2 * SPDX-License-Identifier: BSD-2-Clause
4 * Copyright (C) 1997 John D. Polstra. All rights reserved.
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
9 * 1. Redistributions of source code must retain the above copyright
10 * notice, this list of conditions and the following disclaimer.
11 * 2. Redistributions in binary form must reproduce the above copyright
12 * notice, this list of conditions and the following disclaimer in the
13 * documentation and/or other materials provided with the distribution.
15 * THIS SOFTWARE IS PROVIDED BY JOHN D. POLSTRA AND CONTRIBUTORS ``AS IS'' AND
16 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
17 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
18 * ARE DISCLAIMED. IN NO EVENT SHALL JOHN D. POLSTRA OR CONTRIBUTORS BE LIABLE
19 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
20 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
21 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
22 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
23 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
24 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
28 #include <sys/cdefs.h>
29 #include <sys/types.h>
41 static int acquire_lock(const char *name, int flags);
42 static void cleanup(void);
43 static void killed(int sig);
44 static void timeout(int sig);
45 static void usage(void) __dead2;
46 static void wait_for_lock(const char *name);
48 static const char *lockname;
49 static int lockfd = -1;
51 static volatile sig_atomic_t timed_out;
54 * Execute an arbitrary command while holding a file lock.
57 main(int argc, char **argv)
59 int ch, flags, silent, status, waitsec;
63 flags = O_CREAT | O_RDONLY;
64 waitsec = -1; /* Infinite. */
65 while ((ch = getopt(argc, argv, "sknt:w")) != -1) {
79 waitsec = strtol(optarg, &endptr, 0);
80 if (*optarg == '\0' || *endptr != '\0' || waitsec < 0)
82 "invalid timeout \"%s\"", optarg);
86 flags = (flags & ~O_RDONLY) | O_WRONLY;
92 if (argc - optind < 2)
94 lockname = argv[optind++];
97 if (waitsec > 0) { /* Set up a timeout. */
100 act.sa_handler = timeout;
101 sigemptyset(&act.sa_mask);
102 act.sa_flags = 0; /* Note that we do not set SA_RESTART. */
103 sigaction(SIGALRM, &act, NULL);
107 * If the "-k" option is not given, then we must not block when
108 * acquiring the lock. If we did, then the lock holder would
109 * unlink the file upon releasing the lock, and we would acquire
110 * a lock on a file with no directory entry. Then another
111 * process could come along and acquire the same lock. To avoid
112 * this problem, we separate out the actions of waiting for the
113 * lock to be available and of actually acquiring the lock.
115 * That approach produces behavior that is technically correct;
116 * however, it causes some performance & ordering problems for
117 * locks that have a lot of contention. First, it is unfair in
118 * the sense that a released lock isn't necessarily granted to
119 * the process that has been waiting the longest. A waiter may
120 * be starved out indefinitely. Second, it creates a thundering
121 * herd situation each time the lock is released.
123 * When the "-k" option is used, the unlink race no longer
124 * exists. In that case we can block while acquiring the lock,
125 * avoiding the separate step of waiting for the lock. This
126 * yields fairness and improved performance.
128 lockfd = acquire_lock(lockname, flags | O_NONBLOCK);
129 while (lockfd == -1 && !timed_out && waitsec != 0) {
131 lockfd = acquire_lock(lockname, flags);
133 wait_for_lock(lockname);
134 lockfd = acquire_lock(lockname, flags | O_NONBLOCK);
139 if (lockfd == -1) { /* We failed to acquire the lock. */
142 errx(EX_TEMPFAIL, "%s: already locked", lockname);
144 /* At this point, we own the lock. */
145 if (atexit(cleanup) == -1)
146 err(EX_OSERR, "atexit failed");
147 if ((child = fork()) == -1)
148 err(EX_OSERR, "cannot fork");
149 if (child == 0) { /* The child process. */
151 execvp(argv[0], argv);
155 /* This is the parent process. */
156 signal(SIGINT, SIG_IGN);
157 signal(SIGQUIT, SIG_IGN);
158 signal(SIGTERM, killed);
159 if (waitpid(child, &status, 0) == -1)
160 err(EX_OSERR, "waitpid failed");
161 return (WIFEXITED(status) ? WEXITSTATUS(status) : EX_SOFTWARE);
165 * Try to acquire a lock on the given file, creating the file if
166 * necessary. The flags argument is O_NONBLOCK or 0, depending on
167 * whether we should wait for the lock. Returns an open file descriptor
168 * on success, or -1 on failure.
171 acquire_lock(const char *name, int flags)
175 if ((fd = open(name, O_EXLOCK|flags, 0666)) == -1) {
176 if (errno == EAGAIN || errno == EINTR)
178 else if (errno == ENOENT && (flags & O_CREAT) == 0)
179 err(EX_UNAVAILABLE, "%s", name);
180 err(EX_CANTCREAT, "cannot open %s", name);
186 * Remove the lock file.
193 flock(lockfd, LOCK_UN);
199 * Signal handler for SIGTERM. Cleans up the lock file, then re-raises
207 signal(sig, SIG_DFL);
208 if (kill(getpid(), sig) == -1)
209 err(EX_OSERR, "kill failed");
213 * Signal handler for SIGALRM.
216 timeout(int sig __unused)
227 "usage: lockf [-kns] [-t seconds] file command [arguments]\n");
232 * Wait until it might be possible to acquire a lock on the given file.
233 * If the file does not exist, return immediately without creating it.
236 wait_for_lock(const char *name)
240 if ((fd = open(name, O_RDONLY|O_EXLOCK, 0666)) == -1) {
241 if (errno == ENOENT || errno == EINTR)
243 err(EX_CANTCREAT, "cannot open %s", name);