Multithreading

pratham29 · 10-12-2013, 12:29 PM

Hi ,
I am new to Linux, I had difficulty understanding the following code :

Code:

#include <pthread.h> %p thread library
#include <stdio.h>

void* test(void* s) {
  sleep(2);
  printf("%s\n",(char*)s);
  return NULL;
}
int main (void) %declare 2 variables
 {
  pthread_t t1,t2;
  pthread_create(&t1,NULL,&test,(void*)"thread1");
  pthread_create(&t2,NULL,&test,(void*)"thread2");
  pthread_join(t1,NULL); %join :wait for another thread to exit
  pthread_join(t2,NULL);
  return 0;
}

Output :
Thread 2
Thread 1

I could not understand the logic of this code. Also , will i get different output if executed on different o/s.

Thanks for help in advance. Awaiting response

suicidaleggroll · 10-13-2013, 08:52 PM

What's there not to understand?

main() is executed when you run the program, the first thing it does is create two threads. It spawns t1 and passes the void* argument "thread1", then it spawns t2 and passes the void* argument "thread2". It then waits for t1 to exit, waits for t2 to exit, and then returns.

Each thread is spawned with the "test" routine, which simply sleeps for two seconds and then prints out the void* (recast as a char*) argument that it was called with before exiting.

The output will always be either:
thread 1
thread 2

or

thread 2
thread 1

Since the two threads are in a race condition for output after their 2 second delay.

Where exactly are you confused? I think this is probably the simplest example of multithreading imaginable. How familiar are you with C, and programming in general? This question has nothing to do with Linux, FYI. Execution will be the same on any OS.

pratham29 · 10-14-2013, 04:32 PM

Quote:

Originally Posted by suicidaleggroll

What's there not to understand?

main() is executed when you run the program, the first thing it does is create two threads. It spawns t1 and passes the void* argument "thread1", then it spawns t2 and passes the void* argument "thread2". It then waits for t1 to exit, waits for t2 to exit, and then returns.

Each thread is spawned with the "test" routine, which simply sleeps for two seconds and then prints out the void* (recast as a char*) argument that it was called with before exiting.

The output will always be either:
thread 1
thread 2

or

thread 2
thread 1

Since the two threads are in a race condition for output after their 2 second delay.

Where exactly are you confused? I think this is probably the simplest example of multithreading imaginable. How familiar are you with C, and programming in general? This question has nothing to do with Linux, FYI. Execution will be the same on any OS.

Thanks, was confused in the calling function part but now cleared with this doubt.No background in programming , hence the question.

zeebra · 10-15-2013, 07:55 AM

You should read <pthread.h>