【一】用valgrind对代码进行内存检测的时候，如果提示“Conditional jump or move depends on uninitialised value(s)”，有可能是某些变量未初始化造成的。
例如我遇到的两处这样的提示，一处是由于 struct tm 结构体未初始化，另一处是由于 char tmp[512]未初始化造成的。要初始化，只需memset即可，这样做之后，valgrind不再会提示有问题。
请在90%以上的时间里相信valgrind，而不是坚持自己的代码不需要做任何改动。
【2】
3 内存泄漏监测
3.1 示例代码
1: int main()
2: {
3: char *p = malloc(sizeof(char) * 10);
4: if (p == NULL) {
5: return 0;
6: } 7: 8: *p++ = 'a'; 9: *p++ = 'b'; 10: 11: printf("%s\n", *p); 12: 13: return 0; 14: }
3.3 用Valgrind监测进程的内存泄漏
1: valgrind --leak-check=yes --show-reachable=yes ./core
Valgrind的输出为为：
1: ==25500== Memcheck, a memory error detector
2: ==25500== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
3: ==25500== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
4: ==25500== Command: ./core1
5: ==25500==
6: ==25500== Conditional jump or move depends on uninitialised value(s) 7: ==25500== at 0x36A104546A: vfprintf (in /lib64/libc-2.12.so) 8: ==25500== by 0x36A104EAC9: printf (in /lib64/libc-2.12.so) 9: ==25500== by 0x40055D: main (core1.c:13) 10: ==25500== 11: (null) 12: ==25500== 13: ==25500== HEAP SUMMARY: 14: ==25500== in use at exit: 10 bytes in 1 blocks 15: ==25500== total heap usage: 1 allocs, 0 frees, 10 bytes allocated 16: ==25500== 17: ==25500== 10 bytes in 1 blocks are definitely lost in loss record 1 of 1 18: ==25500== at 0x4A0515D: malloc (vg_replace_malloc.c:195) 19: ==25500== by 0x400515: main (core1.c:5) 20: ==25500== 21: ==25500== LEAK SUMMARY: 22: ==25500== definitely lost: 10 bytes in 1 blocks 23: ==25500== indirectly lost: 0 bytes in 0 blocks 24: ==25500== possibly lost: 0 bytes in 0 blocks 25: ==25500== still reachable: 0 bytes in 0 blocks 26: ==25500== suppressed: 0 bytes in 0 blocks 27: ==25500== 28: ==25500== For counts of detected and suppressed errors, rerun with: -v 29: ==25500== Use --track-origins=yes to see where uninitialised values come from 30: ==25500== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 6 from 6)
可以看到，Valgrind提示在第五行分配的内存未被释放
4 悬挂指针
4.1 示例代码
1: struct elem {
2: int a;
3: double b;
4: };
5:
6: int main() 7: { 8: struct elem *e = malloc(sizeof(struct elem)); 9: if (e == NULL) { 10: return 0; 11: } 12: 13: e->a = 10; 14: e->b = 10.10; 15: 16: double *xx = &e->b; 17: 18: printf("%f\n", *xx); 19: 20: free(e); 21: 22: printf("%f\n", *xx); 23: 24: return 0; 25: }
4.2 Valgrind运行结果
同样用-g编译后valgrind运行的结果：
1: [cobbliu@MacBook]$ valgrind --leak-check=yes --show-reachable=yes ./core2
2: ==26148== Memcheck, a memory error detector
3: ==26148== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
4: ==26148== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
5: ==26148== Command: ./core2
6: ==26148== 7: 10.100000 8: ==26148== Invalid read of size 8 9: ==26148== at 0x4005CA: main (core2.c:26) 10: ==26148== Address 0x502a048 is 8 bytes inside a block of size 16 free'd 11: ==26148== at 0x4A04D72: free (vg_replace_malloc.c:325) 12: ==26148== by 0x4005C5: main (core2.c:24) 13: ==26148== 14: 10.100000 15: ==26148== 16: ==26148== HEAP SUMMARY: 17: ==26148== in use at exit: 0 bytes in 0 blocks 18: ==26148== total heap usage: 1 allocs, 1 frees, 16 bytes allocated 19: ==26148== 20: ==26148== All heap blocks were freed -- no leaks are possible 21: ==26148== 22: ==26148== For counts of detected and suppressed errors, rerun with: -v 23: ==26148== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 6 from 6)
可以看到在free(e)后，指针xx成为了悬挂指针，此后对xx的读，如果xx指向的内存还未被glibc回收，进程不会core掉。valgrind提示在26行做了对xx的 Invalid read.
5 多次释放同一个指针
5.1 示例代码
1: int main()
2: {
3: char *p = malloc(sizeof(char) * 10);
4: if (p == NULL) {
5: return 0;
6: } 7: 8: char *q = p; 9: 10: *p++ = 'a'; 11: *p++ = 'b'; 12: 13: printf("%s\n", *p); 14: 15: free(p); 16: free(q); 17: return 0; 18: }
5.2 Valgrind 监测
1: [cobbliu@MacBook]$ valgrind --leak-check=yes --show-reachable=yes ./core1
2: ==26874== Memcheck, a memory error detector
3: ==26874== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
4: ==26874== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
5: ==26874== Command: ./core1
6: ==26874== 7: ==26874== Conditional jump or move depends on uninitialised value(s) 8: ==26874== at 0x36A104546A: vfprintf (in /lib64/libc-2.12.so) 9: ==26874== by 0x36A104EAC9: printf (in /lib64/libc-2.12.so) 10: ==26874== by 0x4005B5: main (core1.c:15) 11: ==26874== 12: (null) 13: ==26874== Invalid free() / delete / delete[] 14: ==26874== at 0x4A04D72: free (vg_replace_malloc.c:325) 15: ==26874== by 0x4005C1: main (core1.c:17) 16: ==26874== Address 0x502a042 is 2 bytes inside a block of size 10 alloc'd 17: ==26874== at 0x4A0515D: malloc (vg_replace_malloc.c:195) 18: ==26874== by 0x400565: main (core1.c:5) 19: ==26874== 20: ==26874== 21: ==26874== HEAP SUMMARY: 22: ==26874== in use at exit: 0 bytes in 0 blocks 23: ==26874== total heap usage: 1 allocs, 2 frees, 10 bytes allocated 24: ==26874== 25: ==26874== All heap blocks were freed -- no leaks are possible 26: ==26874== 27: ==26874== For counts of detected and suppressed errors, rerun with: -v 28: ==26874== Use --track-origins=yes to see where uninitialised values come from 29: ==26874== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 6 from 6)
可以看到，valgrind提示在17行有一个Invalid Free()
6 Valgrind的优缺点
valgrind默认使用memcheck工具做内存监测。
6.2 Disadvantages
不管valgrind在使用memcheck工具监测内存时，它会接管应用程序，并且读取应用程序可执行文件和库文件中的debug信息来显示详细的出错位置。当valgrind启动后，应用 进程实际上在valgrind的虚拟环境中执行，valgrind会将每行代码传递给memcheck工具，memcheck工具再加入自己的调试信息，之后再将合成的代码真正运行。memcheck工具在 应用进程每个防存操作和每个变量赋值操作时加入额外的统计代码，通常情况下，使用memcheck工具后应用程序的运行时间会比原生代码慢大约10-50倍。
其次，对于一些不停机运行的服务器程序的内存问题，valgrind无能为力。不仅仅是因为valgrind无法使之停止，还有可能是因为服务器进程本身就被设计为申请一些生命周期 与进程生命周期一样长的内存，永远不释放，这些内存会被valgrind报泄漏错误。
再次，valgrind对多线程程序支持得不够好。在多线程程序执行时，valgrind在同一时刻只让其中一个线程执行，它不会充分利用多核的环境。在用valgrind运行您的多线程程序 时，您的宝贵程序的运行情况可能跟不使用valgrind的运行情况千差万别。
7 Valgrind的其他工具
除了memcheck工具外，valgrind工具包还有一些别的好用的工具
7.1 Cachegrind
这个工具模拟 CPU中的一级缓存I1,D1和L2二级缓存，能够精确地指出程序中 cache的丢失和命中。如果需要，它还能够为我们提供cache丢失次数，内存引用次数，以及每行 代码，每个函数，每个模块，整个程序产生的指令数。这对优化程序有很大的帮助。 详情见这里
7.2 Callgrind
Callgrind收集程序运行时的一些数据，函数调用关系等信息，还可以有选择地进行cache 模拟。在运行结束时，它会把分析数据写入一个文件。callgrindannotate可以把 这个文件的内容转化成可读的形式。 详情见这里
7.4 DRD
这也使一款多线程程序监测工具，它提供的监测信息比Helgrind更丰富。 详情见这里
7.6 DHAT
这个工具能详细地显示应用进程如何使用堆栈，以使用户更好地评估程序的设计。 详情见这里
