- initialize an unnamed semaphore
cc [ flag... ] file... -lrt [ library... ] #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(3RT), sem_trywait(3RT), sem_post(3RT), and sem_destroy(3RT). 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(3RT), sem_trywait(3RT), sem_post(3RT), and sem_destroy(3RT) 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(3RT), sem_trywait(3RT), sem_post(3RT), and sem_destroy(3RT) 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.
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: