設置觀察點 · 100-gdb-tips

# 設置觀察點 ## 例子 #include <stdio.h> #include <pthread.h> #include <unistd.h> int a = 0; void *thread1_func(void *p_arg) { while (1) { a++; sleep(10); } } int main(int argc, char* argv[]) { pthread_t t1; pthread_create(&t1, NULL, thread1_func, "Thread 1"); sleep(1000); return 0; } ## 技巧 gdb可以使用“`watch`”命令設置觀察點，也就是當一個變量值發生變化時，程序會停下來。以上面程序為例: (gdb) start Temporary breakpoint 1 at 0x4005a8: file a.c, line 19. Starting program: /data2/home/nanxiao/a [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib64/libthread_db.so.1". Temporary breakpoint 1, main () at a.c:19 19 pthread_create(&t1, NULL, thread1_func, "Thread 1"); (gdb) watch a Hardware watchpoint 2: a (gdb) r Starting program: /data2/home/nanxiao/a [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib64/libthread_db.so.1". [New Thread 0x7ffff782c700 (LWP 8813)] [Switching to Thread 0x7ffff782c700 (LWP 8813)] Hardware watchpoint 2: a Old value = 0 New value = 1 thread1_func (p_arg=0x4006d8) at a.c:11 11 sleep(10); (gdb) c Continuing. Hardware watchpoint 2: a Old value = 1 New value = 2 thread1_func (p_arg=0x4006d8) at a.c:11 11 sleep(10); 可以看到，使用“`watch a`”命令以后，當`a`的值變化：由`0`變成`1`，由`1`變成`2`，程序都會停下來。此外也可以使用“`watch *(data type*)address`”這樣的命令，仍以上面程序為例: (gdb) p &a $1 = (int *) 0x6009c8 <a> (gdb) watch *(int*)0x6009c8 Hardware watchpoint 2: *(int*)0x6009c8 (gdb) r Starting program: /data2/home/nanxiao/a [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib64/libthread_db.so.1". [New Thread 0x7ffff782c700 (LWP 15431)] [Switching to Thread 0x7ffff782c700 (LWP 15431)] Hardware watchpoint 2: *(int*)0x6009c8 Old value = 0 New value = 1 thread1_func (p_arg=0x4006d8) at a.c:11 11 sleep(10); (gdb) c Continuing. Hardware watchpoint 2: *(int*)0x6009c8 Old value = 1 New value = 2 thread1_func (p_arg=0x4006d8) at a.c:11 11 sleep(10); 先得到`a`的地址：`0x6009c8`，接著用“`watch *(int*)0x6009c8`”設置觀察點，可以看到同“`watch a`”命令效果一樣。觀察點可以通過軟件或硬件的方式實現，取決于具體的系統。但是軟件實現的觀察點會導致程序運行很慢，使用時需注意。參見[gdb手冊](https://sourceware.org/gdb/onlinedocs/gdb/Set-Watchpoints.html). 如果系統支持硬件觀測的話，當設置觀測點是會打印如下信息： Hardware watchpoint num: expr 如果不想用硬件觀測點的話可如下設置： set can-use-hw-watchpoints ## 查看斷點列出當前所設置了的所有觀察點： info watchpoints watch 所設置的斷點也可以用控制斷點的命令來控制。如 disable、enable、delete等 ## 貢獻者 nanxiao