Use sem_init(3RT) to initialize the unnamed semaphore variable pointed to by sem to value amount.
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.
#include <semaphore.h> sem_t sem; int ret; int count = 4; /* to be used within this process only */ ret = sem_init(&sem, 0, count);
#include <semaphore.h> sem_t sem; int ret; int count = 4; /* to be shared among processes */ ret = sem_init(&sem, 1, count);
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.
The value argument exceeds SEM_VALUE_MAX .
A resource that is 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.