Condition variables in Linux

1 You may know that a mutex is a necessary tool in a threaded program, but it cannot be omnipotent, such as when a thread is waiting for a condition of shared data to occur. It is possible to repeatedly attempt to lock and unlock the mutex to check the shared data structure.

The 2 thread puts the thread to sleep while waiting for certain conditions to be met, and then wakes up and waits for a thread to sleep if the condition satisfies the specific condition.

3 conditional variables generally allow a thread to block and wait for another thread to send a signal to compensate for the lack of a mutex.

4 Introduction to Functions

(1) static mode using pthread_cond_t Cond=pthread_cond_initializer

Dynamic mode int pthread_cond_init (pthread_cond_t *cond,pthread_condattr_t *cond_attr)

--------> Use the default property when Cond_attr is null.

(2) Unregister a condition variable

int Pthread_cond_destroy (pthread_cond_t *cond) can unregister this condition variable only if no thread is in the condition variable.

(3) Wait there are two ways, conditional wait and time wait. Either wait must mate with a mutex to prevent multiple threads from simultaneously requesting pthread_cond_wait () for this race condition.

(4) Excitation two ways, pthread_cond_signal () activates a thread waiting for the condition. Pthread_cond_broadcast () Activates all waiting threads.

5 examples

(1) A first example

1 #include <iostream>2 #include <pthread.h>3 #include <unistd.h>4 using namespace std;5pthread_cond_t qready = Pthread_cond_initializer; /* Initial CONSTRUCTION condition variable */6pthread_mutex_t qlock = Pthread_mutex_initializer; /* Initial Construction Lock */7int x = 10;8int y = 20;9void *func1 (void *Arg) {Tencout<<"func1 Start"<<Endl; OnePthread_mutex_lock (&qlock); A      while(x<y) -     { -Pthread_cond_wait (&qready,&qlock); the     } -Pthread_mutex_unlock (&qlock); -Sleep (3); -cout<<"Func1 End"<<Endl; + } -void *func2 (void *Arg) { +cout<<"Func2 Start"<<Endl; APthread_mutex_lock (&qlock); at//Modify XY -x = 20; -y = 10; -cout<<"Have change x and y"<<Endl; -Pthread_mutex_unlock (&qlock); -     if(X >y) { inPthread_cond_signal (&qready);//send a signal so that thread 1 does not block -     } tocout<<"Func2 End"<<Endl; + } -int main (int Argc,char * *argv) { the pthread_t Tid1,tid2; * int iRet; $IRet = Pthread_create (&tid1,null,func1,null);Panax Notoginseng     if(iRet) { -cout<<"pthread 1 Create error"<<Endl; the         returnIRet; +     } ASleep (2); theIRet = Pthread_create (&tid2,null,func2,null); +     if(iRet) { -cout<<"pthread 2 Create error"<<Endl; $         returnIRet; $     } -Sleep (5);  -     return0;  the}

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(2) A second example

1 #include <stdio.h>2 #include <stdlib.h>3 #include <string.h>4 #include <unistd.h>5 #include <pthread.h>6 #include <errno.h>7 #include <iostream>8 #include <pthread.h>9 using namespace std;Ten  One/* Prompt for Taxi arrival condition variable */ Apthread_cond_t Taxicond =Pthread_cond_initializer; -/* Sync Lock */ -pthread_mutex_t Taximutex =Pthread_mutex_initializer;  the  -int travelercound=0; -//when the passengers arrive, the number adds 1. -void * Traveler_arrive (void *name) { +cout<<"Traveler:"<< (char *) name<<"needs a taxi now!"<<Endl; -Pthread_mutex_lock (&Taximutex); +travelercound++; APthread_cond_wait (&taxiCond,&Taximutex); atPthread_mutex_unlock (&Taximutex); -cout<<"Traveler:"<< (char *) name<<"Now got a taxi!"<<Endl; -Pthread_exit ((void*) 0); - } -  -void * Taxi_arrive (void *name) { incout<<"Taxi:"<< (char *) name<<"arrives."<<Endl; -//guaranteed first-come detection is there a new customer arrives to      while(1){ +Pthread_mutex_lock (&Taximutex); -//greater than the notification the         if(travelercound>0) { *Pthread_cond_signal (&taxicond); $Pthread_mutex_unlock (&Taximutex); Panax Notoginseng              Break;  -         } thePthread_mutex_unlock (&Taximutex); +     } APthread_exit ((void*) 0); the } +  - int main () { $pthread_t tids[3]; $int iRet = Pthread_create (&tids[0],null,taxi_arrive, (void*) ("Jack")); -     if(iRet) { -printf"pthread_create error:iret=%d\n", IRet); the         returnIRet; -     }Wuyiprintf"Time passing by.\n"); theSleep (1); -IRet = Pthread_create (&tids[1],null,traveler_arrive, (void*) ("Susan")); Wu     if(iRet) { -printf"pthread_create error:iret=%d\n", IRet); About         returnIRet; $     } -printf"Time passing by.\n"); -Sleep (1);  -IRet = Pthread_create (&tids[2],null,taxi_arrive, (void*) ("Mike")); A     if(iRet) { +printf"pthread_create error:iret=%d\n", IRet); the         returnIRet; -     }     $printf"Time passing by.\n"); theSleep (1); the      thevoid *retval;  the      for(int i=0;i<3;i++){ -Iret=pthread_join (tids[i],&retval); in         if(iRet) { theprintf"Pthread_join error:iret=%d\n", IRet); the             returnIRet; About         } theprintf"retval=%ld\n", (long) retval);  the     } the     return0;  +}

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Condition variables in Linux