为什么 valgrind 不会在有泄漏的基本程序中检测到内存泄漏？答案

【问题标题】：Why won’t valgrind detect a memory leak in a basic program with a leak?为什么 valgrind 不会在有泄漏的基本程序中检测到内存泄漏？
【发布时间】：2021-06-27 20:57:19
【问题描述】：

我的代码：


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

int main() {
        char* string = malloc(50 * sizeof(char));
        printf("my code ran\n");
        return 0;

}

我编译它：

gcc -o mem memErr.c

并运行它

valgrind ./mem

和

valgrind --leak-check=yes ./mem

但我的输出是

(cmd)(base) ~/Documents/c/summerC$ valgrind --leak-check=yes ./mem
==65172== Memcheck, a memory error detector
==65172== Copyright (C) 2002-2017, and GNU GPL`d, by Julian Seward et al.
==65172== Using Valgrind-3.17.0.GIT-lbmacos and LibVEX; rerun with -h for copyright info
==65172== Command: ./mem
==65172==
==65172== Warning: set address range perms: large range [0x7fff202d0000, 0x8000200d0000) (defined)
==65172== Warning: set address range perms: large range [0x7fff202d0000, 0x7fff7ff88000) (defined)
==65172== Warning: set address range perms: large range [0x7fff8e23c000, 0x7fffc02d0000) (noaccess)
==65172== Warning: set address range perms: large range [0x7fffc02d0000, 0x7fffe2560000) (defined)
==65172== Warning: set address range perms: large range [0x7fffe2560000, 0x7fffffe00000) (noaccess)
my code ran
==65172==
==65172== HEAP SUMMARY:
==65172==     in use at exit: 0 bytes in 0 blocks
==65172==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==65172==
==65172== All heap blocks were freed -- no leaks are possible
==65172==
==65172== For lists of detected and suppressed errors, rerun with: -s
==65172== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

为什么 valgrind 不在这里报告内存泄漏？

编辑：基于来自@dbush 和@AJIOB 的cmets，我尝试将代码更改为：

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
int main() {
    char* string = malloc(50 * sizeof(char));
    strcpy(string, "my new code ran");
    printf("%s\n", string);
    return 0; 
}

并编译： gcc -g -o mem memErr.c 但结果是一样的：

(cmd)(base) ~/Documents/c/summerC$ valgrind ./mem
==81073== Memcheck, a memory error detector
==81073== Copyright (C) 2002-2017, and GNU GPL`d, by Julian Seward et al.
==81073== Using Valgrind-3.17.0.GIT-lbmacos and LibVEX; rerun with -h for copyright info
==81073== Command: ./mem
==81073==
==81073== Warning: set address range perms: large range [0x7fff202d0000, 0x8000200d0000) (defined)
==81073== Warning: set address range perms: large range [0x7fff202d0000, 0x7fff7ff88000) (defined)
==81073== Warning: set address range perms: large range [0x7fff8e23c000, 0x7fffc02d0000) (noaccess)
==81073== Warning: set address range perms: large range [0x7fffc02d0000, 0x7fffe2560000) (defined)
==81073== Warning: set address range perms: large range [0x7fffe2560000, 0x7fffffe00000) (noaccess)
my new code ran
==81073==
==81073== HEAP SUMMARY:
==81073==     in use at exit: 0 bytes in 0 blocks
==81073==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==81073==
==81073== All heap blocks were freed -- no leaks are possible
==81073==
==81073== For lists of detected and suppressed errors, rerun with: -s
==81073== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

编辑：这个问题似乎是与 valgrind 的 macOS Big Sur 兼容性问题。我目前的解决方法是使用带有 Docker 的虚拟 linux 环境来运行 valgrind。

【问题讨论】：

我想，你应该看看page。

标签： c memory-leaks

【解决方案1】：

由于从 malloc 返回的内存从未使用过，编译器可能优化了调用。

valgrind 输出中的这一行似乎支持了这一点：

total heap usage: 0 allocs, 0 frees, 0 bytes allocated

如果您修改代码以便使用 malloc 的缓冲区，valgrind 应该报告泄漏：

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

int main() {
        char* string = malloc(50 * sizeof(char));
        strcpy(string, "my code ran");
        printf("%s\n", string);
        return 0;

}

【讨论】：

OP 应该检查反汇编。使用默认的-O0，它不会优化我这边的malloc

【解决方案2】：

来自Valgrind Quick start guide：您应该使用-g 标志（调试符号）编译您的程序。

另外，正如@dbush 用户所说，您应该使用您的字符串来告诉编译器，您的变量确实是必需的。

【讨论】：