C 中奇怪的 malloc 行为

Question

我有以下 ANSI C 代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {
    char *buffer = 0;
    int length = 0;
    FILE *f = fopen("text.txt", "r");
    if(f) {
        fseek(f, 0, SEEK_END);
        length = ftell(f);
        fseek(f, 0, SEEK_SET);
        buffer = malloc(length);
        fread(buffer, 1, length, f);
        fclose (f);
    }
    printf("File size: %d\nBuffer size: %d\nContent: %s\n=END=", length, strlen(buffer), buffer);
    return 0;
}

由于某种原因，在 malloc 分配了比需要更多的内存并从内存中输出额外的垃圾，例如：首先运行：

File size: 12
Buffer size: 22
Content: 123456789012les=$#▬rW|
=END=

第二个运行:

File size: 12
Buffer size: 22
Content: 123456789012les↔1↕.'
=END=

第三个运行:

File size: 12
Buffer size: 22
Content: 123456789012les=▬kπà
=END=

有人可以帮我解决这个问题并解释为什么我的版本表现得很奇怪吗？我使用 MingW TDM-GCC 4.9.2 32bit 编译 (gcc)

Answer 1

您有 undefined behavior (this explains why you should be afraid of UB) -because of buffer overflow。您忘记添加终止空字节。

替换 错误的 行：

    // WRONG CODE:
    buffer = malloc(length);
    fread(buffer, 1, length, f);

和

    buffer = malloc(length+1);
    if (!buffer) 
      { perror("malloc"); exit(EXIT_FAILURE); };
    memset (buffer, 0, length+1);
    if (fread(buffer, 1, length, f) < length) 
      { perror("fread"); exit(EXIT_FAILURE); };

^{（您可以只将结束字节归零；我更喜欢用 memset 清除整个缓冲区）}

顺便说一句，ANSI C 已经过时了。你应该使用 C11 compliant compiler (e.g. a recent GCC used as gcc -std=c11 -Wall -Wextra -g) and target C11 compliance (or at least C99). Learn to use the debugger (e.g. gdb)

仔细阅读 malloc(3), fread(3), perror(3) 等的文档....

Answer 2

fseek(f, 0, SEEK_END); 的使用会调用未定义的行为。首先，您不是以二进制模式读取，因此文件中的字节数不一定是要读取的字节数。

但是如果你切换到二进制流，根据 C Standard 的 7.19.9.2:

A binary stream need not meaningfully support fseek calls with a whence value of SEEK_END.

和

Setting the file position indicator to end-of-file, as with fseek(file, 0, SEEK_END), has undefined behavior for a binary stream (because of possible trailing null characters) or for any stream with state-dependent encoding that does not assuredly end in the initial shift state.

C 中奇怪的 malloc 行为

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