memcheck怎麼運行
valgrind --log-file=valgrind.log --tool=memcheck --leak-check=full --show-reachable=no --workaround-gcc296-bugs=yes ./mcsample arg1 arg2
–log-file 表示輸出報告文件，可以是相對路徑或完全路徑
–tool=memcheck 做內存檢測就是memcheck，要知道valgrind是一個工具集
–leak-check=full 完整檢測
–show-reachable=no 是否顯示reachable詳見內存泄露部分，通常是no，也可以改成yes
–workaround-gcc296-bugs=yes 如果你的gcc存在對應的bug，則要設爲yes，否則有誤報
最後是被檢測程序及其參數。
memcheck報告怎麼看
先來一段意外的寫錯
int main(int argc, char *argv[])
{
char* bigBuff = (char*)malloc[1024];
free(bigBuff);
}==3498== Invalid free() / delete / delete[] / realloc()
==3498== at 0x402B06C: free (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==3498== by 0x8048444: main (main.cpp:19)
==3498== Address 0x40c0500 is in the Text segment of /lib/i386-linux-gnu/libc-2.15.so
代碼錯誤的將malloc()寫成了malloc[]，相當於取得了malloc函數指針後面的地址，輸出報告告訴我們這個地址位於.text段。
可以看出報告的基本格式是：
{問題描述}
at {地址、函數名、模塊或代碼行}
by {地址、函數名、代碼行}
by ...{逐層依次顯示調用堆棧}
Address 0x???????? {描述地址的相對關係}
而報告的輸出文檔整體格式則可以總結爲：
1. copyright 版權聲明
2. 異常讀寫報告
2.1 主線程異常讀寫
2.2 線程A異常讀寫報告
2.3 線程B異常讀寫報告
2... 其他線程
3. 堆內存泄露報告
3.1 堆內存使用情況概述(HEAP SUMMARY)
3.2 確信的內存泄露報告(definitely lost)
3.3 可疑內存操作報告 (show-reachable=no關閉)
3.4 泄露情況概述(LEAK SUMMARY)
都有哪些常見異常報告
內存泄漏
int main(int argc, char *argv[])
{
char* bigBuff = (char*)malloc(1024);
}1,024 bytes in 1 blocks are definitely lost in loss record 1 of 1
at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
by 0x8048414: main (main.cpp:17)
definitely lost：內存沒有被釋放，且沒有任何指針指向這裏。肯定泄漏了。報告給出的堆棧是內存被分配時的調用堆棧，它可以基本明確內存是由什麼業務邏輯創建的。
still reachable：是說內存沒有被釋放，儘管如此仍有指針指向，內存仍在使用中，這可以不算泄露。(程序退出時仍在工作的異步系統調用?)
possibly lost：是說可能有泄漏，一般是有二級指針（指針的指針）等複雜情況不易於追蹤時出現。
suppressed：統計了使用valgrind的某些參數取消了特定庫的某些錯誤，會被歸結到這裏
異常釋放
int main(int argc, char *argv[])
{
char* bigBuff = (char*)malloc(1024);
char* offsetBuff = bigBuff + 888;
free(offsetBuff);
} Invalid free() / delete / delete[] / realloc()
at 0x402B06C: free (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
by 0x8048461: main (main.cpp:24)
Address 0x41f23a0 is 888 bytes inside a block of size 1,024 alloc'd
at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
by 0x8048444: main (main.cpp:17)
free() / delete / delete[] / realloc() 四種中的任一種，這裏是free的非法釋放。在描述地址的相對關係時，使用了一個句子，句子的格式是：Address 0x???????? is {x} bytes {inside/before/after} a block of size {y} {alloc’d/free’d}
它表示了釋放的地址與一個y長度塊的相對位置關係。如果地址位於塊前，則用before，位於塊內則用inside，塊後則是after。而最後的alloc’d代表這個y長度的塊處於有效狀態，其分配時的棧如下；而free’d代表y長度塊已刪除，其刪除時的棧如下。
所以上面的報告可以解釋爲：地址0x41f23a0位於一個長度1024的有效塊內+888處，其分配時的調用堆棧如下。
非法讀寫
int main(int argc, char *argv[])
{
char* bigBuff = (char*)malloc(1024);
uint64_t* bigNum = (uint64_t*)(bigBuff+1020);
*bigNum = 0x12345678AABBCCDD;
printf("bigNum is %llu\n",*bigNum);
free(bigBuff);
}Invalid write of size 4
at 0x8048490: main (main.cpp:19)
Address 0x41f2428 is 0 bytes after a block of size 1,024 alloc'd
at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
by 0x8048474: main (main.cpp:17)
Invalid read of size 4
at 0x804849B: main (main.cpp:20)
Address 0x41f2428 is 0 bytes after a block of size 1,024 alloc'd
at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
by 0x8048474: main (main.cpp:17)
對一個內存區的使用超過了分配的大小時，可以觸發Invalid write/read，同時被告知長度。本例中uint64_t有8字節長，訪問超出了4字節。如果將bigBuff+1020改成bigBuff-20，那麼報告中會準確的告訴你Address xxx is 20 bytes before a block of …
另外一個有趣的現象是，我發現對uint64_t的非法訪問會產生2次4字節長度非法訪問的報告，這說明了什麼？
不匹配的釋放
int main(int argc, char *argv[])
{
int unused;
char* bigBuff = (char*)malloc(1024);
delete[] bigBuff;
printf("unused=%d",unused);
}Mismatched free() / delete / delete []
at 0x402A8DC: operator delete[](void*) (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
by 0x80484FB: main (main.cpp:19)
Address 0x4323028 is 0 bytes inside a block of size 1,024 alloc'd
at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
by 0x80484E4: main (main.cpp:18)
Use of uninitialised value of size 4
at 0x416E0DB: _itoa_word (_itoa.c:195)
by 0x417221A: vfprintf (vfprintf.c:1629)
by 0x4178B2E: printf (printf.c:35)
by 0x41454D2: (below main) (libc-start.c:226)
不管malloc分配後用delete還是delete[]，又或者是new[]之後粗心用delete釋放，都會得到Mismatched free() / delete / delete []報告，且報告主體內容基本一致。
使用未初始的值
上例中int unused並未賦值即被使用，得到了Use of uninitialised value of size 4的報告，這樣的問題通常不致命，但是也需要排除。
可以觀察到一個有趣情況，堆棧最後一層首次出現了 (below main)，它表示代碼位於main函數以外被執行，也並非來自於線程，我還不能明確解釋這種現象，但是我做了下面這個測試：…
靜態構造和釋放
class GlobalClass
{
public:
GlobalClass()
{
char* buf = (char*)malloc(10);
*(int*)(buf+8) = 100;
free(buf);
}
~GlobalClass()
{
char* buf = (char*)malloc(10);
*(int*)(buf+8) = 100;
free(buf);
}
void fake(){}
} g_globalClass;
int main(int argc, char *argv[])
{
g_globalClass.fake();
}Invalid write of size 4
at 0x804857B: GlobalClass::GlobalClass() (main.cpp:21)
by 0x804850F: __static_initialization_and_destruction_0(int, int) (main.cpp:31)
by 0x8048551: _GLOBAL__sub_I_g_globalClass (main.cpp:55)
by 0x8048631: __libc_csu_init (in /home/jinzeyu/codelocal/build-mcsample-Desktop_Qt_5_3_GCC_32bit-Debug/mcsample)
by 0x4060469: (below main) (libc-start.c:185)
Address 0x41f2030 is 8 bytes inside a block of size 10 alloc'd
at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
by 0x8048571: GlobalClass::GlobalClass() (main.cpp:20)
by 0x804850F: __static_initialization_and_destruction_0(int, int) (main.cpp:31)
by 0x8048551: _GLOBAL__sub_I_g_globalClass (main.cpp:55)
by 0x8048631: __libc_csu_init (in /home/jinzeyu/codelocal/build-mcsample-Desktop_Qt_5_3_GCC_32bit-Debug/mcsample)
by 0x4060469: (below main) (libc-start.c:185)
Invalid write of size 4
at 0x80485B9: GlobalClass::~GlobalClass() (main.cpp:27)
by 0x4079B80: __run_exit_handlers (exit.c:78)
by 0x4079C0C: exit (exit.c:100)
by 0x40604DA: (below main) (libc-start.c:258)
Address 0x41f2070 is 8 bytes inside a block of size 10 alloc'd
at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
by 0x80485AF: GlobalClass::~GlobalClass() (main.cpp:26)
by 0x4079B80: __run_exit_handlers (exit.c:78)
by 0x4079C0C: exit (exit.c:100)
by 0x40604DA: (below main) (libc-start.c:258)
靜態類的構造和釋放都在main之外，所以都出現了(below main)的字樣，堆棧的函數名也很好的證實了這兩個過程。這裏我聯想到了另一個問題，就是靜態構造的順序不一定按預期，強烈建議靜態對象之間不要有依賴關係。
崩潰
如果在memcheck運行你的程序過程中遇到崩潰，它依然能夠提供一些有用的信息
--16198-- VALGRIND INTERNAL ERROR: Valgrind received a signal 11 (SIGSEGV) - exiting
--16198-- si_code=1; Faulting address: 0x74207972; sp: 0x6564ca5c
valgrind: the 'impossible' happened:
Killed by fatal signal
==16198== at 0x380C0AD4: ??? (in /usr/lib/valgrind/memcheck-x86-linux)
==16198== by 0x380C12C5: ??? (in /usr/lib/valgrind/memcheck-x86-linux)
==16198== by 0x38040A63: ??? (in /usr/lib/valgrind/memcheck-x86-linux)
==16198== by 0x38040B36: ??? (in /usr/lib/valgrind/memcheck-x86-linux)
==16198== by 0x3803EA4B: ??? (in /usr/lib/valgrind/memcheck-x86-linux)
==16198== by 0x20202E78: ???
sched status:
running_tid=3
然後報告中依次羅列崩潰時各線程所處的堆棧和線程的運行狀態
Thread 1: status = VgTs_WaitSys
...
Thread 2: status = VgTs_WaitSys
...
Thread 3: status = VgTs_Runnable
==16198== at 0x402C9B4: operator new(unsigned int) (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==16198== by 0x437D7D3: std::string::_Rep::_S_create(unsigned int, unsigned int, std::allocator<char> const&) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.16)
==16198== by 0x437FBB5: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.16)
==16198== by 0x82A76A3: DataChecker::handle_data_check_resp_msg(void*) (data_checker.c:55)
==16198== by 0x8144411: main_thread(void*) (main_thread.c:198)
==16198== by 0x82839CF: thread_manager_start_routine(void*) (thread_manager.c:72)
==16198== by 0x42D3D4B: start_thread (pthread_create.c:308)
==16198== by 0x450BFDD: clone (clone.S:130)
Thread 4: status = VgTs_WaitSys
...
那麼，運行中的線程自然是嫌疑最大的，我們可以提取它的堆棧信息做進一步分析。