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Message-ID: <409FF38C.7080902@bluetheta.com>
Date: Mon, 10 May 2004 22:26:36 +0100
From: Andre Ben Hamou <andre@bluetheta.com>
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To: linux-kernel@vger.kernel.org
Subject: Multithread select() bug
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Here's the scenario...

- parent thread P creates a connected socket pair S[0, 1]
- P spawns a child thread C and passes it S
- C selects on S[0]
- P closes S[0]

As I understand the semantics of the select call, C should now return 
immediately in response to the closure (and it does on Mac OS X). 
However, the following test code behaves otherwise for the two test 
cases I've tried (2.4.21 and 2.6.5). Compilation command used: 'gcc 
foobar.c -lpthread'.

Cheers,

Andre Ben Hamou
Imperial College London


--- BEGIN TEST CODE (foobar.c)---

#include <assert.h>         // assert
#include <pthread.h>        // pthread_create
#include <sys/select.h>     // select
#include <sys/types.h>      // socketpair
#include <sys/socket.h>     // socketpair
#include <unistd.h>         // sleep
#include <stdio.h>          // printf

void *threadFuntion (void *sockets) {
     int socket = ((int *)sockets)[0];
     struct timeval timeout = {tv_sec: 5, tv_usec: 0};

     // Allocate a file descriptor set with the passed socket
     fd_set fds;
     FD_ZERO (&fds);
     FD_SET (socket, &fds);

     // Select to read / register exceptions on the FD set
     select (socket + 1, &fds, NULL, &fds, &timeout);

     return NULL;
}

int main (void) {
     int sockets[2];
     pthread_t thread;

     // Create a connected pair of sockets
     assert (socketpair (PF_UNIX, SOCK_STREAM, 0, sockets) != -1);
     printf ("sockets: {%i, %i}\n", sockets[0], sockets[1]);

     // Create a POSIX thread
     // - use the default configuration
     // - invoke 'threadFunction' as the root function of the thread
     // - pass the socket array to 'threadFunction'
     assert (pthread_create (&thread,
			    NULL,
                             threadFuntion,
                             sockets) == 0);

     // Wait for a second and then close the socket being selected on
     sleep (1);
     assert (close (sockets[0]) == 0);
     printf ("Socket closed\n");

     // Wait for the thread to exit - SHOULD BE ~ INSTANTANEOUS
     assert (pthread_join (thread, NULL) == 0);
     printf ("Thread joined\n");

     assert (close (sockets[1]) == 0);
     return 0;
}

--- END TEST CODE ---

-- 

...and, on the seventh day, God switched off his Mac.