SEM_INIT(3) Linux Programmer's Manual SEM_INIT(3)
sem_init - initialize an unnamed semaphore
int sem_init(sem_t *sem, int pshared, unsigned int value);
Link with -pthread.
sem_init() initializes the unnamed semaphore at the address pointed to
by sem. The value argument specifies the initial value for the sema-
The pshared argument indicates whether this semaphore is to be shared
between the threads of a process, or between processes.
If pshared has the value 0, then the semaphore is shared between the
threads of a process, and should be located at some address that is
visible to all threads (e.g., a global variable, or a variable allo-
cated dynamically on the heap).
If pshared is nonzero, then the semaphore is shared between processes,
and should be located in a region of shared memory (see shm_open(3),
mmap(2), and shmget(2)). (Since a child created by fork(2) inherits
its parent's memory mappings, it can also access the semaphore.) Any
process that can access the shared memory region can operate on the
semaphore using sem_post(3), sem_wait(3), and so on.
Initializing a semaphore that has already been initialized results in
sem_init() returns 0 on success; on error, -1 is returned, and errno is
set to indicate the error.
EINVAL value exceeds SEM_VALUE_MAX.
ENOSYS pshared is nonzero, but the system does not support process-
shared semaphores (see sem_overview(7)).
For an explanation of the terms used in this section, see
|Interface | Attribute | Value |
|sem_init() | Thread safety | MT-Safe |
Bizarrely, POSIX.1-2001 does not specify the value that should be
returned by a successful call to sem_init(). POSIX.1-2008 rectifies
this, specifying the zero return on success.
sem_destroy(3), sem_post(3), sem_wait(3), sem_overview(7)
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Linux 2017-09-15 SEM_INIT(3)