2 * Copyright (C) 1997 John D. Polstra. All rights reserved.
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions
7 * 1. Redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer.
9 * 2. Redistributions in binary form must reproduce the above copyright
10 * notice, this list of conditions and the following disclaimer in the
11 * documentation and/or other materials provided with the distribution.
13 * THIS SOFTWARE IS PROVIDED BY JOHN D. POLSTRA AND CONTRIBUTORS ``AS IS'' AND
14 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
15 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
16 * ARE DISCLAIMED. IN NO EVENT SHALL JOHN D. POLSTRA OR CONTRIBUTORS BE LIABLE
17 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
18 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
19 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
20 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
21 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
22 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 #include <sys/cdefs.h>
27 __FBSDID("$FreeBSD$");
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);
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, silent, status, waitsec;
63 waitsec = -1; /* Infinite. */
64 while ((ch = getopt(argc, argv, "skt:")) != -1) {
75 waitsec = strtol(optarg, &endptr, 0);
76 if (*optarg == '\0' || *endptr != '\0' || waitsec < 0)
78 "invalid timeout \"%s\"", optarg);
85 if (argc - optind < 2)
87 lockname = argv[optind++];
90 if (waitsec > 0) { /* Set up a timeout. */
93 act.sa_handler = timeout;
94 sigemptyset(&act.sa_mask);
95 act.sa_flags = 0; /* Note that we do not set SA_RESTART. */
96 sigaction(SIGALRM, &act, NULL);
100 * If the "-k" option is not given, then we must not block when
101 * acquiring the lock. If we did, then the lock holder would
102 * unlink the file upon releasing the lock, and we would acquire
103 * a lock on a file with no directory entry. Then another
104 * process could come along and acquire the same lock. To avoid
105 * this problem, we separate out the actions of waiting for the
106 * lock to be available and of actually acquiring the lock.
108 * That approach produces behavior that is technically correct;
109 * however, it causes some performance & ordering problems for
110 * locks that have a lot of contention. First, it is unfair in
111 * the sense that a released lock isn't necessarily granted to
112 * the process that has been waiting the longest. A waiter may
113 * be starved out indefinitely. Second, it creates a thundering
114 * herd situation each time the lock is released.
116 * When the "-k" option is used, the unlink race no longer
117 * exists. In that case we can block while acquiring the lock,
118 * avoiding the separate step of waiting for the lock. This
119 * yields fairness and improved performance.
121 lockfd = acquire_lock(lockname, O_NONBLOCK);
122 while (lockfd == -1 && !timed_out && waitsec != 0) {
124 lockfd = acquire_lock(lockname, 0);
126 wait_for_lock(lockname);
127 lockfd = acquire_lock(lockname, O_NONBLOCK);
132 if (lockfd == -1) { /* We failed to acquire the lock. */
135 errx(EX_TEMPFAIL, "%s: already locked", lockname);
137 /* At this point, we own the lock. */
138 if (atexit(cleanup) == -1)
139 err(EX_OSERR, "atexit failed");
140 if ((child = fork()) == -1)
141 err(EX_OSERR, "cannot fork");
142 if (child == 0) { /* The child process. */
144 execvp(argv[0], argv);
148 /* This is the parent process. */
149 signal(SIGINT, SIG_IGN);
150 signal(SIGQUIT, SIG_IGN);
151 signal(SIGTERM, killed);
152 if (waitpid(child, &status, 0) == -1)
153 err(EX_OSERR, "waitpid failed");
154 return (WIFEXITED(status) ? WEXITSTATUS(status) : EX_SOFTWARE);
158 * Try to acquire a lock on the given file, creating the file if
159 * necessary. The flags argument is O_NONBLOCK or 0, depending on
160 * whether we should wait for the lock. Returns an open file descriptor
161 * on success, or -1 on failure.
164 acquire_lock(const char *name, int flags)
168 if ((fd = open(name, O_RDONLY|O_CREAT|O_EXLOCK|flags, 0666)) == -1) {
169 if (errno == EAGAIN || errno == EINTR)
171 err(EX_CANTCREAT, "cannot open %s", name);
177 * Remove the lock file.
184 flock(lockfd, LOCK_UN);
190 * Signal handler for SIGTERM. Cleans up the lock file, then re-raises
198 signal(sig, SIG_DFL);
199 if (kill(getpid(), sig) == -1)
200 err(EX_OSERR, "kill failed");
204 * Signal handler for SIGALRM.
207 timeout(int sig __unused)
218 "usage: lockf [-ks] [-t seconds] file command [arguments]\n");
223 * Wait until it might be possible to acquire a lock on the given file.
224 * If the file does not exist, return immediately without creating it.
227 wait_for_lock(const char *name)
231 if ((fd = open(name, O_RDONLY|O_EXLOCK, 0666)) == -1) {
232 if (errno == ENOENT || errno == EINTR)
234 err(EX_CANTCREAT, "cannot open %s", name);