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sem_init.2 (2010 09)

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sem_init(2) sem_init(2)
NAME
sem_init - initialize an unnamed POSIX semaphore
SYNOPSIS
#include <sys/semaphore.h>
int sem_init(sem_t *sem, int pshared, unsigned int value);
DESCRIPTION
sem_init() is used to initialize an unnamed semaphore. A successful call to
sem_init() will create
a new unnamed semaphore referred to by sem, if one does not exist, initialize the unnamed semaphore
descriptor, referred to by sem, to the non-negative value specified by value . If the unnamed semaphore
already exists, i.e. created by a previous call to
sem_init(), it is re-initialized only if its current value
is equal to its initial value (set by the last successful call to
sem_init()). If so, the initial value of the
unnamed semaphore is re-initialized to the value argument. Otherwise, the call fails.
The argument pshared specifies if the unnamed semaphore is sharable with other processes. If pshared is
equal to 0, the unnamed semaphore is not shared with other processes. If pshared is non-zero, the
unnamed semaphore is sharable with any processes that can access sem. The access mode specified for
the unnamed semaphore allows read and write permissions to all processes. If the calling process may
attach to the shared
sem_t structure, it is assumed it may operate on the semaphore.
To use this function, link in the realtime library by specifying
-lrt on the compiler or linker command
line.
EXAMPLES
The following call to
sem_init() will create a new unnamed semaphore referred to by sem, if one does
not exist, initialize the unnamed semaphore descriptor, referred to by sem, to the non-negative value
specified by value .
sem_init(sem, pshared, value);
RETURN VALUE
If the semaphore was created and initialized, sem_init() returns 0 to the caller.
If the semaphore could not be created/initialized, the call returns -1 and sets
errno to indicate the error.
ERRORS
sem_init() fails and does not perform the requested operation if any of the following conditions are
encountered:
[EPERM] The calling process does not have the privileges necessary to initialize the sema-
phore.
[EBUSY] There are threads currently blocked on the semaphore or there are outstanding
locks held on the semaphore.
[EINVAL] The argument value is greater than
{_POSIX_SEM_VALUE_MAX}
.
[ENOSPC] There are insufficient resources to perform the operation or the upper limit on the
number of semaphores is reached.
SEE ALSO
sem_destroy(2), sem_post(2), sem_trywait(2), sem_wait(2), <semaphore.h>.
STANDARDS CONFORMANCE
sem_init(): POSIX
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

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