mdw@git.distorted.org.uk Git - mLib/blob - sys/lock.c

   1 /* -*-c-*-
   2  *
   3  * Simplified POSIX locking interface
   4  *
   5  * (c) 1997 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of the mLib utilities library.
  11  *
  12  * mLib is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU Library General Public License as
  14  * published by the Free Software Foundation; either version 2 of the
  15  * License, or (at your option) any later version.
  16  *
  17  * mLib is distributed in the hope that it will be useful,
  18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20  * GNU Library General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU Library General Public
  23  * License along with mLib; if not, write to the Free
  24  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  25  * MA 02111-1307, USA.
  26  */
  27
  28 /*----- Header files ------------------------------------------------------*/
  29
  30 #include <errno.h>
  31 #include <setjmp.h>
  32 #include <signal.h>
  33 #include <stdio.h>
  34 #include <stdlib.h>
  35 #include <string.h>
  36 #include <time.h>
  37
  38 #include <sys/types.h>
  39 #include <unistd.h>
  40 #include <fcntl.h>
  41
  42 #include "lock.h"
  43
  44 /*----- Tunable constants -------------------------------------------------*/
  45
  46 #define LOCK_TIMEOUT 10                 /* Maximum time in seconds to wait */
  47
  48 /*----- Static variables --------------------------------------------------*/
  49
  50 static jmp_buf jmp;                     /* Jump here to interrup @fcntl@ */
  51
  52 /*----- Main code ---------------------------------------------------------*/
  53
  54 /* --- @sigalrm@ --- *
  55  *
  56  * Arguments:   @int sig@ = signal number
  57  *
  58  * Returns:     ---
  59  *
  60  * Use:         Makes sure that a @SIGALRM@ signal interrupts system calls.
  61  */
  62
  63 static void sigalrm(int sig) { longjmp(jmp, 1); }
  64
  65 /* --- @lock_file@ --- *
  66  *
  67  * Arguments:   @int fd@ = file descriptor to lock
  68  *              @unsigned how@ = type of lock required
  69  *
  70  * Returns:     0 if OK, -1 if it failed.
  71  *
  72  * Use:         Acquires a lock on the given file.  The value @how@
  73  *              specifies the type of lock to acquire: @LOCK_EXCL@ gets
  74  *              an exclusive (write) lock; @LOCK_NONEXCL@ gets a non-
  75  *              exclusive (read) lock and @LOCK_UNLOCK@ releases any locks.
  76  *              Acquiring a lock gets timed out after a while with an
  77  *              error.
  78  */
  79
  80 int lock_file(int fd, unsigned how)
  81 {
  82   struct flock fk;
  83   struct sigaction sa, osa;
  84   sigset_t ss, oss;
  85   int e;
  86   int al, d, left;
  87
  88   /* --- Fill in the easy bits --- */
  89
  90   fk.l_whence = SEEK_SET;
  91   fk.l_start = 0;
  92   fk.l_len = 0;
  93
  94   /* --- Unlocking is really easy --- */
  95
  96   if (how == LOCK_UNLOCK) {
  97     fk.l_type = F_UNLCK;
  98     return (fcntl(fd, F_SETLK, &fk));
  99   }
 100
 101   /* --- Decide how to do the locking --- */
 102
 103   if (how == LOCK_EXCL)
 104     fk.l_type = F_WRLCK;
 105   else if (how == LOCK_NONEXCL)
 106     fk.l_type = F_RDLCK;
 107   else {
 108     errno = EINVAL;
 109     return (-1);
 110   }
 111
 112   /* --- Block @SIGALRM@ for a while --- *
 113    *
 114    * I don't want stray alarms going off while I'm busy here.
 115    */
 116
 117   sigemptyset(&ss);
 118   sigaddset(&ss, SIGALRM);
 119   if (sigprocmask(SIG_BLOCK, &ss, &oss))
 120     return (-1);
 121
 122   /* --- Set up the signal handler --- */
 123
 124   sa.sa_handler = sigalrm;
 125   sa.sa_flags = 0;
 126 #ifdef SA_INTERRUPT
 127   sa.sa_flags |= SA_INTERRUPT;
 128 #endif
 129   sigemptyset(&sa.sa_mask);
 130   if (sigaction(SIGALRM, &sa, &osa))
 131     return (-1);
 132
 133   /* --- Set up the alarm, remembering when it's meant to go off --- */
 134
 135   al = alarm(0);
 136   if (al && al < LOCK_TIMEOUT)
 137     d = al;
 138   else
 139     d = LOCK_TIMEOUT;
 140   alarm(d);
 141
 142   /* --- Set up the return context for the signal handler --- */
 143
 144   if (setjmp(jmp)) {
 145     sigprocmask(SIG_SETMASK, &oss, 0);
 146     errno = EINTR;
 147     e = -1;
 148     goto done;
 149   }
 150
 151   /* --- Unblock the signal and we're ready --- */
 152
 153   if (sigprocmask(SIG_SETMASK, &oss, 0)) {
 154     alarm(al);
 155     e = -1;
 156     goto done;
 157   }
 158
 159   /* --- Do it --- */
 160
 161   e = fcntl(fd, F_SETLKW, &fk);
 162
 163   /* --- Tidy up the mess I left --- */
 164
 165   left = alarm(0);
 166   if (al)
 167     alarm(al - d + left);
 168 done:
 169   sigaction(SIGALRM, &osa, 0);
 170   return (e);
 171 }
 172
 173 /*----- That's all, fokls -------------------------------------------------*/