c - pthread execution on linux

Question

I started doing pthread programming on linux and at the very first programme i got totally confused. Below is the programme I am running

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

void *print_message_function( void *ptr );

int main(){
 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);
 printf("amit");
 pthread_join( thread2, NULL); 

 printf("Thread 1 returns: %d
",iret1);
 printf("Thread 2 returns: %d
",iret2);
 exit(0);
}

void *print_message_function( void *ptr ){
 char *message;
 message = (char *) ptr;
 printf("%s 
", message);
}

First one thing i would like to know is order of thread execution is not sequential ??

Second thing is i intensionally put print("amit"); to see main is really halt during thread1 getting terminated but in the output what we can see is printf statement is executed first. The output of whole process is

Thread 1

Thread 2

amitThread 1 returns: 0

Thread 2 returns: 0

Answer 1

You are right in saying that the order of thread execution is not sequential. To some extent, that is the whole point of using threads, i.e. to run other tasks concurrently.

The output you are seeing is as expected, and can possibly be different.

Perhaps this will help:

   main            thread1     thread2
    |                
    |--create--------+-----------
    |                |           |   
    |            "Thread 1"      |   "Thread 2" can
    |                |           |<- occur anywhere
    |               /            |   along this line
   join(1) ---------             |
    |                            |
    |                            |
  "amit"                         |
    |                            |
    |                            |
   join(2) ---------------------/
    |
    |
"Thread 1 returns"
"Thread 2 returns"
    |
  exit(0)

The only guarantee you have is:

• "Thread 1" will always be printed before "amit" (because pthread_join() waits for thread 1 to end before the main program can proceed)
• "Thread X returns ..." statements will always come at the end, after both threads have terminated.