#include <semaphore.h> int sem_init(sem_t *sem, int pshared, unsigned int value);
The sem_init() function is used to initialize the unnamed semaphore referred to by sem. The value of the initialized semaphore is value. Following a successful call to sem_init(), the semaphore may be used in subsequent calls to sem_wait(3C), sem_trywait(3C), sem_post(3C), and sem_destroy(3C). This semaphore remains usable until the semaphore is destroyed.
If the pshared argument has a non-zero value, then the semaphore is shared between processes; in this case, any process that can access the semaphore sem can use sem for performing sem_wait(3C), sem_trywait(3C), sem_post(3C), and sem_destroy(3C) operations.
If the pshared argument is zero, then the semaphore is shared between threads of the process; any thread in this process can use sem for performing sem_wait(3C), sem_trywait(3C), sem_post(3C), and sem_destroy(3C) operations. The use of the semaphore by threads other than those created in the same process is undefined.
Attempting to initialize an already initialized semaphore results in undefined behavior.
The sem_open(3C) function is used with named semaphores.
Upon successful completion, the function initializes the semaphore in sem. Otherwise, it returns −1 and sets errno to indicate the error.
The sem_init() function will fail if:
The value argument exceeds SEM_VALUE_MAX.
A resource required to initialize the semaphore has been exhausted, or the resources have reached the limit on semaphores (SEM_NSEMS_MAX).
The sem_init() function is not supported by the system.
The process lacks the appropriate privileges to initialize the semaphore.
See attributes(5) for descriptions of the following attributes: