* File locker
*
* Usage:
- * with-lock-ex -<mode> <lockfile> <command> <args>...
+ * with-lock-ex -<mode> [-t <secs>] <lockfile> <command> <args>...
* with-lock-ex -l <lockfile>
*
* modes are
* or "write <pid>"; lockfile opened for reading;
* no command may be specified)
*
+ * If -t is specified, then with-lock-ex will wait for up to <secs>
+ * seconds to acquire the lock, and then fail or silently do nothing
+ * (depending on whether -f or -q is specified). You cannot specify
+ * a timeout for modes l or w.
+ *
* with-lock-ex will open and lock the lockfile for writing and
* then feed the remainder of its arguments to exec(2); when
* that process terminates the fd will be closed and the file
#include <unistd.h>
#include <string.h>
#include <sys/stat.h>
+#include <limits.h>
+#include <signal.h>
+#include <sys/time.h>
static const char *cmd;
static int mode;
+/* This signal handler uses unsafe functions, so MUST NOT be callable
+ * during an unsafe function, as that is Undefined Behaviour
+ */
+static void alrm_handler(int signum) {
+ if (mode=='q') {
+ exit(0);
+ } else {
+ fprintf(stderr,
+ "with-lock-ex %s: timer expired while trying to acquire lock\n",
+ cmd);
+ exit(255);
+ }
+}
+
int main(int argc, char **argv) {
- int fd, um, c;
+ int fd, um, c, r;
struct stat stab, fstab;
- long cloexec;
+ long cloexec, secs=0;
struct flock fl;
+ char *endptr;
+ sigset_t sigs, oldsigs;
+ struct sigaction siga;
+ struct itimerval itv;
mode= 'x';
while ((c= getopt(argc,argv,"+wfqlt:")) != -1) {
if (mode != 'x') badusage();
mode= c;
break;
+ case 't':
+ errno = 0;
+ secs = strtol(optarg, &endptr, 0);
+ if (*endptr || endptr==optarg || errno==ERANGE)
+ fail("parsing timeout value");
+ if (secs < 0) {
+ fprintf(stderr,"timeout value must be >=0\n");
+ exit(255);
+ }
+ break;
default:
badusage();
}
}
+ if (secs && (mode=='l' || mode=='w')) {
+ fputs("-t only allowed with -q or -f.\n", stderr);
+ exit(255);
+ }
+
argv += optind-1; argc -= optind-1;
if (argc < 2) badusage();
+ if (secs) {
+ if (sigemptyset(&sigs)) fail("Initialising signal set");
+ if (sigaddset(&sigs,SIGALRM)) fail("Adding SIGALRM to signal set");
+ if (sigprocmask(SIG_BLOCK,&sigs,&oldsigs)) fail("Blocking SIGALRM");
+ memset(&siga,0,sizeof(siga));
+ siga.sa_handler=alrm_handler;
+ if (sigaction(SIGALRM,&siga,NULL)) fail("Installing SIGALRM handler");
+ memset(&itv,0,sizeof(itv));
+ itv.it_value.tv_sec=secs;
+ if (setitimer(ITIMER_REAL,&itv,NULL)) fail("Setting timer");
+ }
+
cmd= argv[2];
um= umask(0777); if (um==-1) fail("find umask");
if (umask(um)==-1) fail("reset umask");
fl.l_whence= SEEK_SET;
fl.l_start= 0;
fl.l_len= mode=='l' ? 0 : 1;
- if (fcntl(fd,
+ if (secs) sigprocmask(SIG_UNBLOCK,&sigs,NULL);
+ r = fcntl(fd,
mode=='l' ? F_GETLK :
- mode=='w' ? F_SETLKW :
+ mode=='w' || secs > 0 ? F_SETLKW :
F_SETLK,
- &fl) != -1) break;
+ &fl);
+ if (secs) sigprocmask(SIG_BLOCK,&sigs,NULL);
+ if (!r) {
+ break;
+ }
if (mode=='q' &&
(errno == EAGAIN || errno == EWOULDBLOCK || errno == EBUSY))
exit(0);
close(fd);
}
+ if (secs) {
+ itv.it_value.tv_sec=0;
+ if (setitimer(ITIMER_REAL,&itv,NULL)) fail("Clearing timer");
+ sigprocmask(SIG_SETMASK,&oldsigs,NULL);
+ siga.sa_handler=SIG_DFL;
+ sigaction(SIGALRM,&siga,NULL);
+ }
+
cloexec= fcntl(fd, F_GETFD); if (cloexec==-1) fail("fcntl F_GETFD");
cloexec &= ~1;
if (fcntl(fd, F_SETFD, cloexec)==-1) fail("fcntl F_SETFD");