C语言 pthread 的生产者-消费者问题

producer-consumer problem with pthreads

提问by Mike

I'm attempting to solve the producer-consumer problem using pthreads and semaphores, but it looks like the producer threads aren't producing, and the consumer threads aren't consuming. It appears that the threads are being created:

我正在尝试使用 pthread 和信号量解决生产者-消费者问题，但看起来生产者线程没有生产，消费者线程没有消费。似乎正在创建线程：

  /* Do actual work from this point forward */
  /* Create the producer threads */
  for(c1=1; c1<=argarray[1]; c1++)
  {
    pthread_create(&tid, &attr, producer, NULL);
    printf("Creating producer #%d\n", c1);    
  }

  /* Create the consumer threads */
  for(c1=1; c1<=argarray[2]; c1++)
  {
    pthread_create(&tid, &attr, consumer, NULL);
    printf("Creating consumer #%d\n", c1);    
  }

because the "Creating producer #x" and "Creating consumer #x" are printed to the screen. It doesn't however, print from inside the threads themselves:

因为“Creating producer #x”和“Creating consumer #x”被打印到屏幕上。但是，它不会从线程本身内部打印：

if(insert_item(item))
{
  fprintf(stderr, "Producer error.");
}
else
{
  printf("Producer produced %d\n", item);
}

likewise for consumer threads. Full code:

对于消费者线程也是如此。完整代码：

#include "buffer.h"
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>

/* Create Buffer */
buffer_item buffer[BUFFER_SIZE];

/* Semaphore and Mutex lock */
sem_t cEmpty;
sem_t cFull;
pthread_mutex_t mutex;

/* Threads */
pthread_t tid; /* Thread ID */
pthread_attr_t attr; /* Thread attributes */

void *producer(void *param);
void *consumer(void *param);
void init();

/* Progress Counter */
int cg;

main(int argc, char *argv[])
{
  /* Variables */
  int argarray[3], c1;

  /* Argument counter checks */
  if(argc != 4)
  {
    fprintf(stderr, "usage: main [sleep time] [# of producer threads] [# of consumer threads]\n");
    return -1;
  }

  /* Get args from command line and change them into integers */
  argarray[0] = atoi(argv[1]); /* How long to sleep before ending */
  argarray[1] = atoi(argv[2]); /* Producer threads */
  argarray[2] = atoi(argv[3]); /* Consumer threads */

  /* Error check */
  if(argarray[1]<1)
  {
    fprintf(stderr, "argument 2 must be > 0\n");
    return -1;
  }
  if(argarray[2]<1)
  {
    fprintf(stderr, "argument 3 must be > 0\n");
    return -1;
  }    

  init();

  /* Do actual work from this point forward */
  /* Create the producer threads */
  for(c1=1; c1<=argarray[1]; c1++)
  {
    pthread_create(&tid, &attr, producer, NULL);
    printf("Creating producer #%d\n", c1);    
  }

  /* Create the consumer threads */
  for(c1=1; c1<=argarray[2]; c1++)
  {
    pthread_create(&tid, &attr, consumer, NULL);
    printf("Creating consumer #%d\n", c1);    
  }

  /* Ending it */
  sleep(argarray[0]);

  printf("Production complete.\n");
  exit(0);
}

void init()
{
  int c2;

  pthread_mutex_init(&mutex, NULL); /* Initialize mutex lock */
  pthread_attr_init(&attr); /* Initialize pthread attributes to default */
  sem_init(&cFull, 0, 0); /* Initialize full semaphore */
  sem_init(&cEmpty, 0, BUFFER_SIZE); /* Initialize empty semaphore */
  cg = 0; /* Initialize global counter */ 
  for(c2=0;c2<BUFFER_SIZE;c2++) /* Initialize buffer */
  {
    buffer[c2] = 0;
  }
}

void *producer(void *param)
{
  /* Variables */
  buffer_item item;

  while(1)
  { 
    sleep(rand());      
    item = (rand()); /* Generates random item */ 

    sem_wait(&cEmpty); /* Lock empty semaphore if not zero */
    pthread_mutex_lock(&mutex);

    if(insert_item(item))
    {
      fprintf(stderr, "Producer error."); 
    }
    else
    {
      printf("Producer produced %d\n", item); 
    }

    pthread_mutex_unlock(&mutex);
    sem_post(&cFull); /* Increment semaphore for # of full */
  }
}

void *consumer(void *param)
{
  buffer_item item;

  while(1)
  {
    sleep(rand());
    sem_wait(&cFull); /* Lock empty semaphore if not zero */
    pthread_mutex_lock(&mutex);
    if(remove_item(&item))
    {
      fprintf(stderr, "Consumer error."); 
    }
    else
    {
      printf("Consumer consumed %d\n", item);
    }

    pthread_mutex_unlock(&mutex);
    sem_post(&cEmpty); /* Increments semaphore for # of empty */
  }
}

int insert_item(buffer_item item)
{
  if(cg < BUFFER_SIZE) /* Buffer has space */
  {
    buffer[cg] = item;
    cg++;
    return 0;
  }
  else /* Buffer full */
  {
    return -1;
  }
}

int remove_item(buffer_item *item)
{
  if(cg > 0) /* Buffer has something in it */
  {
    *item = buffer[(cg-1)];
    cg--;
    return 0;
  }
  else /* Buffer empty */
  {
    return -1;
  }
}

Terminal output:

终端输出：

user@isanacom:~/Desktop/PCthreads$ ./main 10 10 10
Creating producer #1
Creating producer #2
Creating producer #3
Creating producer #4
Creating producer #5
Creating producer #6
Creating producer #7
Creating producer #8
Creating producer #9
Creating producer #10
Creating consumer #1
Creating consumer #2
Creating consumer #3
Creating consumer #4
Creating consumer #5
Creating consumer #6
Creating consumer #7
Creating consumer #8
Creating consumer #9
Creating consumer #10
Production complete.

As a beginner to multithreading, I'm sure its probably something simple I'm overlooking, and I appreciate the help.

作为多线程的初学者，我确信它可能是我忽略的一些简单的东西，我很感激你的帮助。

采纳答案by Rickard

Both the consumer and producer does a sleep(rand()) which will sleep for a random number of seconds between 0 and MAX_INT, in the example you give the main thread will terminate after 10 seconds. If the rand() value of the producers are above 10 they will never have the chance produce anything.

消费者和生产者都执行 sleep(rand()) ，它将在 0 和 MAX_INT 之间休眠随机秒数，在示例中，主线程将在 10 秒后终止。如果生产者的 rand() 值高于 10，他们将永远没有机会生产任何东西。

回答by malcolm

You should start by using an array of threads...

您应该从使用线程数组开始...

pthread_t tid[argarray[1] + argarray[2]];

for(c1 = 0; c1 < argarray[1]; c1++)
{
    pthread_create(&tid[c1], &attr, producer, NULL);
    printf("Creating producer #%d\n", c1); 
}
for(c1 = 0; c1 < argarray[2]; c1++)
{
    pthread_create(&tid[c1 + argarray[1]], &attr, consumer, NULL);
    printf("Creating consumer #%d\n", c1); 
}

There may be other problems but this is the first one I see ...

可能还有其他问题，但这是我看到的第一个问题......

回答by gunan

You should not directly call exit at the end of your main function. You should first call pthread_join for the threads you created at the end, to keep the main thread alive until the other threads die.

您不应在 main 函数的末尾直接调用 exit。您应该首先为您在最后创建的线程调用 pthread_join，以保持主线程处于活动状态，直到其他线程死亡。