Multi-threading in C (Posix Thread) 

What is a Thread?

 Shared Memory Model


POSIX thread (pthread)

The POSIX thread libraries are a standards based thread API for C/C++. It allows one to spawn a new concurrent process flow. It is most effective on multi-processor or multi-core systems where the process flow can be scheduled to run on another processor thus gaining speed through parallel or distributed processing. Threads require less overhead than "forking" or spawning a new process because the system does not initialize a new system virtual memory space and environment for the process.

Thread Creation and Termination:

#include <stdio.h>

#include <stdlib.h>

#include <pthread.h>

void *print_message_function( void *ptr );



     pthread_t thread1, thread2;

     char *message1 = "Thread 1";

     char *message2 = "Thread 2";

     int  iret1, iret2;

    /* Create independent threads each of which will execute function */

     iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);

     iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message2);

     /* Wait till threads are complete before main continues. Unless we  */

     /* wait we run the risk of executing an exit which will terminate   */

     /* the process and all threads before the threads have completed.   */

     pthread_join( thread1, NULL);

     pthread_join( thread2, NULL); 

     printf("Thread 1 returns: %d\n",iret1);

     printf("Thread 2 returns: %d\n",iret2);



void *print_message_function( void *ptr )


     char *message;

     message = (char *) ptr;

     printf("%s \n", message);



Run: ./a.out 


Thread 1

Thread 2

Thread 1 returns: 0

Thread 2 returns: 0


In this example the same function is used in each thread. The arguments are different. The functions need not be the same.

Threads terminate by explicitly calling pthread_exit, by letting the function return, or by a call to the function exit which will terminate the process including any threads.

Function call: pthread_create

int pthread_create(pthread_t * thread, 

                       const pthread_attr_t * attr,

                       void * (*start_routine)(void *), 

                       void *arg);


Function call: pthread_exit

    void pthread_exit(void *retval);



retval - Return value of thread.