C中奇怪的malloc行为 [英] weird malloc behavior in C

问题描述

我有以下ANSI C代码:

I have the following ANSI C code:

#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，malloc分配了比所需更多的内存，并从内存中输出了额外的垃圾，例如: 第一次运行:

Which for some reason after the malloc alocates more memory than needed and output extra garbage from the memory, example: First run:

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

第二次运行:

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

第三次运行:

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

有人可以帮我这个忙，还可以解释为什么我的版本很奇怪吗? 我使用MingW TDM-GCC 4.9.2 32bit进行编译(gcc)

Could someone please help me with this and also explain why my version is behaving weird? I use MingW TDM-GCC 4.9.2 32bit for compilation (gcc)

推荐答案

您有不确定的行为(此解释了为什么您应该害怕UB)-因为

You have undefined behavior (this explains why you should be afraid of UB) -because of buffer overflow. You forgot to add a terminating null byte.

替换有问题的行:

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

使用

    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); };

^{(您可以将结束字节清零；我更喜欢用memset清除整个缓冲区)}

^{(You could zero just the ending byte; I prefer to clear with memset the entire buffer)}

顺便说一句，ANSI C已过时.您应该使用 C11 兼容的编译器(例如， recent GCC 用作gcc -std=c11 -Wall -Wextra -g)并达到C11规范(或至少 gdb )

BTW, ANSI C is obsolete. You should use a 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)

请仔细阅读 malloc(3)的文档， fread(3)，

Read carefully the documentation of malloc(3), fread(3), perror(3) etc....

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