C-valgrind为什么不报告内存泄漏？

Question

我写了这个小malloc：

#include <cstdio>
#include <cstddef>
#include <unistd.h>

#define word_size sizeof(intptr_t)
#define align(n) ((n + word_size - 1) & ~(word_size - 1))

void* my_malloc(size_t size) {
    void* p = sbrk(0);
    printf("before allocation: %p\n", p);
    if (sbrk(align(size)) == (void*) -1) {
        // failed to allocate memory
        return NULL;
    }
    printf("after allocation: %p\n", sbrk(0));
    return p;
}

int main() {
    int* foo = (int*) my_malloc(1);
    *foo = 100;
    printf("after allocation outside: %p\n", sbrk(0));
}

这是代码的输出：

before allocation: 0x1796000
after allocation: 0x1796008
after allocation outside: 0x1796008

如您所见，my_malloc分配的内存尚未释放。但是，当我使用valgrind通过以下命令运行它时：

valgrind --leak-check=yes ./main

我明白了：

==1592== Memcheck, a memory error detector
==1592== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==1592== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==1592== Command: ./main
==1592== HEAP SUMMARY:
==1592==     in use at exit: 0 bytes in 0 blocks
==1592==   total heap usage: 2 allocs, 2 frees, 73,728 bytes allocated
==1592== 
==1592== All heap blocks were freed -- no leaks are possible
==1592== 
==1592== For counts of detected and suppressed errors, rerun with: -v
==1592== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

如您所见，valgrind找不到any内存泄漏！我是错误地使用了valgrind，还是这是一个错误？

Answer 1

Valgrind通过链接其自身的功能代替malloc，calloc，realloc和free来检查内存泄漏。您正在使用sbrk在较低级别分配内存。 Valgrind不会查看这些功能，因此不会检测到泄漏。