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/* ========================================================================
* Copyright 1988-2006 University of Washington
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
*
* ========================================================================
*/
/*
* Program: flock emulation via fcntl() locking
*
* Author: Mark Crispin
* Networks and Distributed Computing
* Computing & Communications
* University of Washington
* Administration Building, AG-44
* Seattle, WA 98195
* Internet: MRC@CAC.Washington.EDU
*
* Date: 10 April 2001
* Last Edited: 30 August 2006
*/
/* Cygwin does not seem to have the design flaw in fcntl() locking that
* most other systems do (see flocksim.c for details). If some cretin
* decides to implement that design flaw, then Cygwin will have to use
* flocksim. Also, we don't test NFS either.
*
* However, Cygwin does have the Windows misfeature (introduced in NT 4.0)
* that you can not write to any segment which has a shared lock, and you
* can't lock a zero-byte segment either. This screws up the shared-write
* mailbox drivers (mbx, mtx, mx, and tenex). As a workaround, we'll only
* lock the first byte of the file, meaning that you can't write that byte
* shared. It's been suggested to lock the maximum off_t type, but that
* risks having a future version of Windows (or Cygwin) deciding that this
* also means "no lock".
*/
#undef flock /* name is used as a struct for fcntl */
/* Emulator for flock() call
* Accepts: file descriptor
* operation bitmask
* Returns: 0 if successful, -1 if failure under BSD conditions
*/
int flocksim (int fd,int op)
{
char tmp[MAILTMPLEN];
int logged = 0;
struct flock fl;
/* lock one bytes at byte 0 */
fl.l_whence = SEEK_SET; fl.l_start = 0; fl.l_len = 1;
fl.l_pid = getpid (); /* shouldn't be necessary */
switch (op & ~LOCK_NB) { /* translate to fcntl() operation */
case LOCK_EX: /* exclusive */
fl.l_type = F_WRLCK;
break;
case LOCK_SH: /* shared */
fl.l_type = F_RDLCK;
break;
case LOCK_UN: /* unlock */
fl.l_type = F_UNLCK;
break;
default: /* default */
errno = EINVAL;
return -1;
}
while (fcntl (fd,(op & LOCK_NB) ? F_SETLK : F_SETLKW,&fl))
if (errno != EINTR) {
/* Can't use switch here because these error codes may resolve to the
* same value on some systems.
*/
if ((errno != EWOULDBLOCK) && (errno != EAGAIN) && (errno != EACCES)) {
sprintf (tmp,"Unexpected file locking failure: %s",strerror (errno));
/* give the user a warning of what happened */
MM_NOTIFY (NIL,tmp,WARN);
if (!logged++) syslog (LOG_ERR,"%s",tmp);
if (op & LOCK_NB) return -1;
sleep (5); /* slow things down for loops */
}
/* return failure for non-blocking lock */
else if (op & LOCK_NB) return -1;
}
return 0; /* success */
}
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