Running pthread on linux

Question

I started programming pthread on linux, and in the first program I was completely confused. Below is the program in which I run

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

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

The first thing I would like to know is that the order of execution of the threads is not sequential.

Secondly, I intentionally put a seal ("amit"); to see that main really stops when thread1 completes, but in the output we see that the printf statement is executed first. Conclusion of the whole process

Theme 1

Theme 2

amitThread 1 returns: 0

Return flow 2: 0

Answer 1

The first thing I would like to know is the thread execution order is not sequential?

. ( Linux ) , , printf, . pthread_join , :

• Thread 1 Amit, 1 Amit1
• Thread 2 Thread 1 returns: - pthread_join. printf main , main.

, . , , .

Answer 2

, . - , .. .

, , , .

, :

   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)

, , :

• "Thread 1" "amit" ( pthread_join() 1 , )
• "Thread X returns ..." , .