How to implement Linux multi-thread programming

Introduction
Conditional variables are a mechanism that uses global variables shared between threads for synchronization. It mainly includes two actions: one thread waits for the condition of the condition variable to be established and hangs (it no longer occupies the CPU at this time); Another thread makes the condition true (gives a signal that the condition is true). To prevent contention, the use of condition variables is always combined with a mutex lock.

Function prototype
1. Define condition variables

#include <pthread.h>

/* 定义两个条件变量 */
pthread_cond_t cond_pro, cond_con;

2. Initialize and destroy condition variables

#include <pthread.h>

int pthread_cond_init(pthread_cond_t *restrict cond, const pthread_condattr_t *restrict attr);int pthread_cond_destroy(pthread_cond_t *cond); /* 初始化条件变量 */
pthread_cond_init(&cond_pro, null);
pthread_cond_init(&cond_con, null);
/* 销毁条件变量 */
pthread_cond_destroy(&cond_pro);
pthread_cond_destroy(&cond_pro);

3. Wait and fire conditions

#include <pthread.h>

int pthread_cond_wait(pthread_cond_t *restrict cond, pthread_mutex_t *restrict mutex);

int pthread_cond_broadcast(pthread_cond_t *cond);
int pthread_cond_signal(pthread_cond_t *cond);
/* 等待条件 */
/* 注意：pthread_cond_wait为阻塞函数。解开锁，再等待。等条件满足时，需要抢到锁，才可以被唤醒*/  
pthread_cond_wait(&cond_pro,&mutex); 

/* 激发条件 */
/* 所有因为不满足条件的线程都会阻塞在条件变量cond_pro中的一个队列中 */
/* 以广播方式，通知所有被阻塞的所有线程 */
pthread_cond_broadcast(&cond_pro);
/* 以signal方式，只通知排在最前面的线程 */
pthread_cond_signal(&cond_pro);

Code

/*************************************************************************
  > file name: my_con.c
  > author: krischou
  > mail:zhoujx0219@163.com 
  > created time: tue 26 aug 2014 10:24:29 am cst
 ************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <unistd.h>
#define cell 10
#define flore 0

 

int i = 0; /* 所有线程共享的全局变量，此处假定至多递增至10，最小减到0 */

pthread_mutex_t mutex;       /* 定义互斥锁 */
pthread_cond_t cond_pro, cond_con; /* 定义两个条件变量 */

/* 生产者线程 */
void* pro_handler(void *arg)
{
  pthread_detach(pthread_self());  /* 由系统回收线程资源，而非主线程回收资源 ，此类情况主线程是个服务器，永久不会退出 */
  
  while(1)
  {
    pthread_mutex_lock(&mutex);
    while(i >= cell)
    {
      pthread_cond_wait(&cond_pro,&mutex); 
      /* continue是轮询，此处是阻塞 */
      /* 把锁放开再等 ，第一个参数是结构体指针，其中有成员存放被阻塞的函数 */
      /*不占cpu*/
      /* 不满足条件时才会等 ，需要别人告诉它，才能唤醒它*//* 当它返回时，锁也要回来了*/
    }
    i++;
    if(i == 1)
    {
      /* 由空到不空，唤醒消费者 */
      pthread_cond_signal(&cond_con);  /*不会立马signal被阻塞的消费者线程，因为其还要等锁抢回来*/
    }
    printf("add i: %d \n", i);
    pthread_mutex_unlock(&mutex);
    sleep(rand() % 5 + 1);
  }
}

/* 消费者线程 */
void* con_handler(void *arg)
{
  pthread_detach(pthread_self());
  while(1)
  {
    pthread_mutex_lock(&mutex);
    while(i <= flore)
    {
      pthread_cond_wait(&cond_cno,&mutex);
    }
    i--;
    if(i == 9) /* 由满到不满，要告诉生产者,以便将其唤醒 *//*此处，直接signal也可以，我们是为了更加精确*/
    {
      pthread_cond_signal(&cond_pro);
    }
    printf("con i: %d \n", i);
    pthread_mutex_unlock(&mutex);
    sleep(rand() % 5 + 1);
  }
}

int main(int argc, char *argv[]) // exe +num -num
{
  srand(getpid());
  int con_cnt, pro_cnt;
  pro_cnt = atoi(argv[1]);
  con_cnt = atoi(argv[2]);
  pthread_mutex_init(&mutex,null);
  pthread_cond_init(&cond_pro,null);
  pthread_cond_init(&cond_con,null);
  pthread_t *arr = (pthread_t*)calloc(con_cnt + pro_cnt , sizeof(pthread_t));
  int index = 0;
  while(pro_cnt > 0)
  {
    pthread_create(arr + index, null, pro_handler, null);
    index++;
    pro_cnt--;
  }
  while(con_cnt > 0)
  {
    pthread_create(arr + index, null, con_handler, null);
    index++;
    con_cnt--;
  }
  while(1);
  pthread_mutex_destroy(&mutex);
  pthread_cond_destroy(&cond_pro);
  pthread_cond_destroy(&cond_con);
  return 0;
}

Note
Whether it is in the producer thread or the consumer thread. The judgment condition marked in yellow must use while. Taking the producer thread as an example, when i>=cell, that is, when i is full, pthread_cond_wait(&cond_cno,&mutex); is executed at this time and the producer thread is suspended. Must wait until the consumer thread pthread_cond_signal(&cond_pro); wakes it up. But it is not enough for the consumer to signal it. The producer thread that is suspended must get the lock again before it can be activated. However, since the producer cannot immediately grab the lock when the consumer signals, the i value may change to greater than or equal to 10 at this time. Therefore you must use while. Otherwise, i>10 may result.

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