# 使用“$_thread”變量 ## 例子 #include <stdio.h> #include <pthread.h> int a = 0; void *thread1_func(void *p_arg) { while (1) { a++; sleep(10); } } void *thread2_func(void *p_arg) { while (1) { a++; sleep(10); } } int main(void) { pthread_t t1, t2; pthread_create(&t1, NULL, thread1_func, "Thread 1"); pthread_create(&t2, NULL, thread2_func, "Thread 2"); sleep(1000); return; } ## 技巧 gdb從7.2版本引入了`$_thread`這個“`convenience variable`”，用來保存當前正在調試的線程號。這個變量在寫斷點命令或是命令腳本時會很有用。以上面程序為例: (gdb) wa a Hardware watchpoint 2: a (gdb) command 2 Type commands for breakpoint(s) 2, one per line. End with a line saying just "end". >printf "thread id=%d\n", $_thread >end 首先設置了觀察點：“wa a”（`wa`是`watch`命令縮寫），也就是當`a`的值發生變化時，程序會暫停，接下來在`commands`語句中打印線程號。 然后繼續執行程序： (gdb) c Continuing. [New Thread 0x7ffff782c700 (LWP 20928)] [Switching to Thread 0x7ffff782c700 (LWP 20928)] Hardware watchpoint 2: a Old value = 0 New value = 1 thread1_func (p_arg=0x400718) at a.c:11 11 sleep(10); thread id=2 (gdb) c Continuing. [New Thread 0x7ffff6e2b700 (LWP 20929)] [Switching to Thread 0x7ffff6e2b700 (LWP 20929)] Hardware watchpoint 2: a Old value = 1 New value = 2 thread2_func (p_arg=0x400721) at a.c:20 20 sleep(10); thread id=3 可以看到程序暫停時，會打印線程號：“`thread id=2`”或者“`thread id=3`”。 參見[gdb手冊](https://sourceware.org/gdb/onlinedocs/gdb/Threads.html). ## 貢獻者 nanxiao