C中奇怪的malloc行为

本文介绍了C中奇怪的malloc行为的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 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)-因为缓冲区溢出.您忘了添加一个终止的空字节.

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 C99 ).学习使用调试器(例如 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BTW, ANSI C is obsolete.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)， perror(3)等.

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