Process Termination

Process Termination

When the main thread completes, all the child threads are terminated and their resources freed. We can see this demonstrated if we build and run the code shown in Listing 5.15.

Listing 5.15   Code to Create a Child Thread

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

void* thread_code( void * param )

{

printf( "In thread code\n" );

}

int main()

{

pthread_t thread;

pthread_create( &thread, 0, &thread_code, 0 ); printf( "In main thread\n" );

}

When the application works, we should see a message printed by both the main and child threads, as shown in Listing 5.16.

Listing 5.16   Output from Both Original and Child Threads

$ cc -mt t.c

$ ./a.out

In thread code

In main thread

However, sometimes the application will produce output only from the main thread, as shown in Listing 5.17.

Listing 5.17   Output from Only the Original Thread

$ ./a.out

In main thread

The reason for this behavior is that sometimes the main thread terminates before the child thread has had time to execute. To avoid this behavior, the main thread needs to call pthread_exit(), which, for the main thread, will wait until all the other threads have terminated before exiting. This is true, even if the child threads have been detached. Listing 5.18 shows a version of the code with this change.

Listing 5.18  Main Thread Calls Waits for Child Threads to Complete

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

void* thread_code( void * param )

{

printf("In thread code\n");

}

int main()

{

pthread_t thread;

pthread_create( &thread, 0, &thread_code, 0 ); pthread_detach( thread );

printf( "In main thread\n" ); pthread_exit( 0 );

}

After this change, all the child threads will print their output before the main thread exits.