C fopen 多种打开文件类型操作 疑问

[yanasdf789]

我需要对二进制文件进行如下操作：如果不存在，就新建文件，并写入数据；如果文件存在，修改原有部分数据的值，并对在末尾添加数据。

操作比较复杂，如何选择文件操作类型？

FILE *fp=fopen("D:\\aa.dat","ab+");//ab+ 只能添加 不能修改数据

"wb+" // 会删除原有所有数据  "rb+" 不能新建文件

该这么做？同时满足要求？急急急急急急急急急

[yanasdf789]

引用 1 楼 adlay 的回复:

[quote=引用 楼主 yanasdf789 的回复:] FILE *fp=fopen("D:\\aa.dat","ab+");//ab+ 只能添加 不能修改数据

用 fseek 把指针移到你要修改的位置再写入就是修改了.[/quote]

FILE *fp=fopen("D:\\aa.dat","ab+");
	if(fp!=NULL)
	{
		fseek(fp,0,0);
		fread(&Ihead,sizeof(head),1,fp);
                 //Ihead.name 获取出来是aaaaa
		Ihead.name="bbbbb";
                fwrite(&Ihead,sizeof(head),1,fp);
                fclose(fp);
     }
		

重新打开获取，Ihead.name 还是aaaaa? 怎么办啊

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引用 3 楼 yanasdf789 的回复:

[quote=引用 1 楼 adlay 的回复:] [quote=引用 楼主 yanasdf789 的回复:] FILE *fp=fopen("D:\\aa.dat","ab+");//ab+ 只能添加 不能修改数据

用 fseek 把指针移到你要修改的位置再写入就是修改了.[/quote]

FILE *fp=fopen("D:\\aa.dat","ab+");
	if(fp!=NULL)
	{
		fseek(fp,0,0);
		fread(&Ihead,sizeof(head),1,fp);
                 //Ihead.name 获取出来是aaaaa
		Ihead.name="bbbbb";
                fwrite(&Ihead,sizeof(head),1,fp);
                fclose(fp);
     }
		

重新打开获取，Ihead.name 还是aaaaa? 怎么办啊[/quote] 你的 Ihead.name 是什么? 如果是 char* 或 std::string, 你那样写入不对呀, 如果是 char name[100]; 这种数组, 你怎么可以这样 Ihead.name="bbbbb"; 赋值的? 另外, read 后指向的位置又变了, wrtie 之前重新 fseek

[慧眼识狗熊]

你fread之后再fwrite时，指针已经后移了. fread(&Ihead,sizeof(head),1,fp); //Ihead.name 获取出来是aaaaa //在这里再fseek Ihead.name="bbbbb"; fwrite(&Ihead,sizeof(head),1,fp); fclose(fp);

[yanasdf789]

引用 5 楼 small21 的回复:

你fread之后再fwrite时，指针已经后移了. fread(&Ihead,sizeof(head),1,fp); //Ihead.name 获取出来是aaaaa //在这里再fseek Ihead.name="bbbbb"; fwrite(&Ihead,sizeof(head),1,fp); fclose(fp);

我加上 fseek(fp,0,0) ab 貌似无法修改 只能在末尾添加？？？

[赵4老师]

fopen, _wfopen Open a file. FILE *fopen( const char *filename, const char *mode ); FILE *_wfopen( const wchar_t *filename, const wchar_t *mode ); Function Required Header Compatibility fopen <stdio.h> ANSI, Win 95, Win NT _wfopen <stdio.h> or <wchar.h> Win NT For additional compatibility information, see Compatibility in the Introduction. Libraries LIBC.LIB Single thread static library, retail version LIBCMT.LIB Multithread static library, retail version MSVCRT.LIB Import library for MSVCRT.DLL, retail version The c, n, and t mode options are Microsoft extensions for fopen and _fdopen and should not be used where ANSI portability is desired. Return Value Each of these functions returns a pointer to the open file. A null pointer value indicates an error. Parameters filename Filename mode Type of access permitted Remarks The fopen function opens the file specified by filename. _wfopen is a wide-character version of fopen; the arguments to _wfopen are wide-character strings. _wfopen and fopen behave identically otherwise. Generic-Text Routine Mappings TCHAR.H Routine _UNICODE & _MBCS Not Defined _MBCS Defined _UNICODE Defined _tfopen fopen fopen _wfopen The character string mode specifies the type of access requested for the file, as follows: "r" Opens for reading. If the file does not exist or cannot be found, the fopen call fails. "w" Opens an empty file for writing. If the given file exists, its contents are destroyed. "a" Opens for writing at the end of the file (appending) without removing the EOF marker before writing new data to the file; creates the file first if it doesn’t exist. "r+" Opens for both reading and writing. (The file must exist.) "w+" Opens an empty file for both reading and writing. If the given file exists, its contents are destroyed. "a+" Opens for reading and appending; the appending operation includes the removal of the EOF marker before new data is written to the file and the EOF marker is restored after writing is complete; creates the file first if it doesn’t exist. When a file is opened with the "a" or "a+" access type, all write operations occur at the end of the file. The file pointer can be repositioned using fseek or rewind, but is always moved back to the end of the file before any write operation is carried out. Thus, existing data cannot be overwritten. The "a" mode does not remove the EOF marker before appending to the file. After appending has occurred, the MS-DOS TYPE command only shows data up to the original EOF marker and not any data appended to the file. The "a+" mode does remove the EOF marker before appending to the file. After appending, the MS-DOS TYPE command shows all data in the file. The "a+" mode is required for appending to a stream file that is terminated with the CTRL+Z EOF marker. When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed (the file is said to be open for “update”). However, when you switch between reading and writing, there must be an intervening fflush, fsetpos, fseek, or rewind operation. The current position can be specified for the fsetpos or fseek operation, if desired. In addition to the above values, the following characters can be included in mode to specify the translation mode for newline characters: t Open in text (translated) mode. In this mode, CTRL+Z is interpreted as an end-of-file character on input. In files opened for reading/writing with "a+", fopen checks for a CTRL+Z at the end of the file and removes it, if possible. This is done because using fseek and ftell to move within a file that ends with a CTRL+Z, may cause fseek to behave improperly near the end of the file. Also, in text mode, carriage return–linefeed combinations are translated into single linefeeds on input, and linefeed characters are translated to carriage return–linefeed combinations on output. When a Unicode stream-I/O function operates in text mode (the default), the source or destination stream is assumed to be a sequence of multibyte characters. Therefore, the Unicode stream-input functions convert multibyte characters to wide characters (as if by a call to the mbtowc function). For the same reason, the Unicode stream-output functions convert wide characters to multibyte characters (as if by a call to the wctomb function). b Open in binary (untranslated) mode; translations involving carriage-return and linefeed characters are suppressed. If t or b is not given in mode, the default translation mode is defined by the global variable _fmode. If t or b is prefixed to the argument, the function fails and returns NULL. For more information about using text and binary modes in Unicode and multibyte stream-I/O, see Text and Binary Mode File I/O and Unicode Stream I/O in Text and Binary Modes. c Enable the commit flag for the associated filename so that the contents of the file buffer are written directly to disk if either fflush or _flushall is called. n Reset the commit flag for the associated filename to “no-commit.” This is the default. It also overrides the global commit flag if you link your program with COMMODE.OBJ. The global commit flag default is “no-commit” unless you explicitly link your program with COMMODE.OBJ. Valid characters for the mode string used in fopen and _fdopen correspond to oflag arguments used in _open and _sopen, as follows. Characters in mode String Equivalent oflag Value for _open/_sopen a _O_WRONLY | _O_APPEND (usually _O_WRONLY | _O_CREAT | _O_APPEND) a+ _O_RDWR | _O_APPEND (usually _O_RDWR | _O_APPEND | _O_CREAT ) r _O_RDONLY r+ _O_RDWR w _O_WRONLY (usually _O_WRONLY | _O_CREAT | _O_TRUNC) w+ _O_RDWR (usually _O_RDWR | _O_CREAT | _O_TRUNC) b _O_BINARY t _O_TEXT c None n None Example /* FOPEN.C: This program opens files named "data" * and "data2".It uses fclose to close "data" and * _fcloseall to close all remaining files. */ #include <stdio.h> FILE *stream, *stream2; void main( void ) { int numclosed; /* Open for read (will fail if file "data" does not exist) */ if( (stream = fopen( "data", "r" )) == NULL ) printf( "The file 'data' was not opened\n" ); else printf( "The file 'data' was opened\n" ); /* Open for write */ if( (stream2 = fopen( "data2", "w+" )) == NULL ) printf( "The file 'data2' was not opened\n" ); else printf( "The file 'data2' was opened\n" ); /* Close stream */ if( fclose( stream ) ) printf( "The file 'data' was not closed\n" ); /* All other files are closed: */ numclosed = _fcloseall( ); printf( "Number of files closed by _fcloseall: %u\n", numclosed ); } Output The file 'data' was opened The file 'data2' was opened Number of files closed by _fcloseall: 1 Stream I/O Routines See Also fclose, _fdopen, ferror, _fileno, freopen, _open, _setmode

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引用 楼主 yanasdf789 的回复:

FILE *fp=fopen("D:\\aa.dat","ab+");//ab+ 只能添加 不能修改数据

用 fseek 把指针移到你要修改的位置再写入就是修改了.

[qzf362269994]

引用 1 楼 adlay 的回复:

[quote=引用 楼主 yanasdf789 的回复:] FILE *fp=fopen("D:\\aa.dat","ab+");//ab+ 只能添加 不能修改数据

用 fseek 把指针移到你要修改的位置再写入就是修改了.[/quote] +1 在你想要操作之前进行定标