why using pthread_exit?

Question

I'm trying to figure out the usage of pthread_exit using this example code:

void* PrintVar(void* arg)
 { 
   int * a = (int *) arg; // we can access memory of a!!!
    printf( "%d\n", *a); 
 } 

int main(int argc, char*argv[]) 
 { 
   int a, rc;
    a = 10; 
   pthread_t thr; 
   pthread_create( &thr, NULL, PrintVar, &a ); 

  //why do I need it here?//
  pthread_exit(&rc); /* process continues until last  
                                threads termintates */

there are two things I'm not quite sure about :

1. when we are using pthread_create - I'm passing 'a' parameter's address, but is this paramter being "saved" under "arg" of the PrintVar function? for example if I was using : PrintVar(void *blabla) , and wanted to pass 2 parameters from main function : int a = 10, int b= 20 .. how can I do that?

2. Why the pthread_exit needed? it means - wait for proccess to end - but what scenario can I get if I won't use that line?

thanks alot!

Answer 1

1. when we are using pthread_create - I'm passing 'a' parameter's address, but is this paramter being "saved" under "arg" of the PrintVar function?

The "original" a (the one defined in main ) is not being copied, you are only passing around a pointer to it.

for example if I was using : PrintVar(void *blabla) , and wanted to pass 2 parameters from main function : int a = 10, int b= 20 .. how can I do that?

Put those two values in a struct and pass a pointer to such struct as argument to pthread_create (PrintVar, thus, will receive such a pointer and will be able to retrieve the two values).

and my second question is why the pthread_exit needed? it means - wait for proccess to end - but what scenario can I get if I won't use that line?

pthread_exit terminates the current thread without terminating the process if other threads are still running; returning from main , instead, is equivalent to calling exit which, as far as the standard is concerned, should "terminate the program" (thus implicitly killing all the threads).

Now, being the C standard thread-agnostic (until C11) and support for threading in the various Unixes a relatively recent addition, depending from libc/kernel/whatever version exit may or may not kill just the current thread or all the threads.

Still, in current versions of libc, exit (and thus return from main ) should terminate the process (and thus all its threads), actually using the syscall exit_group on Linux.

Notice that a similar discussion applies for the Windows CRT.

Answer 2

The detached attribute merely determines the behavior of the system when the thread terminates; it does not prevent the thread from being terminated if the process terminates using exit(3) (or equivalently, if the main thread returns).