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lockf_linux.c
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Authored by allanjude on Mar 14 2021, 1:54 PM.
/*
* Copyright (C) 1997 John D. Polstra. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY JOHN D. POLSTRA AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL JOHN D. POLSTRA OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#include <sys/types.h>
#include <sys/wait.h>
#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sysexits.h>
#include <unistd.h>
static int acquire_lock(const char *name, int flags);
static void cleanup(void);
static void killed(int sig);
static void timeout(int sig);
static void usage(void);
static void wait_for_lock(const char *name);
static const char *lockname;
static int lockfd = -1;
static int keep;
static volatile sig_atomic_t timed_out;
/*
* Execute an arbitrary command while holding a file lock.
*/
int
main(int argc, char **argv)
{
int ch, flags, silent, status, waitsec;
pid_t child;
silent = keep = 0;
flags = O_CREAT;
waitsec = -1; /* Infinite. */
while ((ch = getopt(argc, argv, "sknt:")) != -1) {
switch (ch) {
case 'k':
keep = 1;
break;
case 'n':
flags &= ~O_CREAT;
break;
case 's':
silent = 1;
break;
case 't':
{
char *endptr;
waitsec = strtol(optarg, &endptr, 0);
if (*optarg == '\0' || *endptr != '\0' || waitsec < 0)
errx(EX_USAGE,
"invalid timeout \"%s\"", optarg);
}
break;
default:
usage();
}
}
if (argc - optind < 2)
usage();
lockname = argv[optind++];
argc -= optind;
argv += optind;
if (waitsec > 0) { /* Set up a timeout. */
struct sigaction act;
act.sa_handler = timeout;
sigemptyset(&act.sa_mask);
act.sa_flags = 0; /* Note that we do not set SA_RESTART. */
sigaction(SIGALRM, &act, NULL);
alarm(waitsec);
}
/*
* If the "-k" option is not given, then we must not block when
* acquiring the lock. If we did, then the lock holder would
* unlink the file upon releasing the lock, and we would acquire
* a lock on a file with no directory entry. Then another
* process could come along and acquire the same lock. To avoid
* this problem, we separate out the actions of waiting for the
* lock to be available and of actually acquiring the lock.
*
* That approach produces behavior that is technically correct;
* however, it causes some performance & ordering problems for
* locks that have a lot of contention. First, it is unfair in
* the sense that a released lock isn't necessarily granted to
* the process that has been waiting the longest. A waiter may
* be starved out indefinitely. Second, it creates a thundering
* herd situation each time the lock is released.
*
* When the "-k" option is used, the unlink race no longer
* exists. In that case we can block while acquiring the lock,
* avoiding the separate step of waiting for the lock. This
* yields fairness and improved performance.
*/
lockfd = acquire_lock(lockname, flags | O_NONBLOCK);
while (lockfd == -1 && !timed_out && waitsec != 0) {
if (keep)
lockfd = acquire_lock(lockname, flags);
else {
wait_for_lock(lockname);
lockfd = acquire_lock(lockname, flags | O_NONBLOCK);
}
}
if (waitsec > 0)
alarm(0);
if (lockfd == -1) { /* We failed to acquire the lock. */
if (silent)
exit(EX_TEMPFAIL);
errx(EX_TEMPFAIL, "%s: already locked", lockname);
}
/* At this point, we own the lock. */
if (atexit(cleanup) == -1)
err(EX_OSERR, "atexit failed");
if ((child = fork()) == -1)
err(EX_OSERR, "cannot fork");
if (child == 0) { /* The child process. */
close(lockfd);
execvp(argv[0], argv);
warn("%s", argv[0]);
_exit(1);
}
/* This is the parent process. */
signal(SIGINT, SIG_IGN);
signal(SIGQUIT, SIG_IGN);
signal(SIGTERM, killed);
if (waitpid(child, &status, 0) == -1)
err(EX_OSERR, "waitpid failed");
return (WIFEXITED(status) ? WEXITSTATUS(status) : EX_SOFTWARE);
}
/*
* Try to acquire a lock on the given file, creating the file if
* necessary. The flags argument is O_NONBLOCK or 0, depending on
* whether we should wait for the lock. Returns an open file descriptor
* on success, or -1 on failure.
*/
static int
acquire_lock(const char *name, int flags)
{
int fd;
if ((fd = open(name, O_RDONLY|flags, 0666)) == -1) {
if (errno == EAGAIN || errno == EINTR)
return (-1);
err(EX_CANTCREAT, "cannot open %s", name);
}
if ((flock(fd, LOCK_EX | (flags & O_NONBLOCK ? LOCK_NB : 0))) == -1) {
if (errno == EAGAIN || errno == EINTR)
return (-1);
err(EX_CANTCREAT, "failed to acquire lock %s", name);
}
return (fd);
}
/*
* Remove the lock file.
*/
static void
cleanup(void)
{
if (keep)
flock(lockfd, LOCK_UN);
else
unlink(lockname);
}
/*
* Signal handler for SIGTERM. Cleans up the lock file, then re-raises
* the signal.
*/
static void
killed(int sig)
{
cleanup();
signal(sig, SIG_DFL);
if (kill(getpid(), sig) == -1)
err(EX_OSERR, "kill failed");
}
/*
* Signal handler for SIGALRM.
*/
static void
timeout(int sig)
{
timed_out = 1;
}
static void
usage(void)
{
fprintf(stderr,
"usage: lockf [-kns] [-t seconds] file command [arguments]\n");
exit(EX_USAGE);
}
/*
* Wait until it might be possible to acquire a lock on the given file.
* If the file does not exist, return immediately without creating it.
*/
static void
wait_for_lock(const char *name)
{
int fd;
if ((fd = open(name, O_RDONLY, 0666)) == -1) {
if (errno == ENOENT || errno == EINTR)
return;
err(EX_CANTCREAT, "cannot open %s", name);
}
if ((flock(fd, LOCK_EX)) == -1) {
if (errno == ENOENT || errno == EINTR)
return;
err(EX_CANTCREAT, "cannot lock %s", name);
}
close(fd);
}

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allanjude created this object in space S1 Global.