snapshot 2004-Oct-10: pthread_cancel() for write() to full pipe

[denny joel <jdenny at hubcap dot clemson dot edu>]

Using cygwin 1.5.11-1 or snapshot 2004-Oct-10 on XP Pro SP2, a thread
blocked at a write() to a full pipe does not appear cancellable with
pthread_cancel().  "cygcheck.out" is attached.

I'm guessing that POSIX specifies that write() is supposed to be a
cancellation point, but I do not have a copy of the spec (any pointers
would be appreciated).  I'm guessing this based on (1) the man pages in
Solaris ('man cancellation'), and (2) the fact that it works as I expect
on the Solaris 9 and Redhat 9.0 systems I've tried.  My test code appears
at the end of this e-mail.

Searching with google and searching the cygwin mailing list with keywords
like "pthread_cancel", "thread cancellation", etc., I was unable to find
much relevant information. I did find some old messages claiming that
pthreads were not yet fully implemented in cygwin.  If pthread_cancel() is
known to be incomplete or if this is a known bug, then I'd appreciate a
pointer to the relevant information, and I apologize for the repeat.

If I can be of any assistance in resolving this issue, please let me know.
However, I'm not yet intelligent enough about cygwin to try a patch.

Joel Denny

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

int aFd[2];

/* Set this large enough to fill the pipe on your system. */
#define I_PIPE_FILL 90000

void * foo( void * dummy ) {

  int i_byte;

  /* Use _DISABLE instead to prove that the thread does block on write. */
  pthread_setcancelstate( PTHREAD_CANCEL_ENABLE, NULL );

  printf( "thread is alive\n" );
  fflush( stdout );

  for ( i_byte = 0; i_byte < I_PIPE_FILL; ++i_byte ) {
    write( aFd[1], "0", 1 );
  }

  return NULL;

}

int main() {

  pthread_t thread;
  char a_buf[1];

  pipe( aFd ) ) {
  pthread_create( &thread, NULL, foo, NULL );

  /* Wait until thread has enabled/disabled cancellation. */
  read( aFd[0], a_buf, 1 );

  printf( "Thread created.\n" );
  fflush( stdout );

  /* Will not cancel write() on full pipe in cygwin. */
  pthread_cancel( thread );
  printf( "Thread canceled.\n" );
  fflush( stdout );

  /* Deadlocks here if thread  not cancelled. */
  pthread_join( thread, NULL );
  printf( "Thread joined.\n" );
  fflush( stdout );

  return 0;

}

Attachment: cygcheck.out
Description: Text document

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