pthread, sem_post and gdb

[Pieter Grimmerink]

Hello,

I have been experiencing a really strange problem with the combination
of pthreads and POSIX semaphores when debugged with gdb 5.0
I'm using gcc 2.95.3, I don't know the version of my pthread library, but it 
came with glibc 2.2.2


I wrote the following small testprogram:

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

sem_t semaphore;

void *threadproc(void *arg)
{
  //while (1)
  //{
      sem_wait(&semaphore);
      printf("thread output\n");        
  //}
  return NULL;
};

int main(int argc, char *argv[])
{
  pthread_t thread;
  sem_init(&semaphore,0,0);
  pthread_create(&thread, NULL, threadproc, NULL);
  sleep(5);
  sem_post(&semaphore);
  sleep(5);
  printf("main output\n");      
  return EXIT_SUCCESS;
}

When I do

gcc main.c -lpthread

and then

gdb a.out
> run

the output stays quiet for 5 seconds (the first sleep(5)), then
I get "thread output" and "main output" immediately following
eachother, while the 'program exited normally' immediately
after printing this output.
The second 'sleep(5)' immediately seems to return.

When I compile the code with debug info, and step through
the code in main(), the execution stalls at sem_post(&semaphore),
probably because the pthread ends.
So I added a while(1) loop in the threadproc, now I can step
through the complete main function, and the second 'sleep(5)'
takes 5 seconds like it should.
(adding the while loop in the threadproc does not have any
effect on the results of the first test)

I have not been able to find anything related to this problem,
but please forgive my ignorance, because this is one of
my first experiences with gdb...

Best regards,

Pieter Grimmerink