92 lines
2.2 KiB
C
92 lines
2.2 KiB
C
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/* This file is part of the sample code and exercises
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* used by the class "Advanced Programming in the UNIX
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* Environment" taught by Jan Schaumann
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* <jschauma@netmeister.org> at Stevens Institute of
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* Technology.
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*
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* This file is in the public domain.
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*
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* You don't have to, but if you feel like
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* acknowledging where you got this code, you may
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* reference me by name, email address, or point
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* people to the course website:
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* https://stevens.netmeister.org/631/
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*/
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/* This program illustrates the use of flock(2). Run two concurrent
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* processes to show how a shared lock can be applied by both, but only
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* one process can upgrade to an exclusive lock.
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*
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* Illustrate blocking and non-blocking modes: in blocking mode, we
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* are avoiding the danger of a deadlock by the fact that upgrading a read
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* lock releases that lock, allowing the second process to eventually get
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* the exlusive lock. In non-blocking mode, we avoid a deadlock by
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* explicitly releasing all locks and aborting after 10 tries.
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*/
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#include <sys/file.h>
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#include <errno.h>
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#include <fcntl.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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void
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progress() {
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for (int i=0; i < 10; i++) {
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/* Using write(2), because printf(3) is line-buffered. */
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(void)write(STDOUT_FILENO, ".", 1);
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sleep(1);
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}
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}
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int
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main(int argc, char **argv) {
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int i, fd, flags;
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(void)argv;
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if ((fd = open("/tmp/1", O_CREAT|O_RDWR, 0644)) == -1) {
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(void)fprintf(stderr, "Unable to open /tmp/1: %s", strerror(errno));
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exit(EXIT_FAILURE);
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/* NOTREACHED */
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}
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if (flock(fd, LOCK_SH) < 0) {
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perror("flocking");
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exit(EXIT_FAILURE);
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/* NOTREACHED */
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}
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(void)printf("Shared lock established - sleeping for 10 seconds.\n");
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progress();
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(void)printf("\nNow trying to get an exclusive lock.\n");
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flags = LOCK_EX;
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if (argc > 1) {
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flags |= LOCK_NB;
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}
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for (i=0; i < 10; i++) {
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if (flock(fd, flags) < 0) {
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(void)printf("Unable to get an exclusive lock.\n");
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if (i==9) {
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(void)printf("Giving up all locks.\n");
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flock(fd, LOCK_UN);
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exit(EXIT_FAILURE);
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/* NOTREACHED */
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}
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sleep(1);
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} else {
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(void)printf("Exclusive lock established.\n");
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break;
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}
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}
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progress();
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(void)printf("\n");
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(void)close(fd);
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exit(EXIT_SUCCESS);
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}
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