initial import

from
http://www.informit.com/store/programming-with-posix-threads-9780201633924

semaphore_wait.c

@@ -0,0 +1,82 @@
+/*
+ * semaphore_wait.c
+ *
+ * Demonstrate use of semaphores for synchronization.
+ */
+#include <sys/types.h>
+#include <unistd.h>
+#include <pthread.h>
+#include <semaphore.h>
+#include <signal.h>
+#include "errors.h"
+
+sem_t semaphore;
+
+/*
+ * Thread start routine to wait on a semaphore.
+ */
+void *sem_waiter (void *arg)
+{
+    long num = (long)arg;
+
+    printf ("Thread %d waiting\n", num);
+    if (sem_wait (&semaphore) == -1)
+        errno_abort ("Wait on semaphore");
+    printf ("Thread %d resuming\n", num);
+    return NULL;
+}
+
+int main (int argc, char *argv[])
+{
+    int thread_count;
+    pthread_t sem_waiters[5];
+    int status;
+
+#if !defined(_POSIX_SEMAPHORES)
+    printf ("This system does not support POSIX semaphores\n");
+    return -1;
+#else
+
+    if (sem_init (&semaphore, 0, 0) == -1)
+        errno_abort ("Init semaphore");
+
+    /*
+     * Create 5 threads to wait concurrently on the semaphore.
+     */
+    for (thread_count = 0; thread_count < 5; thread_count++) {
+        status = pthread_create (
+            &sem_waiters[thread_count], NULL,
+            sem_waiter, (void*)thread_count);
+        if (status != 0)
+            err_abort (status, "Create thread");
+    }
+
+    sleep (2);
+
+    /*
+     * "Broadcast" the semaphore by repeatedly posting until the
+     * count of waiters goes to 0.
+     */
+    while (1) {
+        int sem_value;
+
+        if (sem_getvalue (&semaphore, &sem_value) == -1)
+            errno_abort ("Get semaphore value");
+        if (sem_value >= 0)
+            break;
+        printf ("Posting from main: %d\n", sem_value);
+        if (sem_post (&semaphore) == -1)
+            errno_abort ("Post semaphore");
+    }
+
+    /*
+     * Wait for all threads to complete.
+     */
+    for (thread_count = 0; thread_count < 5; thread_count++) {
+        status = pthread_join (sem_waiters[thread_count], NULL);
+        if (status != 0)
+            err_abort (status, "Join thread");
+    }
+    return 0;
+#endif
+}