From Rosetta Code

Semaphore is a synchronization object proposed by Edsger Dijkstra. A semaphore is characterized by a natural number k. A task may atomically increase or decrease k. When k reaches 0 the tasks attempting to decrease it are blocked. These are released in an unspecified order when other tasks increase k, one per increment.

The natural number k works like a count of available slots for resources. When you (a task) want to use something (an object, a file, any resource) that can only be used by a limited number of tasks (usually one, but possibly more), you see if there are available slots (check the value of k). If there are slots available (k > 0), you take one (decrement k). When you're done with the resource, you free your slot up (increment k). If there were no slots available when you checked (k = 0), you wait until one becomes available.

A semaphore is considered a low-level synchronization primitive. They are exposed to deadlocking, like in the problem of dining philosophers.

See also mutex, a variant of semaphore.

Sample implementations / APIs[edit]


Here is an implementation of a semaphore based on protected objects. The implementation provides operations P (seize) and V (release), these names are usually used with semaphores.

protected type Semaphore (K : Positive) is
entry P;
procedure V;
Count : Natural := K;
end Mutex;

The implementation of:

protected body Semaphore is
entry P when Count > 0 is
Count := Count - 1;
end P;
procedure V is
Count := Count + 1;
end V;
end Semaphore;


S : Semaphore (5);
S.P; -- Acquire the semaphore
S.V; -- Release it
S.P; -- Wait no longer than 0.5s
or delay 0.5;
raise Timed_Out;
end select;
S.V; -- Release it

It is also possible to implement semaphore as a monitor task.


Library: pthread

Here is an example of counting semaphores in C, using the "pthread" library. To make the code more readable, no error checks are made. A productive version of this implementation should check all the return values from the various function calls!

The example is divided into two parts: the "Interface" (usually the content of a *.h file)...

// Interface
typedef struct Sema *Sema;
Sema Sema_New (int init);
void Sema_P (Sema s);
void Sema_V (Sema s);

... and the "Implementation" (the *.c file):

// Implementation
#include <stdlib.h>
#include <pthread.h>
struct Sema {
int value;
pthread_mutex_t *mutex;
pthread_cond_t *cond;
Sema Sema_New (int init) {
Sema s;
s = malloc (sizeof (*s));
s->value = init;
s->mutex = malloc (sizeof (*(s->mutex)));
s->cond = malloc (sizeof (*(s->cond)));
pthread_mutex_init (s->mutex, NULL);
pthread_cond_init (s->cond, NULL);
return s;
void Sema_P (Sema s) {
pthread_mutex_lock (s->mutex);
while (s->value == 0) {
pthread_cond_wait (s->cond, s->mutex);
pthread_mutex_unlock (s->mutex);
void Sema_V (Sema s) {
pthread_mutex_lock (s->mutex);
if (s->value == 1) {
pthread_cond_signal (s->cond);
pthread_mutex_unlock (s->mutex);