socket close() -w- pthreads linux 2.6.18.2-34 (suse) SMP Post: 302164685

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Top Forums Programming socket close() -w- pthreads linux 2.6.18.2-34 (suse) SMP Post 302164685 by ramen_noodle on Tuesday 5th of February 2008 03:04:36 PM

02-05-2008

Registered User

socket close() -w- pthreads linux 2.6.18.2-34 (suse) SMP

Interesting issue. There was some discussion on the LKML last year regarding the potential problems in concurrent applications reusing file descriptors in various scenarios. The main issue is that the reuse of a file descriptor and reception of data in a threaded application can be confused pretty easily.

Alan Cox suggested using shutdown() before close() to deal with one of the most glaring reuse problems. This doesn't seem to work 100% either.

If I have code like this:

Code:

extern void *handleclient(void *arg) {
int rd = -1;
long mcount = 0;
VHNDL in;
char buf[BSZ];
                         pthread_detach(pthread_self());       
                         pthread_mutex_lock(&socketstable);  
                         memcpy(&in,arg,sizeof(in));
                         pthread_mutex_unlock(&socketstable);
                         while ( (rd = read(in.c_sock,buf,BSZ)) > 0) {
                                 printf("TID %d: Read message %d from peer at %s = %s\n",pthread_self(),mcount++,inet_ntoa(in.peer.sin_addr),buf);
                                 bzero(buf,BSZ);
                         }
                         /*pthread_mutex_lock(&socketstable);*/
                         shutdown(in.c_sock,SHUT_RDWR);
                         close(in.c_sock);
                        /*pthread_mutex_unlock(&socketstable);*/
                         pthread_exit(NULL);
}

The reuse of file descriptors still causes EBADF in some cases. The parent process is simple in an accept() loop creating handler threads till MAXTHREADS then recycling. As you see the handler does nothing but read till error and then exits.
I'm still getting EBADF on some closes and as a result having incrementing numbers of close-wait connects.

Any ideas?

ramen_noodle

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LEARN ABOUT BSD

close

CLOSE(2)							System Calls Manual							  CLOSE(2)

NAME

       close - delete a descriptor

SYNOPSIS

       close(d)
       int d;

DESCRIPTION

       The  close  call deletes a descriptor from the per-process object reference table.  If this is the last reference to the underlying object,
       then it will be deactivated.  For example, on the last close of a file the current seek pointer associated with the file is  lost;  on  the
       last close of a socket(2) associated naming information and queued data are discarded; on the last close of a file holding an advisory lock
       the lock is released (see further flock(2)).

       A close of all of a process's descriptors is automatic on exit, but since there is a limit on the number of active descriptors per process,
       close is necessary for programs that deal with many descriptors.

       When  a	process forks (see fork(2)), all descriptors for the new child process reference the same objects as they did in the parent before
       the fork.  If a new process is then to be run using execve(2), the process would normally inherit these descriptors.  Most of the  descrip-
       tors  can  be  rearranged with dup2(2) or deleted with close before the execve is attempted, but if some of these descriptors will still be
       needed if the execve fails, it is necessary to arrange for them to be closed if the execve succeeds.  For this reason, the call	``fcntl(d,
       F_SETFD,  1)''  is  provided,  which arranges that a descriptor will be closed after a successful execve; the call ``fcntl(d, F_SETFD, 0)''
       restores the default, which is to not close the descriptor.

RETURN VALUE

       Upon successful completion, a value of 0 is returned.  Otherwise, a value of -1 is returned and the global integer variable errno is set to
       indicate the error.

ERRORS

       Close will fail if:

       [EBADF]	      D is not an active descriptor.

SEE ALSO

       accept(2), flock(2), open(2), pipe(2), socket(2), socketpair(2), execve(2), fcntl(2)

4th Berkeley Distribution					   May 22, 1986 							  CLOSE(2)