Question

Could you please show me the way how I can print the parent first using a mutex…Thanks

void *print_message_function( void *ptr );

main() {

     pthread_t thread1;
     char *message1 = "Thread 1";
     int  iret1;


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


     pthread_join( thread1, NULL);

     printf("(Parent)Thread 1 returns: %d\n",iret1);
     exit(0);
}

void *print_message_function( void *ptr ) {

     char *message;
     message = (char *) ptr;
     printf("(Child)%s \n", message);
}

Answer 1

By acquiring a lock before creating a thread and then immediately trying to acquire that lock in your new thread you can guarantee that some code after the thread is created will execute before the thread can possibly have acquired the lock. Of course it would be simpler just to run the code before creating the thread in the first place.

#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>

void *print_message_function( void *ptr );

static pthread_mutex_t mut;

int main() {

     pthread_t thread1;
     char *message1 = "Thread 1";
     int  iret1;
     pthread_mutex_init(&mut, NULL);
     pthread_mutex_lock(&mut);
     /* lock before creating the thread */    
     iret1 = pthread_create( &thread1, NULL, print_message_function, (void*) message1);


     printf("Parent first\n");
     /* realasing the lock gives the child a chance to acquire it */
     pthread_mutex_unlock(&mut);

     pthread_join( thread1, NULL);

     /* NOTE: iret1 is the result of the create call, not what the thread itself returned */
     printf("(Parent)Thread 1 returns: %d\n",iret1);
     return 0;
}

void *print_message_function( void *ptr ) {
     /* parent already has this lock, so this will always block until they release it */
     pthread_mutex_lock(&mut);
     char *message;
     message = (char *) ptr;
     printf("(Child)%s \n", message);
     pthread_mutex_unlock(&mut);

     return NULL;
}