# 设置观察点 ## 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`”命令设置观察点，也就是当一个变量值发生变化时，程序会停下来。以上面程序为例: (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，当`a`的值变化：由`0`变成`1`，由`1`变成`2`，程序都会停下来。 此外也可以使用“`watch *(data type*)address`”这样的命令，仍以上面程序为例: (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`，接着用“`watch *(int*)0x6009c8`”设置观察点，可以看到同“`watch a`”命令效果一样。 观察点可以通过软件或硬件的方式实现，取决于具体的系统。但是软件实现的观察点会导致程序运行很慢，使用时需注意。参见[gdb手册](https://sourceware.org/gdb/onlinedocs/gdb/Set-Watchpoints.html). 如果系统支持硬件观测的话，当设置观测点是会打印如下信息： Hardware watchpoint num: expr 如果不想用硬件观测点的话可如下设置： set can-use-hw-watchpoints ## 查看断点 列出当前所设置了的所有观察点： info watchpoints watch 所设置的断点也可以用控制断点的命令来控制。如 disable、enable、delete等 ## 贡献者 nanxiao