在C程序中肯定会失去内存泄漏

Question

有人能够阐明为什么Valgrind将此程序归类为“绝对丢失：2个字节的1块”内存泄漏？我了解评论线可以解决问题，但我不了解分类。根据Valgrind文档，内存泄漏应该被分类为“间接可达”。我也很好奇，为什么这甚至被认为是内存泄漏，并会赞赏解释。即使程序在main函数的末尾终止，是否手动释放所有内容是一种好的做法？

#include <stdlib.h> 

struct wrapper { 
    char *data; 
}; 

char *strdup(const char *); 

struct wrapper *walloc(struct wrapper *root) 
{ 
    if (root == NULL){ 
    root = (struct wrapper *) malloc(sizeof(struct wrapper)); 
    root->data = strdup("H"); 
    } 

    return root; 
} 

int main(){ 
    struct wrapper *root; 

    root = NULL; 
    root = walloc(root); 

    //free(root->data); 

    return 0; 
} 

这里是Valgrind的输出：

$ valgrind --leak-check=full ./leak 
==26489== Memcheck, a memory error detector 
==26489== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al. 
==26489== Using Valgrind-3.10.0 and LibVEX; rerun with -h for copyright info 
==26489== Command: ./leak 
==26489== 
==26489== 
==26489== HEAP SUMMARY: 
==26489==  in use at exit: 2 bytes in 1 blocks 
==26489== total heap usage: 2 allocs, 1 frees, 1,790 bytes allocated 
==26489== 
==26489== 2 bytes in 1 blocks are definitely lost in loss record 1 of 1 
==26489== at 0x4C29F90: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) 
==26489== by 0x4EB79C9: strdup (in /usr/lib/libc-2.20.so) 
==26489== by 0x400542: walloc (leak.c:13) 
==26489== by 0x400542: main (leak.c:23) 
==26489== 
==26489== LEAK SUMMARY: 
==26489== definitely lost: 2 bytes in 1 blocks 
==26489== indirectly lost: 0 bytes in 0 blocks 
==26489==  possibly lost: 0 bytes in 0 blocks 
==26489== still reachable: 0 bytes in 0 blocks 
==26489==   suppressed: 0 bytes in 0 blocks 
==26489== 
==26489== For counts of detected and suppressed errors, rerun with: -v 
==26489== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0) 

Answer 1

感谢@sharth指引我在正确的方向。 Valgrind实际上正确地检测到了直接损失，但是由于-O3编译完全删除了root，所以令人困惑。编译没有-O3显示8字节的合适的直接损失和2字节的间接损失。

此外，感谢@SylvainL和@Lundin的最佳实践评论。

FYI：校正Valgrind的输出是这样的：

==30492== Memcheck, a memory error detector 
==30492== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al. 
==30492== Using Valgrind-3.10.0 and LibVEX; rerun with -h for copyright info 
==30492== Command: ./leak 
==30492== 
==30492== 
==30492== HEAP SUMMARY: 
==30492==  in use at exit: 10 bytes in 2 blocks 
==30492== total heap usage: 3 allocs, 1 frees, 1,830 bytes allocated 
==30492== 
==30492== 10 (8 direct, 2 indirect) bytes in 1 blocks are definitely lost in loss record 2 of 2 
==30492== at 0x4C29F90: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) 
==30492== by 0x400687: walloc (leak.c:12) 
==30492== by 0x4006C6: main (leak.c:23) 
==30492== 
==30492== LEAK SUMMARY: 
==30492== definitely lost: 8 bytes in 1 blocks 
==30492== indirectly lost: 2 bytes in 1 blocks 
==30492==  possibly lost: 0 bytes in 0 blocks 
==30492== still reachable: 0 bytes in 0 blocks 
==30492==   suppressed: 0 bytes in 0 blocks 
==30492== 
==30492== For counts of detected and suppressed errors, rerun with: -v 
==30492== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0) 

Answer 2

的strdup（使用malloc），因此你需要的时候它不再像使用你自己的人来释放此内存堆上分配内存直接调用malloc。

即使程序完成，您也必须这样做，因为这是检测内存泄漏的唯一方法。当然，当程序完成时，检查任何内存泄漏的概念可能看起来有点矫枉过正，因为所有分配的内存然后由操作系统自动释放，但不要忘记您的小程序在这里是个例外。通常，大多数程序在运行时会占用非常大的内存，并且可能会耗尽内存，或者如果内部存在多个内存泄漏，则运行速度会更慢。

即使是一个小程序也应该写得很好;否则以后你不可能再编写任何大型程序，因为你的不良习惯会导致大量的编码错误。