RE: sem_trywait never returns EAGAIN

["Bossom, John" <John dot Bossom at Cognos dot COM>]

Try checking the value of errno....

Older UNIX methods would return -1 on failure and set errno to the actual
specific error.


-----Original Message-----
From: Patrick Frants [mailto:patrick@quintiq.com]
Sent: February 26, 2002 12:43 PM
To: pthreads-win32@sourceware.cygnus.com
Subject: sem_trywait never returns EAGAIN


Hi,

>From the Linux man pages I concluded that calling sem_trywait on a semaphore
with a zero count should result in an EAGAIN return value. I get a -1 all
time 
time. Intiializing the count with 1 and calling sem_trywait two times
results in 0 and -1 return codes. Initializing with 0, calling sem_trywait
results in a -1 return 
code. Initializing with 0, calling sem_post, and sem_trywait twice results
in 0 and -1 return codes for the sem_trywait calls.

Are my expectations wrong or is the behaviour of the library wrong?

Here is my sample code:

#include <pthread.h>
#include <semaphore.h>
#include <cassert>
#include <cstdio>

#pragma comment(lib, "pthreadvce")

int main(int argc, char** argv)
{
  sem_t s;
  assert(sem_init(&s, 0, 0) == 0);
  int result = sem_trywait(&s);
  if ( result == -1 )
  {
    perror("sem_wait"); // No error
  }
  else
  {
    printf("ok\n");
  }

  result = sem_post(&s);

  result = sem_trywait(&s);
  if ( result == -1 )
  {
    perror("sem_wait");
  }
  else
  {
    printf("ok\n");
  }

  return 0;
}
  
  

	
--
Patrick Frants
Senior Software Engineer
Quintiq
patrick@quintiq.com
www.quintiq.com


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