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'''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 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=
==[[Ada]]==
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.
<lang ada>
protected type Semaphore (K : Positive) is
entry P;
procedure V;
private
Count : Natural := K;
end Mutex;
</lang>
The implementation of:
<lang ada>
protected body Semaphore is
entry P when Count > 0 is
begin
Count := Count - 1;
end P;
procedure V is
begin
Count := Count + 1;
end V;
end Semaphore;
</lang>
Use:
<lang ada>
declare
S : Semaphore (5);
begin
S.P; -- Acquire the semaphore
...
S.V; -- Release it
...
select
S.P; -- Wait no longer than 0.5s
or delay 0.5;
raise Timed_Out;
end select;
...
S.V; -- Release it
end;
</lang>
It is also possible to implement semaphore as a monitor task.
=={{header|C}}==
{{libheader|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)...
<lang c>
//
// Interface
//
typedef struct Sema *Sema;
Sema Sema_New (int init);
void Sema_P (Sema s);
void Sema_V (Sema s);
</lang>
... and the "Implementation" (the *.c file):
<lang c>
//
// 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);
}
s->value--;
pthread_mutex_unlock (s->mutex);
}
void Sema_V (Sema s) {
pthread_mutex_lock (s->mutex);
s->value++;
if (s->value == 1) {
pthread_cond_signal (s->cond);
}
pthread_mutex_unlock (s->mutex);
}
</lang>
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