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NSPR Reference

IPC Semaphores (Chapter 30)

This chapter describes the NSPR API for using interprocess communication semaphores.

NSPR provides an interprocess communication mechanism using a counting semaphore model similar to that which is provided in Unix and Windows platforms.

See also Named Shared Memory (Chapter 28).

IPC Semaphore Functions

PR_OpenSemaphore

Creates or opens a named semaphore with the specified name

Syntax
#include <pripcsem.h>

#define PR_SEM_CREATE 0x1 /* create if not exist */

#define PR_SEM_EXCL 0x2 /* fail if already exists */

NSPR_API(PRSem *) PR_OpenSemaphore(
   const char *name,
   PRIntn flags,
   PRIntn mode,
   PRUintn value
);
Parameters

The function has the following parameters:

name
The name to be given the semaphore.
flags
How to create or open the semaphore.
mode
Unix style file mode to be used when creating the semaphore.
value
The initial value assigned to the semaphore.
Returns

A pointer to a PRSem structure or NULL on error.

Description

If the named semaphore doesn't exist and the PR_SEM_CREATE flag is specified, the named semaphore is created. The created semaphore needs to be removed from the system with a PR_DeleteSemaphore call.

If PR_SEM_CREATE is specified, the third argument is the access permission bits of the new semaphore (same interpretation as the mode argument to PR_Open) and the fourth argument is the initial value of the new semaphore. If PR_SEM_CREATE is not specified, the third and fourth arguments are ignored.

PR_WaitSemaphore

Returns the value of the environment variable.

Syntax
#include <pripcsem.h>

NSPR_API(PRStatus) PR_WaitSemaphore(PRSem *sem);
Parameters

The function has the following parameter:

sem
A pointer to a PRSem structure returned from a call to PR_OpenSemaphore.
Returns

PRStatus

Description

PR_WaitSemaphore tests the value of the semaphore. If the value of the semaphore is > 0, the value of the semaphore is decremented and the function returns. If the value of the semaphore is 0, the function blocks until the value becomes > 0, then the semaphore is decremented and the function returns.

The "test and decrement" operation is performed atomically.

PR_PostSemaphore

Increments the value of a specified semaphore.

Syntax
#include <pripcsem.h>

NSPR_API(PRStatus) PR_PostSemaphore(PRSem *sem);
Parameters

The function has the following parameter:

sem
A pointer to a PRSem structure returned from a call to PR_OpenSemaphore.
Returns

PRStatus

PR_CloseSemaphore

Closes a specified semaphore.

Syntax
#include <pripcsem.h>

NSPR_API(PRStatus) PR_CloseSemaphore(PRSem *sem);
Parameters

The function has the following parameter:

sem
A pointer to a PRSem structure returned from a call to PR_OpenSemaphore.
Returns

PRStatus

PR_DeleteSemaphore

Removes a semaphore specified by name from the system.

Syntax
#include <pripcsem.h>

NSPR_API(PRStatus) PR_DeleteSemaphore(const char *name);
Parameters

The function has the following parameter:

name
The name of a semaphore that was previously created via a call to PR_OpenSemaphore.
Returns

PRStatus