Table of Contents

• 1. How it works
• 2. How to use it
• 3. Predicates
• 4. Examples
• 5. Common Errors

How it works

Semaphore primitive implementation. Semaphores is a way of syncronize concurrent process. Each semaphore has a internal counter which can be incremented or decremented using sem_wait(Sem) and sem_post(Sem). The counter can never go below zero, if a process try a sem_wait(Sem) and find that the counter is zero, that process will wait until another process calls sem_post(Sem).

How to use it

Just clone the repository and include the module using the repo path:

:- use_module(/home/user/git/semaphores/src/semaphores).

Predicates

pred sem_open(+Name,+Value,-Sem) :: atm * int * semaphore Tries to open a semaphore named Name, if the semaphore does not exist a new semaphore is created with name Name value Value and returned in Sem.

pred sem_wait(+Sem) :: semaphore Decrements in one the value of the semaphore Sem, if the current value is 0, the process will stop till the increment of the value.

pred sem_post(+Sem) :: semaphore Increments in one the value of the semaphore Sem, If te current value is 0, then the new value is 1 and stopped process will try catch the semaphore.

pred sem_close(+Sem) :: semaphore Destroys the semaphore Sem.

Examples

This is a simple example of the semaphore usage with value 1. Two processes write Number times in a file, one process writes the number 1 and the other writes 2. In this test can be only one process writting at time, it means, at the end of the execution cannot be numbers interleaved.

*File: test_writing.pl *

:- module(test_writing, []).

:- use_module(semaphores).
:- use_module(library(strings)).

:- export(recursive_writing/2).
recursive_writing(0,_). 
recursive_writing(Iterations,Number) :-
        open('test.txt',append,Stream),
        number_codes(Number,StrNumber),
        write_string(Stream,StrNumber),
        nl(Stream),
        close(Stream),
        Iterations2 is Iterations-1,
        recursive_writing(Iterations2,Number).

:- export(main/1).
main([ARG1,ARG2]) :-
        atom_number(ARG1,Iterations),
        atom_number(ARG2,Number),
        sem_open(test,1,Sem),
        sem_wait(Sem),
        recursive_writing(Iterations,Number),
        sem_post(Sem).

File: test.pl

:- use_module(semaphores).
:- use_module(library(process)).

:- export(test_exclusive_writing/1).
test_exclusive_writing(Number) :-
        process_call(path(ciaoc),['test_writing'],[]),
        atom_number(Atom,Number),
        sem_open(test,1,Sem),
        process_call('test_writing',[Atom,'1'],[background(P1)]),
        process_call('test_writing',[Atom,'2'],[background(P2)]),
        process_join(P1),process_join(P2),
        sem_destroy(Sem).

Common Errors

The incorrect use of semaphores can result in several errors that are very difficult to detect.

In this case, execute a sem_post before a sem_wait may allow other proccesses execute a critical section simultaneously:

sem_open(semaphore,1,Sem),
sem_post(Sem),
%% CRITICAL SECTION
sem_wait(Sem).

Another common error is execute two sem_wait consecutively in a semaphore with value 1, the result is a deadlock and no process can continue executing:

sem_open(semaphore,1,Sem),
sem_wait(Sem),
%% CRITICAL SECTION
sem_wait(Sem).

Take care when are you closing the semaphore, maybe you close a semaphore at the end of a process but there are one or more processes using the same semaphore, this can result in serious errors:

Process 1

sem_open(semaphore,1,Sem),
sem_wait(Sem),
%% CRITICAL SECTION
sem_post(Sem),
sem_close(Sem).

Process 2

sem_open(semaphore,1,Sem),
sem_wait(Sem),
%% CRITICAL SECTION
sem_post(Sem),
sem_close(Sem).

The correct way is:

Process 1

sem_open(semaphore,1,Sem),
sem_wait(Sem),
%% CRITICAL SECTION
sem_post(Sem).

Process 2

sem_open(semaphore,1,Sem),
sem_wait(Sem),
%% CRITICAL SECTION
sem_post(Sem).

Main process

sem_open(semaphore,1,Sem),
process_call('process_1',[],[background(P1)]),
process_call('process_2',[],[background(P2)]),
process_join(P1), process_join(P2),
sem_close(Sem).

Remember that sem_open create a new semaphore or open the semaphore if it exists