POSIX semaphore demo

Author: Vladimir Kotal

semaphores/sem.c

@@ -0,0 +1,88 @@
+/*
+ * POSIX semaphores demonstration. Note especially how sem_wait() should be
+ * treated with regards to interruptions (like signals in this case or I/O
+ * delivery).
+ *
+ * Run with and without argument to see the difference.
+ *
+ * vlada@devnull.cz, 2013
+ */
+
+#include <stdio.h>
+#include <signal.h>
+#include <pthread.h>
+#include <errno.h>
+#include <semaphore.h>
+
+sem_t sem;
+
+#define	PUTCHARF(x)	putc(x, stdout); fflush(stdout);
+
+int
+sem_wait_nointr(sem_t *sem) {
+	while (sem_wait(sem) != 0) {
+		if (errno != EINTR) {
+			return (-1);
+		} else {
+			PUTCHARF('!');
+		}
+	}
+
+	return (0);
+}
+
+void *
+usher(void *x)
+{
+	while (1) {
+		sem_post(&sem);
+		PUTCHARF('-');
+		poll(NULL, 0, 100);
+	}
+}
+
+void
+alarm_handler(int s)
+{
+	alarm(1);
+}
+
+int
+main(int argc, char *argv[])
+{
+	pthread_t t;
+	int nointr = 0;
+	struct sigaction act;
+
+	if (argc == 2)
+		nointr = 1;
+
+	/* Allow to enter 5 times. */
+	if (sem_init(&sem, 0, 5) == -1)
+		err(1, "sem_init");
+
+	pthread_create(&t, NULL, usher, NULL);
+
+	act.sa_handler = alarm_handler;
+	sigemptyset(&act.sa_mask);
+	act.sa_flags = 0;
+
+	sigaction(SIGALRM, &act, NULL);
+	alarm(1);
+
+	while (1) {
+		if (nointr) {
+			sem_wait_nointr(&sem);
+		} else {
+			if (sem_wait(&sem) != 0)
+				err(1, "sem_wait");
+		}
+		PUTCHARF('_');
+		/* Run more frequently than usher. */
+		poll(NULL, 0, 10);
+	}
+
+	sem_destroy(&sem);
+
+	return (0);
+}