`backtrace`

簡介
- 有的系統沒有實現backtrace這個函數.
- 常見的就有linux還有一些docker的庫.

判斷是否支持`backtrace`

#include<stdio.h>
#include<dlfcn.h>
int main()
{
   void * bt = dlsym(NULL,"backtrace");
   if(NULL != bt)
   {
       printf("support\n");
   }
   else
   {
       printf("not support\n");
   }
   return 0;
}

輸出

[root@localhost bt_support]# gcc test.c -ldl -g
[root@localhost bt_support]# ./a.out
support

堆棧
- 說明
  - 查看某一個時刻,函數調用棧.
  - 在崩潰的時候輸出,可以精確的查看在什麼地方出的問題.
- 常見工具
  - pstack -p pid查看正在執行的進程某一時刻的堆棧.
  - gdb.
- 案例
```
(gdb) bt
#0  show () at test.c:3
#1  0x0000000000400501 in cool () at test.c:7
#2  0x0000000000400511 in main () at test.c:12
```
- 注意
  - 必須要有調試信息.
  - 否則結果是??:00這類無意義數據.

獲取堆棧

說明
- 堆棧一般是代碼地址.
- 也可能是共享庫的映射地址.

獲取地址

#include <execinfo.h>
void show()
{
   void * s[30] = {0};
   backtrace(s,30);
   return;
}

void cool()
{
show();
}

int main()
{
   cool();
}

輸出

(gdb) bt
#0  show () at test.c:6
#1  0x0000000000400587 in cool () at test.c:11
#2  0x0000000000400597 in main () at test.c:16
(gdb) x /3xg s
0x7fffffffe320: 0x0000000000400576      0x0000000000400587
0x7fffffffe330: 0x0000000000400597
(gdb) p show
$2 = {void ()} 0x40053d <show>

backtrace獲取到的地址和gdb bt指令獲取到的一樣.

addr2line

[root@localhost bt_support]# addr2line -fap 0x0000000000400576 0x0000000000400587 0x0000000000400597
0x0000000000400576: show at /root/cfile/bt_support/test.c:6
0x0000000000400587: cool at /root/cfile/bt_support/test.c:12
0x0000000000400597: main at /root/cfile/bt_support/test.c:17

得到的結果基本相同.

動態庫

獲取堆棧

test.c

#define _GNU_SOURCE
#include <dlfcn.h>
#include <execinfo.h>
void show()
{
   void * s[30] = {0};
   int n = backtrace(s,30);
   int i;
   Dl_info info;
   for(i = 0 ; i < n; i++)
   {
       dladdr(s[i],&info);
       i+=0;
   }
   return;
}

void cool()
{
show();
}

main.c

extern void cool();
int main()
{
   cool();
}

編譯

gcc -fPIC -shared -o test.so test.c -g
gcc ./test.so main.c -g -ldl

調試輸出

(gdb) p info
$1 = {dli_fname = 0x7ffff7ff9640 "./test.so", dli_fbase = 0x7ffff7bd9000,
 dli_sname = 0x7ffff7bd942d "show", dli_saddr = 0x7ffff7bd9785 <show>}

(gdb) printf "%x\n",info.dli_saddr - info.dli_fbase
785

(gdb) !addr2line -e ./test.so 806
/root/cfile/bt_support/test.c:21

(gdb) bt
#0  show () at test.c:10
#1  0x00007ffff7bd9806 in cool () at test.c:20
#2  0x000000000040063b in main () at main.c:4

addr信息需要藉助dladdr.獲取某個地址的信息.

實現`backtrace`

參考鏈接

https://stackoverflow.com/questions/8115192/android-ndk-getting-the-backtrace

應用場景

有的會沒有這個函數,比如android,docker.
堆棧
#include <unwind.h>

struct BacktraceState
{
   void** current;
   void** end;
};

static _Unwind_Reason_Code unwindCallback(struct _Unwind_Context* context, void* arg)
{
   BacktraceState* state = static_cast<BacktraceState*>(arg);
   uintptr_t pc = _Unwind_GetIP(context);
   if (pc) {
       if (state->current == state->end) {
           return _URC_END_OF_STACK;
       } else {
           *state->current++ = reinterpret_cast<void*>(pc);
       }
   }
   return _URC_NO_REASON;
}

}

size_t captureBacktrace(void** buffer, size_t max)
{
   BacktraceState state = {buffer, buffer + max};
   _Unwind_Backtrace(unwindCallback, &state);

   return state.current - buffer;
}
struct BacktraceState,頭尾指針的數組.存放void*類型數據的指針數組.

_Unwind_Reason_Code 回調函數返回值.

struct _Unwind_Context* context回調函數第一參數.

void* arg用戶用於存放數據的結構體,任意類型.

unwindCallback自定義的回調函數,傳入堆棧信息content,和用戶的容器指針.

_Unwind_GetIP獲取棧指針.

_Unwind_Backtrace 實際函數,回調和容器.
pipeline

`backtrace`實現

原型
- int backtrace(void **buffer, int size);

實現

#include<stdio.h>
#include <unwind.h>
struct BacktraceState {
   void** start;
   void** current;
   void** end;
};

static _Unwind_Reason_Code unwindCallback(struct _Unwind_Context* context, void* arg) {
  BacktraceState* state = (BacktraceState*)(arg);
  _Unwind_Ptr pc = _Unwind_GetIP(context);
  if (pc) {
       if (state->current == state->end) {
           return _URC_END_OF_STACK;
       }
       if(NULL != state->start) {
           *state->current = (void*)(pc);
       }
      state->current++;
   }
   return _URC_NO_REASON;
}

int Backtrace(void** buffer, int size) {
   BacktraceState state = {buffer, buffer, buffer + size};
   _Unwind_Backtrace(unwindCallback, &state);
   return state.current - state.start;
}

int main()
{
   void * s[10] = {NULL};
   Backtrace(s,10);
   return 0;
}

編譯

gcc -g backtrace_demo.c

`backtrace + addr2line`

說明
- 將地址轉成可讀.

案例

#include<stdio.h>
#include<unwind.h>
typedef struct BacktraceState {
   void** start;
   void** current;
   void** end;
} BacktraceState;

static _Unwind_Reason_Code unwindCallback(struct _Unwind_Context* context, void* arg) {
  BacktraceState* state = (BacktraceState*)(arg);
  _Unwind_Ptr pc = _Unwind_GetIP(context);
  if (pc) {
       if (state->current == state->end) {
           return _URC_END_OF_STACK;
       }
       if(NULL != state->start) {
           *state->current = (void*)(pc);
       }
      state->current++;
   }
   return _URC_NO_REASON;
}

int Backtrace(void** buffer, int size) {
   BacktraceState state = {buffer, buffer, buffer + size};
   _Unwind_Backtrace(unwindCallback, &state);
   return state.current - state.start;
}

int show()
{
   void * s[10] = {NULL};
   int i = 0 , n = Backtrace(s,10);
   for( i = 0 ; i < n ;i ++)
   {
       printf("%p\n",s[i]);
   }
   return 0;
}

int main()
{
   show();
   return 0;
}

編譯

gcc backtrace_demo.c -g -o demo.out
./demo.out | addr2line -fapCe ./demo.out

最終版本

簡介
- 利用Backtrace獲取堆棧,再通過dladdr辨別是否爲可執行文件代碼.
- 最後通過addr2line將結果輸出.

案例

#define _GNU_SOURCE
#include<stdio.h>
#include <unistd.h>
#include<stdlib.h>
#include<unwind.h>
#include<dlfcn.h>
typedef struct BacktraceState {
   void** start;
   void** current;
   void** end;
} BacktraceState;

static _Unwind_Reason_Code unwindCallback(struct _Unwind_Context* context, void* arg) {
  BacktraceState* state = (BacktraceState*)(arg);
  _Unwind_Ptr pc = _Unwind_GetIP(context);
  if (pc) {
       if (state->current == state->end) {
           return _URC_END_OF_STACK;
       }
       if(NULL != state->start) {
           *state->current = (void*)(pc);
       }
      state->current++;
   }
   return _URC_NO_REASON;
}

int Backtrace(void** buffer, int size) {
   BacktraceState state = {buffer, buffer, buffer + size};
   _Unwind_Backtrace(unwindCallback, &state);
   return state.current - state.start;
}

void * main_base;
extern int main();
void __attribute__((constructor)) ms()
{
   Dl_info info;
   dladdr(main,&info);
   main_base = info.dli_fbase;
}

int show()
{
   void * s[10] = {NULL};
   int i = 0 , n = Backtrace(s,10);
   for( i = 0 ; i < n ;i ++)
   {
       char buf[100]={0};
       Dl_info info;
       if(dladdr(s[i],&info)) {
           sprintf(buf,"addr2line -fpia -e %s %p",info.dli_fname,info.dli_fbase == main_base ? (unsigned long)s[i] : s[i] - info.dli_fbase);
       }
       //printf("%s %p\n",buf,s[i]);
       system(buf);
   }
   printf("\n");
   return 0;
}

int cool()
{
   show();
   return 0;
}

`test.c`

extern int cool();
int main()
{
   cool();
   return 0;
}

編譯

gcc -fPIC backtrace_demo.c -shared -o backtrace_demo.so -ldl -g
gcc ./backtrace_demo.so test.c -ldl -g

輸出

[root@localhost chap01]# ./a.out
0x0000000000000ab0: Backtrace at xxxxxx/backtrace_demo.c:31
0x0000000000000b2f: show at xxxxxx/backtrace_demo.c:46
0x0000000000000c1a: cool at xxxxxx/backtrace_demo.c:64
0x000000000040063b: main at xxxxxx/test.c:5
0x0000000000022555: __libc_start_main at /usr/src/debug/glibc-2.17-c758a686/csu/../csu/libc-start.c:300
0x0000000000400569: _start at ??:?

anddoird 實現backtrace

backtrace

簡介

判斷是否支持backtrace

輸出

堆棧

說明

常見工具

案例

注意

獲取堆棧

說明

獲取地址

輸出

addr2line

動態庫

獲取堆棧

test.c

main.c

編譯

調試輸出

實現backtrace

參考鏈接

應用場景

堆棧

pipeline

backtrace實現

原型

實現

backtrace + addr2line

說明

案例

編譯

最終版本

簡介

案例

test.c

編譯

輸出

`backtrace`

判斷是否支持`backtrace`

實現`backtrace`

`pipeline`

`backtrace`實現

`backtrace + addr2line`

`test.c`