Multithreaded Programming Guide


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); 

Use sem_init(3R) to initialize the semaphore variable pointed to by sem to value amount. 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. (For Solaris threads, see "sema_init(3T)".)

Multiple threads must not initialize the same semaphore.

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

Return Values

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


The value argument exceeds SEM_VALUE_MAX.


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


The process lacks the appropriate privileges to initialize 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);