请问下C++中，char*指向的内存缓冲区最多能存放多少字节？

Fairchild_X 2014-07-17 02:43:20

在编译的时候我看提示是：

 char* ch = new char [unsigned int]

按理说unsigned int最大也不过就是65536嘛，但是，我用如下代码：

long long size = 100000000;

char* ch = new char[static_cast<unsigned int>(size + 1)];

memset(ch, 0, size + 1);

memset(ch, 1, size);

string str = ch;

cout << str.length();

得到的结果是100000000呢，也就是说实际上char*指向的缓冲区大小远远不止65536字节，甚至可以到1亿字节。。

那么，请问下char*指向的缓冲区最多能存多少字节呢，一般来说，使用缓冲区存放的字节数的范围在多少比较安全呢？
我在做一个工作，就是把一个文件内容以二进制形式读取了，然后按字节，进行拷贝和修改，每次拷贝时，用char*来做缓冲区，我想知道这个缓冲区究竟能存多大，一次搬运多大才能使程序比较安全呢？

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幻夢之葉 2014-07-17

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引用 7 楼 u012135569 的回复:

[quote=引用 6 楼 jianwen0529 的回复:] 有谁告诉你int最大是65536的？

咱们教科书上写的。。。说int 是-32768 - 32767....[/quote] 对内置类型存储宽度这些C++不做强制规定，只做了最小位宽的规定，就如int最少需要2字节（16位）这根据编译器的实现和操作系统。可以用sizeof来查看如sizeof(int);

赵4老师 2014-07-17

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C:\Program Files\Microsoft Visual Studio 10.0\VC\crt\src\limits.h:

/***
*limits.h - implementation dependent values
*
*       Copyright (c) Microsoft Corporation.  All rights reserved.
*
*Purpose:
*       Contains defines for a number of implementation dependent values
*       which are commonly used in C programs.
*       [ANSI]
*
*       [Public]
*
****/

#pragma once

#include <crtdefs.h>

#ifndef _INC_LIMITS
#define _INC_LIMITS

#ifndef _CRTBLD
/* This version of the header files is NOT for user programs.
 * It is intended for use when building the C runtimes ONLY.
 * The version intended for public use will not have this message.
 */
#error ERROR: Use of C runtime library internal header file.
#endif  /* _CRTBLD */

#define CHAR_BIT      8         /* number of bits in a char */
#define SCHAR_MIN   (-128)      /* minimum signed char value */
#define SCHAR_MAX     127       /* maximum signed char value */
#define UCHAR_MAX     0xff      /* maximum unsigned char value */

#ifndef _CHAR_UNSIGNED
#define CHAR_MIN    SCHAR_MIN   /* mimimum char value */
#define CHAR_MAX    SCHAR_MAX   /* maximum char value */
#else  /* _CHAR_UNSIGNED */
#define CHAR_MIN      0
#define CHAR_MAX    UCHAR_MAX
#endif  /* _CHAR_UNSIGNED */

#define MB_LEN_MAX    5             /* max. # bytes in multibyte char */
#define SHRT_MIN    (-32768)        /* minimum (signed) short value */
#define SHRT_MAX      32767         /* maximum (signed) short value */
#define USHRT_MAX     0xffff        /* maximum unsigned short value */
#define INT_MIN     (-2147483647 - 1) /* minimum (signed) int value */
#define INT_MAX       2147483647    /* maximum (signed) int value */
#define UINT_MAX      0xffffffff    /* maximum unsigned int value */
#define LONG_MIN    (-2147483647L - 1) /* minimum (signed) long value */
#define LONG_MAX      2147483647L   /* maximum (signed) long value */
#define ULONG_MAX     0xffffffffUL  /* maximum unsigned long value */
#define LLONG_MAX     9223372036854775807i64       /* maximum signed long long int value */
#define LLONG_MIN   (-9223372036854775807i64 - 1)  /* minimum signed long long int value */
#define ULLONG_MAX    0xffffffffffffffffui64       /* maximum unsigned long long int value */

#define _I8_MIN     (-127i8 - 1)    /* minimum signed 8 bit value */
#define _I8_MAX       127i8         /* maximum signed 8 bit value */
#define _UI8_MAX      0xffui8       /* maximum unsigned 8 bit value */

#define _I16_MIN    (-32767i16 - 1) /* minimum signed 16 bit value */
#define _I16_MAX      32767i16      /* maximum signed 16 bit value */
#define _UI16_MAX     0xffffui16    /* maximum unsigned 16 bit value */

#define _I32_MIN    (-2147483647i32 - 1) /* minimum signed 32 bit value */
#define _I32_MAX      2147483647i32 /* maximum signed 32 bit value */
#define _UI32_MAX     0xffffffffui32 /* maximum unsigned 32 bit value */

/* minimum signed 64 bit value */
#define _I64_MIN    (-9223372036854775807i64 - 1)
/* maximum signed 64 bit value */
#define _I64_MAX      9223372036854775807i64
/* maximum unsigned 64 bit value */
#define _UI64_MAX     0xffffffffffffffffui64

#if _INTEGRAL_MAX_BITS >= 128
/* minimum signed 128 bit value */
#define _I128_MIN   (-170141183460469231731687303715884105727i128 - 1)
/* maximum signed 128 bit value */
#define _I128_MAX     170141183460469231731687303715884105727i128
/* maximum unsigned 128 bit value */
#define _UI128_MAX    0xffffffffffffffffffffffffffffffffui128
#endif  /* _INTEGRAL_MAX_BITS >= 128 */

#ifndef SIZE_MAX
#ifdef _WIN64
#define SIZE_MAX _UI64_MAX
#else  /* _WIN64 */
#define SIZE_MAX UINT_MAX
#endif  /* _WIN64 */
#endif  /* SIZE_MAX */

#if __STDC_WANT_SECURE_LIB__
/* While waiting to the C standard committee to finalize the decision on RSIZE_MAX and rsize_t,
 * we define RSIZE_MAX as SIZE_MAX
 */
#ifndef RSIZE_MAX
#define RSIZE_MAX SIZE_MAX
#endif  /* RSIZE_MAX */
#endif  /* __STDC_WANT_SECURE_LIB__ */


#endif  /* _INC_LIMITS */

chenkangapple 2014-07-17

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换本书看看吧；

Fairchild_X 2014-07-17

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引用 4 楼 zhao4zhong1 的回复:

我的例子是提醒你读写文件将缓冲区开的和磁盘簇大小即可。 char *指向的缓冲区最大值： 16位系统：2的15次方 32位系统：2的31次方 64位系统：2的63次方

哦，原来这样啊，我去搜索下磁盘簇的含义，谢谢啊。

Fairchild_X 2014-07-17

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引用 6 楼 jianwen0529 的回复:

有谁告诉你int最大是65536的？

咱们教科书上写的。。。说int 是-32768 - 32767....

幻夢之葉 2014-07-17

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有谁告诉你int最大是65536的？

赵4老师 2014-07-17

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读写文件将缓冲区开的和磁盘簇大小一样即可。

赵4老师 2014-07-17

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我的例子是提醒你读写文件将缓冲区开的和磁盘簇大小即可。 char *指向的缓冲区最大值： 16位系统：2的15次方 32位系统：2的31次方 64位系统：2的63次方

Fairchild_X 2014-07-17

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引用 2 楼 zhao4zhong1 的回复:

仅供参考

#include <sys\stat.h>
#include <io.h>
#include <fcntl.h>
#include <share.h>
#include <stdlib.h>
#include <stdio.h>
#include <conio.h>
#include <string.h>
#define MAX_CLU_BYTES 65536
FILE *fo;
int fh;
__int64 offs,offs1;
__int64 rvi64;
int rv,wrv;
unsigned char buf[MAX_CLU_BYTES];
char ofn[_MAX_PATH];
char offstr[80];
void strcpybutcomma(char *t,char *s) {
    char c;

    while (1) {
        c = *s++;
        if (','!=c) *t++ = c;
        if (0==c) break;
    }
}
void main(int argc,char **argv) {
    if (argc<3) {
        printf("Copy File Tail.\n");
        printf("Usage:\n");
        printf("    cft filename.ext offset_begin[-offset_end]\n");
        printf("Copy filename.ext offset_begin[-offset_end] to offset_begin[-offset_end]-filename.ext\n");
        printf("Note: Byte at offset_end is NOT included.\n");
        printf("Example:\n");
        printf("    cft abc.rar 12345\n");
        printf("Copy abc.rar offset 12345-end to 12345-abc.rar\n");
        printf("    cft abc.rar 123-12345\n");
        printf("Copy abc.rar offset 123-12345 to 123-12345-abc.rar\n");
        printf("    cft abc.rar 0xAB-0xCD\n");
        printf("Copy abc.rar offset 0xAB-0xCD to 0xAB-0xCD-abc.rar\n");
        return;
    }
    strcpybutcomma(offstr,argv[2]);
    rv=sscanf(offstr,"%I64i-%I64i",&offs,&offs1);
    if (rv==0) {
        printf("offset %s is not number\n",argv[2]);
        return;
    }
    fh=_sopen(argv[1],_O_BINARY|_O_RDONLY|_O_RANDOM,_SH_DENYNO);
    if (fh==-1) {
        printf("_sopen %s errno=%d\n",argv[1],errno);
        return;
    }
    if (rv==1) {
        offs1=_filelengthi64(fh);
        if (offs1==-1i64) {
            printf("%I64=_filelengthi64 errno=%d\n",offs1,errno);
            _close(fh);
            return;
        }
    } else {//rv==2
        if (offs1<offs) {
            printf("%s offset_begin>offset_end error\n",argv[2]);
            _close(fh);
            return;
        }
    }
    rvi64=_lseeki64(fh,offs,SEEK_SET);
    if (rvi64!=offs) {
        printf("%I64u=_lseeki64 %I64u errno=%d\n",rvi64,offs,errno);
        _close(fh);
        return;
    }
    sprintf(ofn,"%s-",offstr);
    strcat(ofn,argv[1]);
    fo=fopen(ofn,"wb");
    if (fo==NULL) {
        _close(fh);
        printf("fopen %s error\n",ofn);
        return;
    }
    cprintf("\n%I64u\r",offs);
    while (1) {
        rv=_read(fh,buf,(unsigned int)__min(offs1-offs,MAX_CLU_BYTES));
        if (rv==0) break;//
        if (rv<0) {
            fclose(fo);
            _close(fh);
            printf("_read %s offset %I64u error\n",argv[1],offs);
            return;
        }
        wrv=fwrite(buf,1,rv,fo);
        if (wrv!=rv) {
            fclose(fo);
            _close(fh);
            printf("fwrite %s error\n",ofn);
            return;
        } else {
            offs+=rv;
            cprintf("%I64u\r",offs);
            if (offs>=offs1) break;//
        }
    }
    fclose(fo);
    _close(fh);
    printf("Copy %s offset %s to %s OK.\n",argv[1],argv[2],ofn);
}

不是太能看懂额，我初学C++还没多久，官方的代码总觉得看不太明白，只看懂了以逗号为分割的拷贝那几段。。。没明白它哪里说明了char*指向的缓冲区有多大呢= =

赵4老师 2014-07-17

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仅供参考

#include <sys\stat.h>
#include <io.h>
#include <fcntl.h>
#include <share.h>
#include <stdlib.h>
#include <stdio.h>
#include <conio.h>
#include <string.h>
#define MAX_CLU_BYTES 65536
FILE *fo;
int fh;
__int64 offs,offs1;
__int64 rvi64;
int rv,wrv;
unsigned char buf[MAX_CLU_BYTES];
char ofn[_MAX_PATH];
char offstr[80];
void strcpybutcomma(char *t,char *s) {
    char c;

    while (1) {
        c = *s++;
        if (','!=c) *t++ = c;
        if (0==c) break;
    }
}
void main(int argc,char **argv) {
    if (argc<3) {
        printf("Copy File Tail.\n");
        printf("Usage:\n");
        printf("    cft filename.ext offset_begin[-offset_end]\n");
        printf("Copy filename.ext offset_begin[-offset_end] to offset_begin[-offset_end]-filename.ext\n");
        printf("Note: Byte at offset_end is NOT included.\n");
        printf("Example:\n");
        printf("    cft abc.rar 12345\n");
        printf("Copy abc.rar offset 12345-end to 12345-abc.rar\n");
        printf("    cft abc.rar 123-12345\n");
        printf("Copy abc.rar offset 123-12345 to 123-12345-abc.rar\n");
        printf("    cft abc.rar 0xAB-0xCD\n");
        printf("Copy abc.rar offset 0xAB-0xCD to 0xAB-0xCD-abc.rar\n");
        return;
    }
    strcpybutcomma(offstr,argv[2]);
    rv=sscanf(offstr,"%I64i-%I64i",&offs,&offs1);
    if (rv==0) {
        printf("offset %s is not number\n",argv[2]);
        return;
    }
    fh=_sopen(argv[1],_O_BINARY|_O_RDONLY|_O_RANDOM,_SH_DENYNO);
    if (fh==-1) {
        printf("_sopen %s errno=%d\n",argv[1],errno);
        return;
    }
    if (rv==1) {
        offs1=_filelengthi64(fh);
        if (offs1==-1i64) {
            printf("%I64=_filelengthi64 errno=%d\n",offs1,errno);
            _close(fh);
            return;
        }
    } else {//rv==2
        if (offs1<offs) {
            printf("%s offset_begin>offset_end error\n",argv[2]);
            _close(fh);
            return;
        }
    }
    rvi64=_lseeki64(fh,offs,SEEK_SET);
    if (rvi64!=offs) {
        printf("%I64u=_lseeki64 %I64u errno=%d\n",rvi64,offs,errno);
        _close(fh);
        return;
    }
    sprintf(ofn,"%s-",offstr);
    strcat(ofn,argv[1]);
    fo=fopen(ofn,"wb");
    if (fo==NULL) {
        _close(fh);
        printf("fopen %s error\n",ofn);
        return;
    }
    cprintf("\n%I64u\r",offs);
    while (1) {
        rv=_read(fh,buf,(unsigned int)__min(offs1-offs,MAX_CLU_BYTES));
        if (rv==0) break;//
        if (rv<0) {
            fclose(fo);
            _close(fh);
            printf("_read %s offset %I64u error\n",argv[1],offs);
            return;
        }
        wrv=fwrite(buf,1,rv,fo);
        if (wrv!=rv) {
            fclose(fo);
            _close(fh);
            printf("fwrite %s error\n",ofn);
            return;
        } else {
            offs+=rv;
            cprintf("%I64u\r",offs);
            if (offs>=offs1) break;//
        }
    }
    fclose(fo);
    _close(fh);
    printf("Copy %s offset %s to %s OK.\n",argv[1],argv[2],ofn);
}

赵4老师 2014-07-17