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[mLib] / lock.c
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1/* -*-c-*-
2 *
3 * $Id: lock.c,v 1.6 2004/04/08 01:36:13 mdw Exp $
4 *
5 * Simplified POSIX locking interface
6 *
7 * (c) 1997 Straylight/Edgeware
8 */
9
10/*----- Licensing notice --------------------------------------------------*
11 *
12 * This file is part of the mLib utilities library.
13 *
14 * mLib is free software; you can redistribute it and/or modify
15 * it under the terms of the GNU Library General Public License as
16 * published by the Free Software Foundation; either version 2 of the
17 * License, or (at your option) any later version.
18 *
19 * mLib is distributed in the hope that it will be useful,
20 * but WITHOUT ANY WARRANTY; without even the implied warranty of
21 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
22 * GNU Library General Public License for more details.
23 *
24 * You should have received a copy of the GNU Library General Public
25 * License along with mLib; if not, write to the Free
26 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
27 * MA 02111-1307, USA.
28 */
29
30/*----- Header files ------------------------------------------------------*/
31
32#include <errno.h>
33#include <setjmp.h>
34#include <signal.h>
35#include <stdio.h>
36#include <stdlib.h>
37#include <string.h>
38#include <time.h>
39
40#include <sys/types.h>
41#include <unistd.h>
42#include <fcntl.h>
43
44#include "lock.h"
45
46/*----- Tunable constants -------------------------------------------------*/
47
48#define LOCK_TIMEOUT 10 /* Maximum time in seconds to wait */
49
50/*----- Static variables --------------------------------------------------*/
51
52static jmp_buf jmp; /* Jump here to interrup @fcntl@ */
53
54/*----- Main code ---------------------------------------------------------*/
55
56/* --- @sigalrm@ --- *
57 *
58 * Arguments: @int sig@ = signal number
59 *
60 * Returns: ---
61 *
62 * Use: Makes sure that a @SIGALRM@ signal interrupts system calls.
63 */
64
65static void sigalrm(int sig) { longjmp(jmp, 1); }
66
67/* --- @lock_file@ --- *
68 *
69 * Arguments: @int fd@ = file descriptor to lock
70 * @unsigned how@ = type of lock required
71 *
72 * Returns: 0 if OK, -1 if it failed.
73 *
74 * Use: Acquires a lock on the given file. The value @how@
75 * specifies the type of lock to acquire: @LOCK_EXCL@ gets
76 * an exclusive (write) lock; @LOCK_NONEXCL@ gets a non-
77 * exclusive (read) lock and @LOCK_UNLOCK@ releases any locks.
78 * Acquiring a lock gets timed out after a while with an
79 * error.
80 */
81
82int lock_file(int fd, unsigned how)
83{
84 struct flock fk;
85 struct sigaction sa, osa;
86 sigset_t ss, oss;
87 int e;
88 int al, d, left;
89
90 /* --- Fill in the easy bits --- */
91
92 fk.l_whence = SEEK_SET;
93 fk.l_start = 0;
94 fk.l_len = 0;
95
96 /* --- Unlocking is really easy --- */
97
98 if (how == LOCK_UNLOCK) {
99 fk.l_type = F_UNLCK;
100 return (fcntl(fd, F_SETLK, &fk));
101 }
102
103 /* --- Decide how to do the locking --- */
104
105 if (how == LOCK_EXCL)
106 fk.l_type = F_WRLCK;
107 else if (how == LOCK_NONEXCL)
108 fk.l_type = F_RDLCK;
109 else {
110 errno = EINVAL;
111 return (-1);
112 }
113
114 /* --- Block @SIGALRM@ for a while --- *
115 *
116 * I don't want stray alarms going off while I'm busy here.
117 */
118
119 sigemptyset(&ss);
120 sigaddset(&ss, SIGALRM);
121 if (sigprocmask(SIG_BLOCK, &ss, &oss))
122 return (-1);
123
124 /* --- Set up the signal handler --- */
125
126 sa.sa_handler = sigalrm;
127 sa.sa_flags = 0;
128#ifdef SA_INTERRUPT
129 sa.sa_flags |= SA_INTERRUPT;
130#endif
131 sigemptyset(&sa.sa_mask);
132 if (sigaction(SIGALRM, &sa, &osa))
133 return (-1);
134
135 /* --- Set up the alarm, remembering when it's meant to go off --- */
136
137 al = alarm(0);
138 if (al && al < LOCK_TIMEOUT)
139 d = al;
140 else
141 d = LOCK_TIMEOUT;
142 alarm(d);
143
144 /* --- Set up the return context for the signal handler --- */
145
146 if (setjmp(jmp)) {
147 sigprocmask(SIG_SETMASK, &oss, 0);
148 errno = EINTR;
149 e = -1;
150 goto done;
151 }
152
153 /* --- Unblock the signal and we're ready --- */
154
155 if (sigprocmask(SIG_SETMASK, &oss, 0)) {
156 alarm(al);
157 e = -1;
158 goto done;
159 }
160
161 /* --- Do it --- */
162
163 e = fcntl(fd, F_SETLKW, &fk);
164
165 /* --- Tidy up the mess I left --- */
166
167 left = alarm(0);
168 if (al)
169 alarm(al - d + left);
170done:
171 sigaction(SIGALRM, &osa, 0);
172 return (e);
173}
174
175/*----- That's all, fokls -------------------------------------------------*/