# 设置读写观察点 ## 例子 #include <stdio.h> #include <pthread.h> int a = 0; void *thread1_func(void *p_arg) { while (1) { a++; sleep(10); } } void *thread2_func(void *p_arg) { while (1) { printf("%d\n", a);; sleep(10); } } int main(void) { pthread_t t1, t2; pthread_create(&t1, NULL, thread1_func, "Thread 1"); pthread_create(&t2, NULL, thread2_func, "Thread 2"); sleep(1000); return; } ## 技巧 gdb可以使用“`awatch`”命令设置读写观察点，也就是当发生读取变量或改变变量值的行为时，程序就会暂停住。以上面程序为例: (gdb) aw a Hardware access (read/write) watchpoint 1: 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 16938)] [Switching to Thread 0x7ffff782c700 (LWP 16938)] Hardware access (read/write) watchpoint 1: a Value = 0 0x00000000004005c6 in thread1_func (p_arg=0x40076c) at a.c:10 10 a++; (gdb) c Continuing. Hardware access (read/write) watchpoint 1: a Old value = 0 New value = 1 thread1_func (p_arg=0x40076c) at a.c:11 11 sleep(10); (gdb) c Continuing. [New Thread 0x7ffff6e2b700 (LWP 16939)] [Switching to Thread 0x7ffff6e2b700 (LWP 16939)] Hardware access (read/write) watchpoint 1: a Value = 1 0x00000000004005f2 in thread2_func (p_arg=0x400775) at a.c:19 19 printf("%d\n", a);; (gdb) c Continuing. 1 [Switching to Thread 0x7ffff782c700 (LWP 16938)] Hardware access (read/write) watchpoint 1: a Value = 1 0x00000000004005c6 in thread1_func (p_arg=0x40076c) at a.c:10 10 a++; 可以看到，使用“`aw a`”命令（`aw`是`awatch`命令的缩写）以后，每次读取或改变`a`的值都会让程序停下来。 需要注意的是`awatch`命令只对硬件观察点才生效，参见[gdb手册](https://sourceware.org/gdb/onlinedocs/gdb/Set-Watchpoints.html). ## 贡献者 nanxiao