Initializing a Semaphore

Use sem_init(3RT) to initialize the unnamed semaphore variable pointed to by sem to value amount.

sem_init Syntax

int sem_init(sem_t *sem, int pshared, unsigned int value);

#include <semaphore.h>

sem_t sem;
int pshared;
int ret;
int value;

/* initialize a private semaphore */
pshared = 0;
value = 1;
ret = sem_init(&sem, pshared, value); 

If the value of pshared is zero, then the semaphore cannot be shared between processes. If the value of pshared is nonzero, then the semaphore can be shared between processes.

Multiple threads must not initialize the same semaphore.

A semaphore must not be reinitialized while other threads might be using the semaphore.

Initializing Semaphores With Intraprocess Scope

When pshared is 0, the semaphore can be used by all the threads in this process only.

#include <semaphore.h>

sem_t sem;
int ret;
int count = 4;

/* to be used within this process only */
ret = sem_init(&sem, 0, count); 

Initializing Semaphores With Interprocess Scope

When pshared is nonzero, the semaphore can be shared by other processes.

#include <semaphore.h>

sem_t sem;
int ret;
int count = 4;

/* to be shared among processes */
ret = sem_init(&sem, 1, count);

sem_init Return Values

sem_init() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.

EINVAL

Description:

The value argument exceeds SEM_VALUE_MAX .

ENOSPC

Description:

A resource that is required to initialize the semaphore has been exhausted. The limit on semaphores SEM_NSEMS_MAX has been reached.

EPERM

Description:

The process lacks the appropriate privileges to initialize the semaphore.