Subj : Is this valid or not?
To   : comp.programming.threads
From : Randy Howard
Date : Tue Feb 15 2005 07:49 am


I recently ran into some problems with gcc and pthreads under cygwin.  
I was trying out a little hello world to see it build, specify the 
correct compiler flags, libs, etc.  This will hang when run on an SMP 
system, but not a single CPU (or a when bash.exe is pinned to a 
single processor via task manager).  

Running it repeatedly, as in:

while [ 1 ]
do 
   ./hello.exe
done

.... is a good way to make it fail quickly.  


Seems to run as expected on other pthread systems, such as SuSE 9.1
pro.  I was hoping that somebody could point out the obvious thing I 
must be doing wrong before I submit it to cygwin as a bug.

I've tried using both of these command lines:

gcc -O0 -g0 -D_REENTRANT -Wall hello.c -o hello.exe -lpthread
gcc -mthreads -O0 -Wall hello.c -o hello.exe

Note that if I comment out the call to printf() labeled "bad guy"
the lockups go away.

<------- snip --------->

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

#define THREADS   8  
pthread_mutex_t   io_mutex = PTHREAD_MUTEX_INITIALIZER;

void Lock(pthread_mutex_t *mtx);
void Unlock(pthread_mutex_t *mtx);

void * PrintHello(void *unused_arg)
{
   Lock(&io_mutex);
      printf("Hello World!\n"); 
   Unlock(&io_mutex);
   return NULL;
}

int main(void)
{
   pthread_t threads[THREADS] = {0};
   int       rc = 0;
   int       t = 0;
  
   for (t=0; t < THREADS; t++) {
      Lock(&io_mutex);
         printf("Creating thread %d\n", t); /* bad guy */
      Unlock(&io_mutex);

      rc = pthread_create(&threads[t], NULL, PrintHello, NULL);
      if (rc != 0) {
         fprintf(stderr, "error from pthread_create(): %d\n", rc);
         exit(EXIT_FAILURE);
      }
   }

   for (t=0; t < THREADS; t++) {
      rc = pthread_join(threads[t], NULL);
      if (rc != 0) {
         fprintf(stderr, "error from pthread_join(): %d\n", rc);
         exit(EXIT_FAILURE);
      }
   }

   puts("All threads joined, seeya.\n\n"); 
   return EXIT_SUCCESS;
}

void Lock(pthread_mutex_t *mtx)
{
   int rc = 0;
   rc = pthread_mutex_lock(mtx);
   if (rc != 0) {
      fprintf(stderr, "locking mutex, error = %d\n", rc);
      exit(EXIT_FAILURE);
   }
}

void Unlock(pthread_mutex_t *mtx)
{
   int rc = 0;
   rc = pthread_mutex_unlock(mtx);
   if (rc != 0) {
      fprintf(stderr, "unlocking mutex, error = %d\n", rc);
      exit(EXIT_FAILURE);
   }
}

<------- snip --------->


-- 
Randy Howard (2reply remove FOOBAR)
"Making it hard to do stupid things often makes it hard
 to do smart ones too." -- Andrew Koenig

.