New example on POSIX semaphores.

Author: Jan Pechanec

semaphores/posix-sem-fixed-race.c

@@ -0,0 +1,138 @@
+/*
+ * See ../race/race.c and ../race/sem-fixed-race.c for more information.
+ *
+ * This version uses POSIX semaphores for synchronization.  After you run this
+ * and kill it with Ctrl-C you see some statistics as to how many loops were
+ * run.  Compare with implementations mentioned above.  You will POSIX
+ * semaphores are an order of magnitude more efficient than Sys V semaphores.
+ *
+ * You must link with -pthread on Linux.
+ *
+ * (c) jp@devnull.cz
+ */
+
+#define	_XOPEN_SOURCE	700
+
+#include <assert.h>
+#include <err.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <semaphore.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+sem_t *sem;
+int fd, parent;
+char *addr = NULL;
+unsigned long loops_done;
+unsigned int races_detected;
+volatile sig_atomic_t run = 1;
+
+static void
+print_stats(void)
+{
+	printf("\nstats: inconsistencies %u out of %lu\n", races_detected,
+	    loops_done);
+}
+
+void
+finish(int sig)
+{
+	run = 0;
+}
+
+static void
+cleanup(void)
+{
+	if (parent)
+		sem_destroy(sem);
+
+	munmap(addr, 2);
+	close(fd);
+}
+
+int
+main(int argc, char **argv)
+{
+	char c = 0;
+	int dbg = 0;
+	pid_t pid, ret_pid;
+	struct sigaction act = { 0 };
+
+	if (argc == 1)
+		printf("Run with any argument to see some debugging info.\n");
+	else
+		dbg = 1;
+
+	/* Get a semaphore. */
+	sem = sem_open("/mysem", O_CREAT, S_IRWXU, 1);
+	if (sem == SEM_FAILED)
+		err(1, "sem_open");
+
+	act.sa_handler = finish;
+	sigaction(SIGINT, &act, NULL);
+
+	if ((fd = open("test.dat", O_CREAT | O_RDWR | O_TRUNC, 0666)) == -1)
+		err(1, "open");
+
+	/* extend the file to 2 bytes */
+	write(fd, &c, 1);
+	write(fd, &c, 1);
+
+	addr = mmap(0, 2, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+	if (addr == MAP_FAILED)
+		err(1, "mmap");
+
+	if ((pid = fork()) == -1)
+		err(1, "fork");
+
+	/* Child */
+	if (pid == 0) {
+		while (run) {
+			sem_wait(sem);
+			if (addr[0] != addr[1]) {
+				if (dbg) {
+					fprintf(stderr, "[child (%d/%d)] ",
+					    addr[0], addr[1]);
+				}
+				++races_detected;
+			}
+			addr[0] = addr[1] = 2;
+			sem_post(sem);
+			++loops_done;
+		}
+		print_stats();
+		cleanup();
+		return (0);
+	}
+
+	/* Parent only. */
+	assert(pid > 0);
+	parent = 1;
+	while (run) {
+		sem_wait(sem);
+		if (addr[0] != addr[1]) {
+			if (dbg) {
+				fprintf(stderr, "[PARENT (%d/%d)] ",
+				    addr[0], addr[1]);
+			}
+			++races_detected;
+		}
+		addr[0] = addr[1] = 1;
+		sem_post(sem);
+		++loops_done;
+	}
+	ret_pid = waitpid(pid, NULL, 0);
+	assert(ret_pid == pid);
+
+	print_stats();
+	if (sem_destroy(sem) == -1)
+		warn("sem_destroy");
+	cleanup();
+
+	return (0);
+}