查看线程信息

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

pthread_mutex_t mutex_1;
pthread_mutex_t mutex_2;

void *child1(void* arg)
{
    printf("Thread1 is running
");
    while (1) {
        pthread_mutex_lock(&mutex_1);
        sleep(3);
        pthread_mutex_lock(&mutex_2);
        pthread_mutex_unlock(&mutex_2);
        pthread_mutex_unlock(&mutex_1);
        sleep(5);
    }
}

void *child2(void* arg)
{
    printf("Thread2 is running
");
    while (1) {
        pthread_mutex_lock(&mutex_2);
        sleep(3);
        pthread_mutex_lock(&mutex_1);
        pthread_mutex_unlock(&mutex_1);
        pthread_mutex_unlock(&mutex_2);
        sleep(5);
    }
}

int main()
{
    //sleep(10);
    pthread_mutex_init(&mutex_1, NULL);
    pthread_mutex_init(&mutex_2, NULL);
    pthread_t tid1, tid2;
    pthread_create(&tid1, NULL, child1, NULL);
    pthread_create(&tid2, NULL, child2, NULL);
    pthread_join(tid1, NULL);
    pthread_join(tid2, NULL);
    return 0;
}

编译:

gcc thread_test.c -pthread -g

gdb attach上去

sudo gdb -pid=6005

(gdb) i threads
  Id   Target Id         Frame 
* 1    Thread 0x7fb775f6f700 (LWP 6005) "a.out" 0x00007fb775b7b98d in pthread_join (threadid=140425926706944, thread_return=0x0) at pthread_join.c:90
  2    Thread 0x7fb7757a8700 (LWP 6006) "a.out" __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
  3    Thread 0x7fb774fa7700 (LWP 6007) "a.out" __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135

有3个线程

切到线程2(LWP 6006)

(gdb) thread 2
[Switching to thread 2 (Thread 0x7fb7757a8700 (LWP 6006))]
#0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
135    ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: No such file or directory.

看下backtrace

(gdb) bt
#0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
#1  0x00007fb775b7cdbd in __GI___pthread_mutex_lock (mutex=0x6010e0 <mutex_2>) at ../nptl/pthread_mutex_lock.c:80
#2  0x000000000040087a in child1 (arg=0x0) at thread_test.c:15
#3  0x00007fb775b7a6ba in start_thread (arg=0x7fb7757a8700) at pthread_create.c:333
#4  0x00007fb7758b03dd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109

从#2看出在child1处出现问题，查看代码

(gdb) l child1
5    
6    pthread_mutex_t mutex_1;
7    pthread_mutex_t mutex_2;
8    
9    void *child1(void* arg)
10    {
11        printf("Thread1 is running
");
12        while (1) {
13            pthread_mutex_lock(&mutex_1);
14            sleep(3);
(gdb) 
15            pthread_mutex_lock(&mutex_2);
16            pthread_mutex_unlock(&mutex_2);
17            pthread_mutex_unlock(&mutex_1);
18            sleep(5);
19        }
20    }

看出15行pthread_mutex_lock(&mutex_2)，在等mutex_2，看下mutex_2的owner是谁

(gdb) p mutex_2
$1 = {__data = {__lock = 2, __count = 0, __owner = 6007, __nusers = 1, __kind = 0, __spins = 0, __elision = 0, __list = {__prev = 0x0, __next = 0x0}}, 
  __size = "0200000000000000w27000001", '00' <repeats 26 times>, __align = 2}

__owner = 6007 是线程3，切到线程3

(gdb) thread 3
[Switching to thread 3 (Thread 0x7fb774fa7700 (LWP 6007))]
#0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
135    ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S: No such file or directory.
(gdb) bt
#0  __lll_lock_wait () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
#1  0x00007fb775b7cdbd in __GI___pthread_mutex_lock (mutex=0x6010a0 <mutex_1>) at ../nptl/pthread_mutex_lock.c:80
#2  0x00000000004008ce in child2 (arg=0x0) at thread_test.c:28
#3  0x00007fb775b7a6ba in start_thread (arg=0x7fb774fa7700) at pthread_create.c:333
#4  0x00007fb7758b03dd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
(gdb) l child2
18            sleep(5);
19        }
20    }
21    
22    void *child2(void* arg)
23    {
24        printf("Thread2 is running
");
25        while (1) {
26            pthread_mutex_lock(&mutex_2);
27            sleep(3);
(gdb) 
28            pthread_mutex_lock(&mutex_1);
29            pthread_mutex_unlock(&mutex_1);
30            pthread_mutex_unlock(&mutex_2);
31            sleep(5);
32        }
33    }
34    
35    int main()
36    {
37        //sleep(10);
(gdb) p mutex_1
$2 = {__data = {__lock = 2, __count = 0, __owner = 6006, __nusers = 1, __kind = 0, __spins = 0, __elision = 0, __list = {__prev = 0x0, __next = 0x0}}, 
  __size = "0200000000000000v27000001", '00' <repeats 26 times>, __align = 2}

__owner = 6006 是线程2

说明6006（线程2）在等mutex_2, mutex_2被6007（线程3）拿着， 6007（线程3）在等mutex_1，mutex_1被6006（线程2）拿着 成了死锁了。