# 設置讀觀察點 ## 例子 #include <stdio.h> #include <pthread.h> int a = 0; void *thread1_func(void *p_arg) { while (1) { printf("%d\n", a); sleep(10); } } int main(void) { pthread_t t1; pthread_create(&t1, NULL, thread1_func, "Thread 1"); sleep(1000); return; } ## 技巧 gdb可以使用“`rwatch`”命令設置讀觀察點，也就是當發生讀取變量行為時，程序就會暫停住。以上面程序為例: (gdb) start Temporary breakpoint 1 at 0x4005f3: 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) rw a Hardware read watchpoint 2: a (gdb) c Continuing. [New Thread 0x7ffff782c700 (LWP 5540)] [Switching to Thread 0x7ffff782c700 (LWP 5540)] Hardware read watchpoint 2: a Value = 0 0x00000000004005c6 in thread1_func (p_arg=0x40071c) at a.c:10 10 printf("%d\n", a); (gdb) c Continuing. 0 Hardware read watchpoint 2: a Value = 0 0x00000000004005c6 in thread1_func (p_arg=0x40071c) at a.c:10 10 printf("%d\n", a); (gdb) c Continuing. 0 Hardware read watchpoint 2: a Value = 0 0x00000000004005c6 in thread1_func (p_arg=0x40071c) at a.c:10 10 printf("%d\n", a); 可以看到，使用“`rw a`”命令（`rw`是`rwatch`命令的縮寫）以后，每次訪問`a`的值都會讓程序停下來。 需要注意的是`rwatch`命令只對硬件觀察點才生效，參見[gdb手冊](https://sourceware.org/gdb/onlinedocs/gdb/Set-Watchpoints.html). ## 貢獻者 nanxiao