tool? Could I modify apktool sources? Do I have to credit you? Actually the Apache License, which apktool uses, answers all these questions. Yes you can redistribute and/or modify apktool without my permission. However, if you do it would be nice to add our contributors (brut.all, iBotPeaches and JesusFreke) into your credits but it's not required. Where does apktool store its framework files? unix - $HOME/.local/share/apktool mac - $HOME/Library/apktool windows - $HOME/AppData/Local/apktool Options Utility Options that can be executed at any time. -version, --version Outputs current version. (Ex: 1.5.2) -v, --verbose Verbose output. Must be first parameter -q, --quiet Quiet output. Must be first parameter -advance, --advanced Advance usage output Decode These are all the options when decoding an apk. --api The numeric api-level of the smali files to generate (defaults to targetSdkVersion) -b, --no-debug-info Prevents baksmali from writing out debug info (.local, .param, .line, etc). Preferred to use if you are comparing smali from the same APK of different versions. The line numbers and debug will change among versions, which can make DIFF reports a pain. -f, --force Force delete destination directory. Use when trying to decode to a folder that already exists --keep-broken-res - Advanced If there is an error like "Invalid Config Flags Detected. Dropping Resources...". This means that APK has a different structure then Apktool can handle. This might be a newer Android version or a random APK that doesn't match standards. Running this will allow the decode, but then you have to manually fix the folders with -ERR in them. -m, --match-original - Used for analysis Matches files closest as possible to original, but prevents rebuild. -o, --output The name of the folder that apk gets written to -p, --frame-path The folder location where framework files should be stored/read from -r, --no-res This will prevent the decompile of resources. This keeps the resources.arsc intact without any decode. If only editing Java (smali) then this is the recommend for faster decompile & rebuild -s, --no-src This will prevent the disassemble of the dex files. This keeps the apk classes.dex file and simply moves it during build. If your only editing the resources. This is recommended for faster decompile & rebuild -t, --frame-tag Uses framework files tagged via Rebuild These are all the options when building an apk. -a, --aapt Loads aapt from the specified file location, instead of relying on path. Falls back to $PATH loading, if no file found -c, --copy-original - Will still require signature resign post API18 Copies original AndroidManifest.xml and META-INF folder into built apk -d, --debug Adds debuggable="true" to AndroidManifest file. -f, --force-all Overwrites existing files during build, reassembling the resources.arsc file and classes.dex file -o, --output The name and location of the apk that gets written -p, --frame-path The location where framework files are loaded from
Table of Contents Header Files The #define Guard Header File Dependencies Inline Functions The -inl.h Files Function Parameter Ordering Names and Order of Includes Scoping Namespaces Nested Classes Nonmember, Static Member, and Global Functions Local Variables Static and Global Variables Classes Doing Work in Constructors Default Constructors Explicit Constructors Copy Constructors Structs vs. Classes Inheritance Multiple Inheritance Interfaces Operator Overloading Access Control Declaration Order Write Short Functions Google-Specific Magic Smart Pointers cpplint Other C++ Features Reference Arguments Function Overloading Default Arguments Variable-Length Arrays and alloca() Friends Exceptions Run-Time Type Information (RTTI) Casting Streams Preincrement and Predecrement Use of const Integer Types 64-bit Portability Preprocessor Macros 0 and NULL sizeof Boost C++0x Naming General Naming Rules File Names Type Names Variable Names Constant Names Function Names Namespace Names Enumerator Names Macro Names Exceptions to Naming Rules Comments Comment Style File Comments Class Comments Function Comments Variable Comments Implementation Comments Punctuation, Spelling and Grammar TODO Comments Deprecation Comments Formatting Line Length Non-ASCII Characters Spaces vs. Tabs Function Declarations and Definitions Function Calls Conditionals Loops and Switch Statements Pointer and Reference Expressions Boolean Expressions Return Values Variable and Array Initialization Preprocessor Directives Class Format Constructor Initializer Lists Namespace Formatting Horizontal Whitespace Vertical Whitespace Exceptions to the Rules Existing Non-conformant Code Windows Code Important Note Displaying Hidden Details in this Guide link ▶This style guide contains many details that are initially hidden from view. They are marked by the triangle icon, which you see here on your left. Click it now. You should see "Hooray" appear below. Hooray! Now you know you can expand points to get more details. Alternatively, there's an "expand all" at the top of this document. Background C++ is the main development language used by many of Google's open-source projects. As every C++ programmer knows, the language has many powerful features, but this power brings with it complexity, which in turn can make code more bug-prone and harder to read and maintain. The goal of this guide is to manage this complexity by describing in detail the dos and don'ts of writing C++ code. These rules exist to keep the code base manageable while still allowing coders to use C++ language features productively. Style, also known as readability, is what we call the conventions that govern our C++ code. The term Style is a bit of a misnomer, since these conventions cover far more than just source file formatting. One way in which we keep the code base manageable is by enforcing consistency. It is very important that any programmer be able to look at another's code and quickly understand it. Maintaining a uniform style and following conventions means that we can more easily use "pattern-matching" to infer what various symbols are and what invariants are true about them. Creating common, required idioms and patterns makes code much easier to understand. In some cases there might be good arguments for changing certain style rules, but we nonetheless keep things as they are in order to preserve consistency. Another issue this guide addresses is that of C++ feature bloat. C++ is a huge language with many advanced features. In some cases we constrain, or even ban, use of certain features. We do this to keep code simple and to avoid the various common errors and problems that these features can cause. This guide lists these features and explains why their use is restricted. Open-source projects developed by Google conform to the requirements in this guide. Note that this guide is not a C++ tutorial: we assume that the reader is familiar with the language. Header Files In general, every .cc file should have an associated .h file. There are some common exceptions, such as unittests and small .cc files containing just a main() function. Correct use of header files can make a huge difference to the readability, size and performance of your code. The following rules will guide you through the various pitfalls of using header files. The #define Guard link ▶All header files should have #define guards to prevent multiple inclusion. The format of the symbol name should be ___H_. To guarantee uniqueness, they should be based on the full path in a project's source tree. For example, the file foo/src/bar/baz.h in project foo should have the following guard: #ifndef FOO_BAR_BAZ_H_ #define FOO_BAR_BAZ_H_ ... #endif // FOO_BAR_BAZ_H_ Header File Dependencies link ▶Don't use an #include when a forward declaration would suffice. When you include a header file you introduce a dependency that will cause your code to be recompiled whenever the header file changes. If your header file includes other header files, any change to those files will cause any code that includes your header to be recompiled. Therefore, we prefer to minimize includes, particularly includes of header files in other header files. You can significantly minimize the number of header files you need to include in your own header files by using forward declarations. For example, if your header file uses the File class in ways that do not require access to the declaration of the File class, your header file can just forward declare class File; instead of having to #include "file/base/file.h". How can we use a class Foo in a header file without access to its definition? We can declare data members of type Foo* or Foo&. We can declare (but not define) functions with arguments, and/or return values, of type Foo. (One exception is if an argument Foo or const Foo& has a non-explicit, one-argument constructor, in which case we need the full definition to support automatic type conversion.) We can declare static data members of type Foo. This is because static data members are defined outside the class definition. On the other hand, you must include the header file for Foo if your class subclasses Foo or has a data member of type Foo. Sometimes it makes sense to have pointer (or better, scoped_ptr) members instead of object members. However, this complicates code readability and imposes a performance penalty, so avoid doing this transformation if the only purpose is to minimize includes in header files. Of course, .cc files typically do require the definitions of the classes they use, and usually have to include several header files. Note: If you use a symbol Foo in your source file, you should bring in a definition for Foo yourself, either via an #include or via a forward declaration. Do not depend on the symbol being brought in transitively via headers not directly included. One exception is if Foo is used in myfile.cc, it's ok to #include (or forward-declare) Foo in myfile.h, instead of myfile.cc. Inline Functions link ▶Define functions inline only when they are small, say, 10 lines or less. Definition: You can declare functions in a way that allows the compiler to expand them inline rather than calling them through the usual function call mechanism. Pros: Inlining a function can generate more efficient object code, as long as the inlined function is small. Feel free to inline accessors and mutators, and other short, performance-critical functions. Cons: Overuse of inlining can actually make programs slower. Depending on a function's size, inlining it can cause the code size to increase or decrease. Inlining a very small accessor function will usually decrease code size while inlining a very large function can dramatically increase code size. On modern processors smaller code usually runs faster due to better use of the instruction cache. Decision: A decent rule of thumb is to not inline a function if it is more than 10 lines long. Beware of destructors, which are often longer than they appear because of implicit member- and base-destructor calls! Another useful rule of thumb: it's typically not cost effective to inline functions with loops or switch statements (unless, in the common case, the loop or switch statement is never executed). It is important to know that functions are not always inlined even if they are declared as such; for example, virtual and recursive functions are not normally inlined. Usually recursive functions should not be inline. The main reason for making a virtual function inline is to place its definition in the class, either for convenience or to document its behavior, e.g., for accessors and mutators. The -inl.h Files link ▶You may use file names with a -inl.h suffix to define complex inline functions when needed. The definition of an inline function needs to be in a header file, so that the compiler has the definition available for inlining at the call sites. However, implementation code properly belongs in .cc files, and we do not like to have much actual code in .h files unless there is a readability or performance advantage. If an inline function definition is short, with very little, if any, logic in it, you should put the code in your .h file. For example, accessors and mutators should certainly be inside a class definition. More complex inline functions may also be put in a .h file for the convenience of the implementer and callers, though if this makes the .h file too unwieldy you can instead put that code in a separate -inl.h file. This separates the implementation from the class definition, while still allowing the implementation to be included where necessary. Another use of -inl.h files is for definitions of function templates. This can be used to keep your template definitions easy to read. Do not forget that a -inl.h file requires a #define guard just like any other header file. Function Parameter Ordering link ▶When defining a function, parameter order is: inputs, then outputs. Parameters to C/C++ functions are either input to the function, output from the function, or both. Input parameters are usually values or const references, while output and input/output parameters will be non-const pointers. When ordering function parameters, put all input-only parameters before any output parameters. In particular, do not add new parameters to the end of the function just because they are new; place new input-only parameters before the output parameters. This is not a hard-and-fast rule. Parameters that are both input and output (often classes/structs) muddy the waters, and, as always, consistency with related functions may require you to bend the rule. Names and Order of Includes link ▶Use standard order for readability and to avoid hidden dependencies: C library, C++ library, other libraries' .h, your project's .h. All of a project's header files should be listed as descentants of the project's source directory without use of UNIX directory shortcuts . (the current directory) or .. (the parent directory). For example, google-awesome-project/src/base/logging.h should be included as #include "base/logging.h" In dir/foo.cc, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows: dir2/foo2.h (preferred location — see details below). C system files. C++ system files. Other libraries' .h files. Your project's .h files. The preferred ordering reduces hidden dependencies. We want every header file to be compilable on its own. The easiest way to achieve this is to make sure that every one of them is the first .h file #included in some .cc. dir/foo.cc and dir2/foo2.h are often in the same directory (e.g. base/basictypes_test.cc and base/basictypes.h), but can be in different directories too. Within each section it is nice to order the includes alphabetically. For example, the includes in google-awesome-project/src/foo/internal/fooserver.cc might look like this: #include "foo/public/fooserver.h" // Preferred location. #include #include #include #include #include "base/basictypes.h" #include "base/commandlineflags.h" #include "foo/public/bar.h" Scoping Namespaces link ▶Unnamed namespaces in .cc files are encouraged. With named namespaces, choose the name based on the project, and possibly its path. Do not use a using-directive. Definition: Namespaces subdivide the global scope into distinct, named scopes, and so are useful for preventing name collisions in the global scope. Pros: Namespaces provide a (hierarchical) axis of naming, in addition to the (also hierarchical) name axis provided by classes. For example, if two different projects have a class Foo in the global scope, these symbols may collide at compile time or at runtime. If each project places their code in a namespace, project1::Foo and project2::Foo are now distinct symbols that do not collide. Cons: Namespaces can be confusing, because they provide an additional (hierarchical) axis of naming, in addition to the (also hierarchical) name axis provided by classes. Use of unnamed spaces in header files can easily cause violations of the C++ One Definition Rule (ODR). Decision: Use namespaces according to the policy described below. Unnamed Namespaces Unnamed namespaces are allowed and even encouraged in .cc files, to avoid runtime naming conflicts: namespace { // This is in a .cc file. // The content of a namespace is not indented enum { kUnused, kEOF, kError }; // Commonly used tokens. bool AtEof() { return pos_ == kEOF; } // Uses our namespace's EOF. } // namespace However, file-scope declarations that are associated with a particular class may be declared in that class as types, static data members or static member functions rather than as members of an unnamed namespace. Terminate the unnamed namespace as shown, with a comment // namespace. Do not use unnamed namespaces in .h files. Named Namespaces Named namespaces should be used as follows: Namespaces wrap the entire source file after includes, gflags definitions/declarations, and forward declarations of classes from other namespaces: // In the .h file namespace mynamespace { // All declarations are within the namespace scope. // Notice the lack of indentation. class MyClass { public: ... void Foo(); }; } // namespace mynamespace // In the .cc file namespace mynamespace { // Definition of functions is within scope of the namespace. void MyClass::Foo() { ... } } // namespace mynamespace The typical .cc file might have more complex detail, including the need to reference classes in other namespaces. #include "a.h" DEFINE_bool(someflag, false, "dummy flag"); class C; // Forward declaration of class C in the global namespace. namespace a { class A; } // Forward declaration of a::A. namespace b { ...code for b... // Code goes against the left margin. } // namespace b Do not declare anything in namespace std, not even forward declarations of standard library classes. Declaring entities in namespace std is undefined behavior, i.e., not portable. To declare entities from the standard library, include the appropriate header file. You may not use a using-directive to make all names from a namespace available. // Forbidden -- This pollutes the namespace. using namespace foo; You may use a using-declaration anywhere in a .cc file, and in functions, methods or classes in .h files. // OK in .cc files. // Must be in a function, method or class in .h files. using ::foo::bar; Namespace aliases are allowed anywhere in a .cc file, anywhere inside the named namespace that wraps an entire .h file, and in functions and methods. // Shorten access to some commonly used names in .cc files. namespace fbz = ::foo::bar::baz; // Shorten access to some commonly used names (in a .h file). namespace librarian { // The following alias is available to all files including // this header (in namespace librarian): // alias names should therefore be chosen consistently // within a project. namespace pd_s = ::pipeline_diagnostics::sidetable; inline void my_inline_function() { // namespace alias local to a function (or method). namespace fbz = ::foo::bar::baz; ... } } // namespace librarian Note that an alias in a .h file is visible to everyone #including that file, so public headers (those available outside a project) and headers transitively #included by them, should avoid defining aliases, as part of the general goal of keeping public APIs as small as possible. Nested Classes link ▶Although you may use public nested classes when they are part of an interface, consider a namespace to keep declarations out of the global scope. Definition: A class can define another class within it; this is also called a member class. class Foo { private: // Bar is a member class, nested within Foo. class Bar { ... }; }; Pros: This is useful when the nested (or member) class is only used by the enclosing class; making it a member puts it in the enclosing class scope rather than polluting the outer scope with the class name. Nested classes can be forward declared within the enclosing class and then defined in the .cc file to avoid including the nested class definition in the enclosing class declaration, since the nested class definition is usually only relevant to the implementation. Cons: Nested classes can be forward-declared only within the definition of the enclosing class. Thus, any header file manipulating a Foo::Bar* pointer will have to include the full class declaration for Foo. Decision: Do not make nested classes public unless they are actually part of the interface, e.g., a class that holds a set of options for some method. Nonmember, Static Member, and Global Functions link ▶Prefer nonmember functions within a namespace or static member functions to global functions; use completely global functions rarely. Pros: Nonmember and static member functions can be useful in some situations. Putting nonmember functions in a namespace avoids polluting the global namespace. Cons: Nonmember and static member functions may make more sense as members of a new class, especially if they access external resources or have significant dependencies. Decision: Sometimes it is useful, or even necessary, to define a function not bound to a class instance. Such a function can be either a static member or a nonmember function. Nonmember functions should not depend on external variables, and should nearly always exist in a namespace. Rather than creating classes only to group static member functions which do not share static data, use namespaces instead. Functions defined in the same compilation unit as production classes may introduce unnecessary coupling and link-time dependencies when directly called from other compilation units; static member functions are particularly susceptible to this. Consider extracting a new class, or placing the functions in a namespace possibly in a separate library. If you must define a nonmember function and it is only needed in its .cc file, use an unnamed namespace or static linkage (eg static int Foo() {...}) to limit its scope. Local Variables link ▶Place a function's variables in the narrowest scope possible, and initialize variables in the declaration. C++ allows you to declare variables anywhere in a function. We encourage you to declare them in as local a scope as possible, and as close to the first use as possible. This makes it easier for the reader to find the declaration and see what type the variable is and what it was initialized to. In particular, initialization should be used instead of declaration and assignment, e.g. int i; i = f(); // Bad -- initialization separate from declaration. int j = g(); // Good -- declaration has initialization. Note that gcc implements for (int i = 0; i < 10; ++i) correctly (the scope of i is only the scope of the for loop), so you can then reuse i in another for loop in the same scope. It also correctly scopes declarations in if and while statements, e.g. while (const char* p = strchr(str, '/')) str = p + 1; There is one caveat: if the variable is an object, its constructor is invoked every time it enters scope and is created, and its destructor is invoked every time it goes out of scope. // Inefficient implementation: for (int i = 0; i < 1000000; ++i) { Foo f; // My ctor and dtor get called 1000000 times each. f.DoSomething(i); } It may be more efficient to declare such a variable used in a loop outside that loop: Foo f; // My ctor and dtor get called once each. for (int i = 0; i < 1000000; ++i) { f.DoSomething(i); } Static and Global Variables link ▶Static or global variables of class type are forbidden: they cause hard-to-find bugs due to indeterminate order of construction and destruction. Objects with static storage duration, including global variables, static variables, static class member variables, and function static variables, must be Plain Old Data (POD): only ints, chars, floats, or pointers, or arrays/structs of POD. The order in which class constructors and initializers for static variables are called is only partially specified in C++ and can even change from build to build, which can cause bugs that are difficult to find. Therefore in addition to banning globals of class type, we do not allow static POD variables to be initialized with the result of a function, unless that function (such as getenv(), or getpid()) does not itself depend on any other globals. Likewise, the order in which destructors are called is defined to be the reverse of the order in which the constructors were called. Since constructor order is indeterminate, so is destructor order. For example, at program-end time a static variable might have been destroyed, but code still running -- perhaps in another thread -- tries to access it and fails. Or the destructor for a static 'string' variable might be run prior to the destructor for another variable that contains a reference to that string. As a result we only allow static variables to contain POD data. This rule completely disallows vector (use C arrays instead), or string (use const char []). If you need a static or global variable of a class type, consider initializing a pointer (which will never be freed), from either your main() function or from pthread_once(). Note that this must be a raw pointer, not a "smart" pointer, since the smart pointer's destructor will have the order-of-destructor issue that we are trying to avoid. Classes Classes are the fundamental unit of code in C++. Naturally, we use them extensively. This section lists the main dos and don'ts you should follow when writing a class. Doing Work in Constructors link ▶In general, constructors should merely set member variables to their initial values. Any complex initialization should go in an explicit Init() method. Definition: It is possible to perform initialization in the body of the constructor. Pros: Convenience in typing. No need to worry about whether the class has been initialized or not. Cons: The problems with doing work in constructors are: There is no easy way for constructors to signal errors, short of using exceptions (which are forbidden). If the work fails, we now have an object whose initialization code failed, so it may be an indeterminate state. If the work calls virtual functions, these calls will not get dispatched to the subclass implementations. Future modification to your class can quietly introduce this problem even if your class is not currently subclassed, causing much confusion. If someone creates a global variable of this type (which is against the rules, but still), the constructor code will be called before main(), possibly breaking some implicit assumptions in the constructor code. For instance, gflags will not yet have been initialized. Decision: If your object requires non-trivial initialization, consider having an explicit Init() method. In particular, constructors should not call virtual functions, attempt to raise errors, access potentially uninitialized global variables, etc. Default Constructors link ▶You must define a default constructor if your class defines member variables and has no other constructors. Otherwise the compiler will do it for you, badly. Definition: The default constructor is called when we new a class object with no arguments. It is always called when calling new[] (for arrays). Pros: Initializing structures by default, to hold "impossible" values, makes debugging much easier. Cons: Extra work for you, the code writer. Decision: If your class defines member variables and has no other constructors you must define a default constructor (one that takes no arguments). It should preferably initialize the object in such a way that its internal state is consistent and valid. The reason for this is that if you have no other constructors and do not define a default constructor, the compiler will generate one for you. This compiler generated constructor may not initialize your object sensibly. If your class inherits from an existing class but you add no new member variables, you are not required to have a default constructor. Explicit Constructors link ▶Use the C++ keyword explicit for constructors with one argument. Definition: Normally, if a constructor takes one argument, it can be used as a conversion. For instance, if you define Foo::Foo(string name) and then pass a string to a function that expects a Foo, the constructor will be called to convert the string into a Foo and will pass the Foo to your function for you. This can be convenient but is also a source of trouble when things get converted and new objects created without you meaning them to. Declaring a constructor explicit prevents it from being invoked implicitly as a conversion. Pros: Avoids undesirable conversions. Cons: None. Decision: We require all single argument constructors to be explicit. Always put explicit in front of one-argument constructors in the class definition: explicit Foo(string name); The exception is copy constructors, which, in the rare cases when we allow them, should probably not be explicit. Classes that are intended to be transparent wrappers around other classes are also exceptions. Such exceptions should be clearly marked with comments. Copy Constructors link ▶Provide a copy constructor and assignment operator only when necessary. Otherwise, disable them with DISALLOW_COPY_AND_ASSIGN. Definition: The copy constructor and assignment operator are used to create copies of objects. The copy constructor is implicitly invoked by the compiler in some situations, e.g. passing objects by value. Pros: Copy constructors make it easy to copy objects. STL containers require that all contents be copyable and assignable. Copy constructors can be more efficient than CopyFrom()-style workarounds because they combine construction with copying, the compiler can elide them in some contexts, and they make it easier to avoid heap allocation. Cons: Implicit copying of objects in C++ is a rich source of bugs and of performance problems. It also reduces readability, as it becomes hard to track which objects are being passed around by value as opposed to by reference, and therefore where changes to an object are reflected. Decision: Few classes need to be copyable. Most should have neither a copy constructor nor an assignment operator. In many situations, a pointer or reference will work just as well as a copied value, with better performance. For example, you can pass function parameters by reference or pointer instead of by value, and you can store pointers rather than objects in an STL container. If your class needs to be copyable, prefer providing a copy method, such as CopyFrom() or Clone(), rather than a copy constructor, because such methods cannot be invoked implicitly. If a copy method is insufficient in your situation (e.g. for performance reasons, or because your class needs to be stored by value in an STL container), provide both a copy constructor and assignment operator. If your class does not need a copy constructor or assignment operator, you must explicitly disable them. To do so, add dummy declarations for the copy constructor and assignment operator in the private: section of your class, but do not provide any corresponding definition (so that any attempt to use them results in a link error). For convenience, a DISALLOW_COPY_AND_ASSIGN macro can be used: // A macro to disallow the copy constructor and operator= functions // This should be used in the private: declarations for a class #define DISALLOW_COPY_AND_ASSIGN(TypeName) \ TypeName(const TypeName&); \ void operator=(const TypeName&) Then, in class Foo: class Foo { public: Foo(int f); ~Foo(); private: DISALLOW_COPY_AND_ASSIGN(Foo); }; Structs vs. Classes link ▶Use a struct only for passive objects that carry data; everything else is a class. The struct and class keywords behave almost identically in C++. We add our own semantic meanings to each keyword, so you should use the appropriate keyword for the data-type you're defining. structs should be used for passive objects that carry data, and may have associated constants, but lack any functionality other than access/setting the data members. The accessing/setting of fields is done by directly accessing the fields rather than through method invocations. Methods should not provide behavior but should only be used to set up the data members, e.g., constructor, destructor, Initialize(), Reset(), Validate(). If more functionality is required, a class is more appropriate. If in doubt, make it a class. For consistency with STL, you can use struct instead of class for functors and traits. Note that member variables in structs and classes have different naming rules. Inheritance link ▶Composition is often more appropriate than inheritance. When using inheritance, make it public. Definition: When a sub-class inherits from a base class, it includes the definitions of all the data and operations that the parent base class defines. In practice, inheritance is used in two major ways in C++: implementation inheritance, in which actual code is inherited by the child, and interface inheritance, in which only method names are inherited. Pros: Implementation inheritance reduces code size by re-using the base class code as it specializes an existing type. Because inheritance is a compile-time declaration, you and the compiler can understand the operation and detect errors. Interface inheritance can be used to programmatically enforce that a class expose a particular API. Again, the compiler can detect errors, in this case, when a class does not define a necessary method of the API. Cons: For implementation inheritance, because the code implementing a sub-class is spread between the base and the sub-class, it can be more difficult to understand an implementation. The sub-class cannot override functions that are not virtual, so the sub-class cannot change implementation. The base class may also define some data members, so that specifies physical layout of the base class. Decision: All inheritance should be public. If you want to do private inheritance, you should be including an instance of the base class as a member instead. Do not overuse implementation inheritance. Composition is often more appropriate. Try to restrict use of inheritance to the "is-a" case: Bar subclasses Foo if it can reasonably be said that Bar "is a kind of" Foo. Make your destructor virtual if necessary. If your class has virtual methods, its destructor should be virtual. Limit the use of protected to those member functions that might need to be accessed from subclasses. Note that data members should be private. When redefining an inherited virtual function, explicitly declare it virtual in the declaration of the derived class. Rationale: If virtual is omitted, the reader has to check all ancestors of the class in question to determine if the function is virtual or not. Multiple Inheritance link ▶Only very rarely is multiple implementation inheritance actually useful. We allow multiple inheritance only when at most one of the base classes has an implementation; all other base classes must be pure interface classes tagged with the Interface suffix. Definition: Multiple inheritance allows a sub-class to have more than one base class. We distinguish between base classes that are pure interfaces and those that have an implementation. Pros: Multiple implementation inheritance may let you re-use even more code than single inheritance (see Inheritance). Cons: Only very rarely is multiple implementation inheritance actually useful. When multiple implementation inheritance seems like the solution, you can usually find a different, more explicit, and cleaner solution. Decision: Multiple inheritance is allowed only when all superclasses, with the possible exception of the first one, are pure interfaces. In order to ensure that they remain pure interfaces, they must end with the Interface suffix. Note: There is an exception to this rule on Windows. Interfaces link ▶Classes that satisfy certain conditions are allowed, but not required, to end with an Interface suffix. Definition: A class is a pure interface if it meets the following requirements: It has only public pure virtual ("= 0") methods and static methods (but see below for destructor). It may not have non-static data members. It need not have any constructors defined. If a constructor is provided, it must take no arguments and it must be protected. If it is a subclass, it may only be derived from classes that satisfy these conditions and are tagged with the Interface suffix. An interface class can never be directly instantiated because of the pure virtual method(s) it declares. To make sure all implementations of the interface can be destroyed correctly, they must also declare a virtual destructor (in an exception to the first rule, this should not be pure). See Stroustrup, The C++ Programming Language, 3rd edition, section 12.4 for details. Pros: Tagging a class with the Interface suffix lets others know that they must not add implemented methods or non static data members. This is particularly important in the case of multiple inheritance. Additionally, the interface concept is already well-understood by Java programmers. Cons: The Interface suffix lengthens the class name, which can make it harder to read and understand. Also, the interface property may be considered an implementation detail that shouldn't be exposed to clients. Decision: A class may end with Interface only if it meets the above requirements. We do not require the converse, however: classes that meet the above requirements are not required to end with Interface. Operator Overloading link ▶Do not overload operators except in rare, special circumstances. Definition: A class can define that operators such as + and / operate on the class as if it were a built-in type. Pros: Can make code appear more intuitive because a class will behave in the same way as built-in types (such as int). Overloaded operators are more playful names for functions that are less-colorfully named, such as Equals() or Add(). For some template functions to work correctly, you may need to define operators. Cons: While operator overloading can make code more intuitive, it has several drawbacks: It can fool our intuition into thinking that expensive operations are cheap, built-in operations. It is much harder to find the call sites for overloaded operators. Searching for Equals() is much easier than searching for relevant invocations of ==. Some operators work on pointers too, making it easy to introduce bugs. Foo + 4 may do one thing, while &Foo + 4 does something totally different. The compiler does not complain for either of these, making this very hard to debug. Overloading also has surprising ramifications. For instance, if a class overloads unary operator&, it cannot safely be forward-declared. Decision: In general, do not overload operators. The assignment operator (operator=), in particular, is insidious and should be avoided. You can define functions like Equals() and CopyFrom() if you need them. Likewise, avoid the dangerous unary operator& at all costs, if there's any possibility the class might be forward-declared. However, there may be rare cases where you need to overload an operator to interoperate with templates or "standard" C++ classes (such as operator<<(ostream&, const T&) for logging). These are acceptable if fully justified, but you should try to avoid these whenever possible. In particular, do not overload operator== or operator< just so that your class can be used as a key in an STL container; instead, you should create equality and comparison functor types when declaring the container. Some of the STL algorithms do require you to overload operator==, and you may do so in these cases, provided you document why. See also Copy Constructors and Function Overloading. Access Control link ▶Make data members private, and provide access to them through accessor functions as needed (for technical reasons, we allow data members of a test fixture class to be protected when using Google Test). Typically a variable would be called foo_ and the accessor function foo(). You may also want a mutator function set_foo(). Exception: static const data members (typically called kFoo) need not be private. The definitions of accessors are usually inlined in the header file. See also Inheritance and Function Names. Declaration Order link ▶Use the specified order of declarations within a class: public: before private:, methods before data members (variables), etc. Your class definition should start with its public: section, followed by its protected: section and then its private: section. If any of these sections are empty, omit them. Within each section, the declarations generally should be in the following order: Typedefs and Enums Constants (static const data members) Constructors Destructor Methods, including static methods Data Members (except static const data members) Friend declarations should always be in the private section, and the DISALLOW_COPY_AND_ASSIGN macro invocation should be at the end of the private: section. It should be the last thing in the class. See Copy Constructors. Method definitions in the corresponding .cc file should be the same as the declaration order, as much as possible. Do not put large method definitions inline in the class definition. Usually, only trivial or performance-critical, and very short, methods may be defined inline. See Inline Functions for more details. Write Short Functions link ▶Prefer small and focused functions. We recognize that long functions are sometimes appropriate, so no hard limit is placed on functions length. If a function exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program. Even if your long function works perfectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your functions short and simple makes it easier for other people to read and modify your code. You could find long and complicated functions when working with some code. Do not be intimidated by modifying existing code: if working with such a function proves to be difficult, you find that errors are hard to debug, or you want to use a piece of it in several different contexts, consider breaking up the function into smaller and more manageable pieces. Google-Specific Magic There are various tricks and utilities that we use to make C++ code more robust, and various ways we use C++ that may differ from what you see elsewhere. Smart Pointers link ▶If you actually need pointer semantics, scoped_ptr is great. You should only use std::tr1::shared_ptr under very specific conditions, such as when objects need to be held by STL containers. You should never use auto_ptr. "Smart" pointers are objects that act like pointers but have added semantics. When a scoped_ptr is destroyed, for instance, it deletes the object it's pointing to. shared_ptr is the same way, but implements reference-counting so only the last pointer to an object deletes it. Generally speaking, we prefer that we design code with clear object ownership. The clearest object ownership is obtained by using an object directly as a field or local variable, without using pointers at all. On the other extreme, by their very definition, reference counted pointers are owned by nobody. The problem with this design is that it is easy to create circular references or other strange conditions that cause an object to never be deleted. It is also slow to perform atomic operations every time a value is copied or assigned. Although they are not recommended, reference counted pointers are sometimes the simplest and most elegant way to solve a problem. cpplint link ▶Use cpplint.py to detect style errors. cpplint.py is a tool that reads a source file and identifies many style errors. It is not perfect, and has both false positives and false negatives, but it is still a valuable tool. False positives can be ignored by putting // NOLINT at the end of the line. Some projects have instructions on how to run cpplint.py from their project tools. If the project you are contributing to does not, you can download cpplint.py separately. Other C++ Features Reference Arguments link ▶All parameters passed by reference must be labeled const. Definition: In C, if a function needs to modify a variable, the parameter must use a pointer, eg int foo(int *pval). In C++, the function can alternatively declare a reference parameter: int foo(int &val). Pros: Defining a parameter as reference avoids ugly code like (*pval)++. Necessary for some applications like copy constructors. Makes it clear, unlike with pointers, that NULL is not a possible value. Cons: References can be confusing, as they have value syntax but pointer semantics. Decision: Within function parameter lists all references must be const: void Foo(const string &in, string *out); In fact it is a very strong convention in Google code that input arguments are values or const references while output arguments are pointers. Input parameters may be const pointers, but we never allow non-const reference parameters. One case when you might want an input parameter to be a const pointer is if you want to emphasize that the argument is not copied, so it must exist for the lifetime of the object; it is usually best to document this in comments as well. STL adapters such as bind2nd and mem_fun do not permit reference parameters, so you must declare functions with pointer parameters in these cases, too. Function Overloading link ▶Use overloaded functions (including constructors) only if a reader looking at a call site can get a good idea of what is happening without having to first figure out exactly which overload is being called. Definition: You may write a function that takes a const string& and overload it with another that takes const char*. class MyClass { public: void Analyze(const string &text); void Analyze(const char *text, size_t textlen); }; Pros: Overloading can make code more intuitive by allowing an identically-named function to take different arguments. It may be necessary for templatized code, and it can be convenient for Visitors. Cons: If a function is overloaded by the argument types alone, a reader may have to understand C++'s complex matching rules in order to tell what's going on. Also many people are confused by the semantics of inheritance if a derived class overrides only some of the variants of a function. Decision: If you want to overload a function, consider qualifying the name with some information about the arguments, e.g., AppendString(), AppendInt() rather than just Append(). Default Arguments link ▶We do not allow default function parameters, except in a few uncommon situations explained below. Pros: Often you have a function that uses lots of default values, but occasionally you want to override the defaults. Default parameters allow an easy way to do this without having to define many functions for the rare exceptions. Cons: People often figure out how to use an API by looking at existing code that uses it. Default parameters are more difficult to maintain because copy-and-paste from previous code may not reveal all the parameters. Copy-and-pasting of code segments can cause major problems when the default arguments are not appropriate for the new code. Decision: Except as described below, we require all arguments to be explicitly specified, to force programmers to consider the API and the values they are passing for each argument rather than silently accepting defaults they may not be aware of. One specific exception is when default arguments are used to simulate variable-length argument lists. // Support up to 4 params by using a default empty AlphaNum. string StrCat(const AlphaNum &a, const AlphaNum &b = gEmptyAlphaNum, const AlphaNum &c = gEmptyAlphaNum, const AlphaNum &d = gEmptyAlphaNum); Variable-Length Arrays and alloca() link ▶We do not allow variable-length arrays or alloca(). Pros: Variable-length arrays have natural-looking syntax. Both variable-length arrays and alloca() are very efficient. Cons: Variable-length arrays and alloca are not part of Standard C++. More importantly, they allocate a data-dependent amount of stack space that can trigger difficult-to-find memory overwriting bugs: "It ran fine on my machine, but dies mysteriously in production". Decision: Use a safe allocator instead, such as scoped_ptr/scoped_array. Friends link ▶We allow use of friend classes and functions, within reason. Friends should usually be defined in the same file so that the reader does not have to look in another file to find uses of the private members of a class. A common use of friend is to have a FooBuilder class be a friend of Foo so that it can construct the inner state of Foo correctly, without exposing this state to the world. In some cases it may be useful to make a unittest class a friend of the class it tests. Friends extend, but do not break, the encapsulation boundary of a class. In some cases this is better than making a member public when you want to give only one other class access to it. However, most classes should interact with other classes solely through their public members. Exceptions link ▶We do not use C++ exceptions. Pros: Exceptions allow higher levels of an application to decide how to handle "can't happen" failures in deeply nested functions, without the obscuring and error-prone bookkeeping of error codes. Exceptions are used by most other modern languages. Using them in C++ would make it more consistent with Python, Java, and the C++ that others are familiar with. Some third-party C++ libraries use exceptions, and turning them off internally makes it harder to integrate with those libraries. Exceptions are the only way for a constructor to fail. We can simulate this with a factory function or an Init() method, but these require heap allocation or a new "invalid" state, respectively. Exceptions are really handy in testing frameworks. Cons: When you add a throw statement to an existing function, you must examine all of its transitive callers. Either they must make at least the basic exception safety guarantee, or they must never catch the exception and be happy with the program terminating as a result. For instance, if f() calls g() calls h(), and h throws an exception that f catches, g has to be careful or it may not clean up properly. More generally, exceptions make the control flow of programs difficult to evaluate by looking at code: functions may return in places you don't expect. This causes maintainability and debugging difficulties. You can minimize this cost via some rules on how and where exceptions can be used, but at the cost of more that a developer needs to know and understand. Exception safety requires both RAII and different coding practices. Lots of supporting machinery is needed to make writing correct exception-safe code easy. Further, to avoid requiring readers to understand the entire call graph, exception-safe code must isolate logic that writes to persistent state into a "commit" phase. This will have both benefits and costs (perhaps where you're forced to obfuscate code to isolate the commit). Allowing exceptions would force us to always pay those costs even when they're not worth it. Turning on exceptions adds data to each binary produced, increasing compile time (probably slightly) and possibly increasing address space pressure. The availability of exceptions may encourage developers to throw them when they are not appropriate or recover from them when it's not safe to do so. For example, invalid user input should not cause exceptions to be thrown. We would need to make the style guide even longer to document these restrictions! Decision: On their face, the benefits of using exceptions outweigh the costs, especially in new projects. However, for existing code, the introduction of exceptions has implications on all dependent code. If exceptions can be propagated beyond a new project, it also becomes problematic to integrate the new project into existing exception-free code. Because most existing C++ code at Google is not prepared to deal with exceptions, it is comparatively difficult to adopt new code that generates exceptions. Given that Google's existing code is not exception-tolerant, the costs of using exceptions are somewhat greater than the costs in a new project. The conversion process would be slow and error-prone. We don't believe that the available alternatives to exceptions, such as error codes and assertions, introduce a significant burden. Our advice against using exceptions is not predicated on philosophical or moral grounds, but practical ones. Because we'd like to use our open-source projects at Google and it's difficult to do so if those projects use exceptions, we need to advise against exceptions in Google open-source projects as well. Things would probably be different if we had to do it all over again from scratch. There is an exception to this rule (no pun intended) for Windows code. Run-Time Type Information (RTTI) link ▶We do not use Run Time Type Information (RTTI). Definition: RTTI allows a programmer to query the C++ class of an object at run time. Pros: It is useful in some unittests. For example, it is useful in tests of factory classes where the test has to verify that a newly created object has the expected dynamic type. In rare circumstances, it is useful even outside of tests. Cons: A query of type during run-time typically means a design problem. If you need to know the type of an object at runtime, that is often an indication that you should reconsider the design of your class. Decision: Do not use RTTI, except in unittests. If you find yourself in need of writing code that behaves differently based on the class of an object, consider one of the alternatives to querying the type. Virtual methods are the preferred way of executing different code paths depending on a specific subclass type. This puts the work within the object itself. If the work belongs outside the object and instead in some processing code, consider a double-dispatch solution, such as the Visitor design pattern. This allows a facility outside the object itself to determine the type of class using the built-in type system. If you think you truly cannot use those ideas, you may use RTTI. But think twice about it. :-) Then think twice again. Do not hand-implement an RTTI-like workaround. The arguments against RTTI apply just as much to workarounds like class hierarchies with type tags. Casting link ▶Use C++ casts like static_cast(). Do not use other cast formats like int y = (int)x; or int y = int(x);. Definition: C++ introduced a different cast system from C that distinguishes the types of cast operations. Pros: The problem with C casts is the ambiguity of the operation; sometimes you are doing a conversion (e.g., (int)3.5) and sometimes you are doing a cast (e.g., (int)"hello"); C++ casts avoid this. Additionally C++ casts are more visible when searching for them. Cons: The syntax is nasty. Decision: Do not use C-style casts. Instead, use these C++-style casts. Use static_cast as the equivalent of a C-style cast that does value conversion, or when you need to explicitly up-cast a pointer from a class to its superclass. Use const_cast to remove the const qualifier (see const). Use reinterpret_cast to do unsafe conversions of pointer types to and from integer and other pointer types. Use this only if you know what you are doing and you understand the aliasing issues. Do not use dynamic_cast except in test code. If you need to know type information at runtime in this way outside of a unittest, you probably have a design flaw. Streams link ▶Use streams only for logging. Definition: Streams are a replacement for printf() and scanf(). Pros: With streams, you do not need to know the type of the object you are printing. You do not have problems with format strings not matching the argument list. (Though with gcc, you do not have that problem with printf either.) Streams have automatic constructors and destructors that open and close the relevant files. Cons: Streams make it difficult to do functionality like pread(). Some formatting (particularly the common format string idiom %.*s) is difficult if not impossible to do efficiently using streams without using printf-like hacks. Streams do not support operator reordering (the %1s directive), which is helpful for internationalization. Decision: Do not use streams, except where required by a logging interface. Use printf-like routines instead. There are various pros and cons to using streams, but in this case, as in many other cases, consistency trumps the debate. Do not use streams in your code. Extended Discussion There has been debate on this issue, so this explains the reasoning in greater depth. Recall the Only One Way guiding principle: we want to make sure that whenever we do a certain type of I/O, the code looks the same in all those places. Because of this, we do not want to allow users to decide between using streams or using printf plus Read/Write/etc. Instead, we should settle on one or the other. We made an exception for logging because it is a pretty specialized application, and for historical reasons. Proponents of streams have argued that streams are the obvious choice of the two, but the issue is not actually so clear. For every advantage of streams they point out, there is an equivalent disadvantage. The biggest advantage is that you do not need to know the type of the object to be printing. This is a fair point. But, there is a downside: you can easily use the wrong type, and the compiler will not warn you. It is easy to make this kind of mistake without knowing when using streams. cout << this; // Prints the address cout << *this; // Prints the contents The compiler does not generate an error because << has been overloaded. We discourage overloading for just this reason. Some say printf formatting is ugly and hard to read, but streams are often no better. Consider the following two fragments, both with the same typo. Which is easier to discover? cerr << "Error connecting to '" hostname.first << ":" hostname.second << ": " hostname.first, foo->bar()->hostname.second, strerror(errno)); And so on and so forth for any issue you might bring up. (You could argue, "Things would be better with the right wrappers," but if it is true for one scheme, is it not also true for the other? Also, remember the goal is to make the language smaller, not add yet more machinery that someone has to learn.) Either path would yield different advantages and disadvantages, and there is not a clearly superior solution. The simplicity doctrine mandates we settle on one of them though, and the majority decision was on printf + read/write. Preincrement and Predecrement link ▶Use prefix form (++i) of the increment and decrement operators with iterators and other template objects. Definition: When a variable is incremented (++i or i++) or decremented (--i or i--) and the value of the expression is not used, one must decide whether to preincrement (decrement) or postincrement (decrement). Pros: When the return value is ignored, the "pre" form (++i) is never less efficient than the "post" form (i++), and is often more efficient. This is because post-increment (or decrement) requires a copy of i to be made, which is the value of the expression. If i is an iterator or other non-scalar type, copying i could be expensive. Since the two types of increment behave the same when the value is ignored, why not just always pre-increment? Cons: The tradition developed, in C, of using post-increment when the expression value is not used, especially in for loops. Some find post-increment easier to read, since the "subject" (i) precedes the "verb" (++), just like in English. Decision: For simple scalar (non-object) values there is no reason to prefer one form and we allow either. For iterators and other template types, use pre-increment. Use of const link ▶We strongly recommend that you use const whenever it makes sense to do so. Definition: Declared variables and parameters can be preceded by the keyword const to indicate the variables are not changed (e.g., const int foo). Class functions can have the const qualifier to indicate the function does not change the state of the class member variables (e.g., class Foo { int Bar(char c) const; };). Pros: Easier for people to understand how variables are being used. Allows the compiler to do better type checking, and, conceivably, generate better code. Helps people convince themselves of program correctness because they know the functions they call are limited in how they can modify your variables. Helps people know what functions are safe to use without locks in multi-threaded programs. Cons: const is viral: if you pass a const variable to a function, that function must have const in its prototype (or the variable will need a const_cast). This can be a particular problem when calling library functions. Decision: const variables, data members, methods and arguments add a level of compile-time type checking; it is better to detect errors as soon as possible. Therefore we strongly recommend that you use const whenever it makes sense to do so: If a function does not modify an argument passed by reference or by pointer, that argument should be const. Declare methods to be const whenever possible. Accessors should almost always be const. Other methods should be const if they do not modify any data members, do not call any non-const methods, and do not return a non-const pointer or non-const reference to a data member. Consider making data members const whenever they do not need to be modified after construction. However, do not go crazy with const. Something like const int * const * const x; is likely overkill, even if it accurately describes how const x is. Focus on what's really useful to know: in this case, const int** x is probably sufficient. The mutable keyword is allowed but is unsafe when used with threads, so thread safety should be carefully considered first. Where to put the const Some people favor the form int const *foo to const int* foo. They argue that this is more readable because it's more consistent: it keeps the rule that const always follows the object it's describing. However, this consistency argument doesn't apply in this case, because the "don't go crazy" dictum eliminates most of the uses you'd have to be consistent with. Putting the const first is arguably more readable, since it follows English in putting the "adjective" (const) before the "noun" (int). That said, while we encourage putting const first, we do not require it. But be consistent with the code around you! Integer Types link ▶Of the built-in C++ integer types, the only one used is int. If a program needs a variable of a different size, use a precise-width integer type from , such as int16_t. Definition: C++ does not specify the sizes of its integer types. Typically people assume that short is 16 bits, int is 32 bits, long is 32 bits and long long is 64 bits. Pros: Uniformity of declaration. Cons: The sizes of integral types in C++ can vary based on compiler and architecture. Decision: defines types like int16_t, uint32_t, int64_t, etc. You should always use those in preference to short, unsigned long long and the like, when you need a guarantee on the size of an integer. Of the C integer types, only int should be used. When appropriate, you are welcome to use standard types like size_t and ptrdiff_t. We use int very often, for integers we know are not going to be too big, e.g., loop counters. Use plain old int for such things. You should assume that an int is at least 32 bits, but don't assume that it has more than 32 bits. If you need a 64-bit integer type, use int64_t or uint64_t. For integers we know can be "big", use int64_t. You should not use the unsigned integer types such as uint32_t, unless the quantity you are representing is really a bit pattern rather than a number, or unless you need defined twos-complement overflow. In particular, do not use unsigned types to say a number will never be negative. Instead, use assertions for this. On Unsigned Integers Some people, including some textbook authors, recommend using unsigned types to represent numbers that are never negative. This is intended as a form of self-documentation. However, in C, the advantages of such documentation are outweighed by the real bugs it can introduce. Consider: for (unsigned int i = foo.Length()-1; i >= 0; --i) ... This code will never terminate! Sometimes gcc will notice this bug and warn you, but often it will not. Equally bad bugs can occur when comparing signed and unsigned variables. Basically, C's type-promotion scheme causes unsigned types to behave differently than one might expect. So, document that a variable is non-negative using assertions. Don't use an unsigned type. 64-bit Portability link ▶Code should be 64-bit and 32-bit friendly. Bear in mind problems of printing, comparisons, and structure alignment. printf() specifiers for some types are not cleanly portable between 32-bit and 64-bit systems. C99 defines some portable format specifiers. Unfortunately, MSVC 7.1 does not understand some of these specifiers and the standard is missing a few, so we have to define our own ugly versions in some cases (in the style of the standard include file inttypes.h): // printf macros for size_t, in the style of inttypes.h #ifdef _LP64 #define __PRIS_PREFIX "z" #else #define __PRIS_PREFIX #endif // Use these macros after a % in a printf format string // to get correct 32/64 bit behavior, like this: // size_t size = records.size(); // printf("%"PRIuS"\n", size); #define PRIdS __PRIS_PREFIX "d" #define PRIxS __PRIS_PREFIX "x" #define PRIuS __PRIS_PREFIX "u" #define PRIXS __PRIS_PREFIX "X" #define PRIoS __PRIS_PREFIX "o" Type DO NOT use DO use Notes void * (or any pointer) %lx %p int64_t %qd, %lld %"PRId64" uint64_t %qu, %llu, %llx %"PRIu64", %"PRIx64" size_t %u %"PRIuS", %"PRIxS" C99 specifies %zu ptrdiff_t %d %"PRIdS" C99 specifies %zd Note that the PRI* macros expand to independent strings which are concatenated by the compiler. Hence if you are using a non-constant format string, you need to insert the value of the macro into the format, rather than the name. It is still possible, as usual, to include length specifiers, etc., after the % when using the PRI* macros. So, e.g. printf("x = %30"PRIuS"\n", x) would expand on 32-bit Linux to printf("x = %30" "u" "\n", x), which the compiler will treat as printf("x = %30u\n", x). Remember that sizeof(void *) != sizeof(int). Use intptr_t if you want a pointer-sized integer. You may need to be careful with structure alignments, particularly for structures being stored on disk. Any class/structure with a int64_t/uint64_t member will by default end up being 8-byte aligned on a 64-bit system. If you have such structures being shared on disk between 32-bit and 64-bit code, you will need to ensure that they are packed the same on both architectures. Most compilers offer a way to alter structure alignment. For gcc, you can use __attribute__((packed)). MSVC offers #pragma pack() and __declspec(align()). Use the LL or ULL suffixes a

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