C：生产者/消费者

Question

我在学校的任务之一是修复这段代码。生产者和消费者线程应该交替增加他们的局部变量并将其打印出来，但我不知道如何修复它。

生产者和消费者线程应该可以无限期地这样做，但如果我启动可执行文件，它会任意停止。

如果有人能向我解释这种行为，那就太好了。

代码如下：

#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>

static pthread_mutex_t mutex;
static pthread_cond_t cond_consumed = PTHREAD_COND_INITIALIZER;
static pthread_cond_t cond_produced = PTHREAD_COND_INITIALIZER;

static void *producer( void *arg )
{
        unsigned long count=0;

        while( 1 ) {
                printf("producer: wait...\n");
                pthread_mutex_lock( &mutex ); // enter critical section
                pthread_cond_wait( &cond_consumed, &mutex );
                printf("producer: PRODUCE %ld...\n", count++);
                pthread_cond_signal( &cond_produced );
                pthread_mutex_unlock( &mutex ); // leave critical section
        }
        return NULL;
}

static void *consumer( void *arg )
{
        unsigned long count=0; 

        sleep( 1 );
        pthread_cond_signal( &cond_consumed );
        while( 1 ) {
                printf("consumer: wait...\n");
                pthread_mutex_lock( &mutex );
                pthread_cond_wait( &cond_produced, &mutex );
                printf("consumer: CONSUME %ld...\n", count++);
                pthread_cond_signal( &cond_consumed );
                pthread_mutex_unlock( &mutex );
        }
        return NULL;
}

int main( int argc, char **argv, char **envp )
{
        pthread_t p1, p2;

        if (pthread_mutex_init( &mutex, NULL )) {
                perror("pthread_mutex_init");
                return -1;
        }

        pthread_create( &p2, NULL, consumer, NULL );
        pthread_create( &p1, NULL, producer, NULL );

        pthread_join( p1, NULL );
        pthread_join( p2, NULL );

        pthread_mutex_destroy( &mutex );
        return 0;
}

输出应如下例所示：

 $ ./event
 producer: wait...
 consumer: wait...
 producer: PRODUCE 0...
 producer: wait...
 consumer: CONSUME 0...
 consumer: wait...
 producer: PRODUCE 1...
 producer: wait...
 consumer: CONSUME 1...
 consumer: wait...
 producer: PRODUCE 2...
 producer: wait...
 consumer: CONSUME 2...
 consumer: wait...
 ....

Answer 1

可能出现的问题是 thread_A 在 thread_B 到达 pthread_cond_wait 语句之前发出信号。 Thread_B 永远不会看到信号，而是继续等待。

根据 specs:

The pthread_cond_signal() and pthread_cond_broadcast() functions have no effect if there are no threads currently blocked on cond.

这会导致死锁，这就是您遇到的情况。

Answer 2

我已经执行了游览代码。这导致了僵局。两个线程都在等待条件变量。下面是 gdb 跟踪。

(gdb) bt
#0  0x00000032b2e0822d in pthread_join () from /lib64/libpthread.so.0
#1  0x0000000000400a1c in main ()
(gdb) info thread
 3 Thread 0x7f78bfdb4700 (LWP 8125)  0x00000032b2e0b5bc in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
 2 Thread 0x7f78bf3b3700 (LWP 8126)  0x00000032b2e0b5bc in  pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
 * 1 Thread 0x7f78bfdb6700 (LWP 8124)  0x00000032b2e0822d in pthread_join () from /lib64/libpthread.so.0
(gdb) thread 2
[Switching to thread 2 (Thread 0x7f78bf3b3700 (LWP 8126))]#0  0x00000032b2e0b5bc in pthread_cond_wait@@GLIBC_2.3.2 ()
from /lib64/libpthread.so.0
(gdb) bt
#0  0x00000032b2e0b5bc in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00000000004008eb in producer ()
#2  0x00000032b2e079d1 in start_thread () from /lib64/libpthread.so.0
#3  0x00000032b26e8b6d in clone () from /lib64/libc.so.6
(gdb) thread 2
[Switching to thread 2 (Thread 0x7f78bf3b3700 (LWP 8126))]#0  0x00000032b2e0b5bc in pthread_cond_wait@@GLIBC_2.3.2 ()
from /lib64/libpthread.so.0
(gdb) bt
#0  0x00000032b2e0b5bc in pthread_cond_wait@@GLIBC_2.3.2 () from  /lib64/libpthread.so.0
#1  0x00000000004008eb in producer ()
#2  0x00000032b2e079d1 in start_thread () from /lib64/libpthread.so.0
#3  0x00000032b26e8b6d in clone () from /lib64/libc.so.6
gdb) thread 3
[Switching to thread 3 (Thread 0x7f78bfdb4700 (LWP 8125))]#0  0x00000032b2e0b5bc in pthread_cond_wait@@GLIBC_2.3.2 ()
from /lib64/libpthread.so.0
(gdb) bt
#0  0x00000032b2e0b5bc in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x000000000040096a in consumer ()
#2  0x00000032b2e079d1 in start_thread () from /lib64/libpthread.so.0
#3  0x00000032b26e8b6d in clone () from /lib64/libc.so.6

O/P:
smadala@gateway2:myExperiments$ ./a.out
producer: wait...
consumer: wait...
producer: PRODUCE 0...
producer: wait...
consumer: CONSUME 0...
consumer: wait...
producer: PRODUCE 1...
producer: wait...
consumer: CONSUME 1...
consumer: wait...
Blocked after this.

请正确使用条件变量或使用信号量。信号量在生产者消费者问题中使用起来很简单。

C：生产者/消费者

C: Producer / Consumer

c

mutex

pthreads

consumer

producer