sem_init — initialize an unnamed semaphore
|unsigned int value
sem_init() initializes the
unnamed semaphore at the address pointed to by
value argument specifies the
initial value for the semaphore.
argument indicates whether this semaphore is to be shared
between the threads of a process, or between processes.
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 allocated dynamically on the heap).
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 undefined behavior.
sem_init() returns 0 on
success; on error, −1 is returned, and
errno is set to indicate the error.
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 attributes(7).
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.
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