Use of condition variables in linux

Conditional variables allow us to sleep and wait for certain conditions to appear. (In windows, the interlock function can be used to simulate the conditional variables in linux)

A condition variable is a mechanism for synchronizing global variables shared by threads. It mainly includes two actions: one thread waits for the condition variable to be established and suspends; another thread sets "condition to true" (a signal indicating condition to true ).

To prevent competition, the use of condition variables is always combined with a mutex lock.

The condition variable type is pthread_cond _

Like mutex lock, conditional variables have two static and dynamic creation methods:

Use the PTHREAD_COND_INITIALIZER constant in static mode, as follows: pthread_cond_t cond = PTHREAD_COND_INITIALIZER

Call the pthread_cond_init () function dynamically. The API is defined as follows: int pthread_cond_init (pthread_cond_t * cond, pthread_condattr_t * cond_attr). Although the POSIX standard defines attributes for condition variables, they are not implemented in Lin, therefore, the cond_attr value is usually NULL and ignored.

To cancel a condition variable, you must call pthread_cond_destroy (). This condition variable can be canceled only when no thread is waiting for the condition variable. Otherwise, EBUSY is returned. The API is defined as follows: int pthread_cond_destroy (pthread_cond_t * cond)

The following is the code for using conditional variables extracted from a program written by a bird:

#include <stdlib.h>#include <queue>#include <pthread.h>struct InitIndexTask{int x;int y;int z;InitIndexTask(int a, int b, int c){this->x = a;this->y = b;this->z = c;}};/*******global variables******/const uint8_t THREAD_COUNT = 8;pthread_mutex_t init_mutex     = PTHREAD_MUTEX_INITIALIZER;pthread_cond_t  condition_init   = PTHREAD_COND_INITIALIZER;queue<InitIndexTask*> InitIndexQ;/*******init thread body******/void* InitIndexThread(void* data){char* thread_name = (char*)data;cout << "created an init_index_thread(tid:" << (unsigned int)pthread_self() << " tname:" << thread_name << ")." << endl;InitIndexTask* initTask = NULL;while (true){pthread_mutex_lock( &init_mutex );while (InitIndexQ.empty())pthread_cond_wait( &condition_init, &init_mutex );initTask = InitIndexQ.front();InitIndexQ.pop();pthread_mutex_unlock( &init_mutex );if (!initTask)break;//...delete initTask;}cout << thread_name << " exited." << endl;delete[] BUFF;delete[] thread_name;}/******* main******/int main(int argc, char** argv){pthread_t* threadArr = new pthread_t[THREAD_COUNT];for (uint8_t i = 0; i < THREAD_COUNT; ++i){char* thread_name = new char[50];snprintf(thread_name, 50, "Thread-%d", i);pthread_create(&threadArr[i], 0, &InitIndexThread, thread_name);}for (int i = 0; i < 100; i++) {InitIndexTask* task = new InitIndexTask(i, 2*i, 3*i);pthread_mutex_lock( &init_mutex );InitIndexQ.push(task);pthread_mutex_unlock( &init_mutex );pthread_cond_signal( &condition_init );}for (uint8_t i = 0; i < THREAD_COUNT; ++i){pthread_mutex_lock( &init_mutex );InitIndexQ.push(NULL);pthread_mutex_unlock( &init_mutex );pthread_cond_signal( &condition_init );}for (uint8_t i = 0; i < THREAD_COUNT; ++i)pthread_join(threadArr[i], NULL);delete[] threadArr;cout << "main thread exited." << endl;}