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msem_init.2 (2010 09)

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msem_init(2) msem_init(2)
NAME
msem_init - initialize a semaphore in a mapped file or anonymous memory region
SYNOPSIS
#include <sys/mman.h>
msemaphore *msem_init(msemaphore *sem, int initial_value);
DESCRIPTION
msem_init() allocates a new binary semaphore and initializes the state of the new semaphore.
sem points to an
msemaphore structure in which the state of the semaphore is to be stored.
If initial_value is
MSEM_LOCKED, the new semaphore is initialized in the locked state. If initial_value is
MSEM_UNLOCKED
, the new semaphore is initialized in the unlocked state.
The
msemaphore structure must be located within a mapped file or anonymous memory region created
by a successful call to mmap() and have both read and write access.
If a semaphore is created in a mapped file region, any reference by a process that has mapped the same
file, using a
(struct msemaphore *) pointer that resolves to the same file offset is interpreted as a
reference to the same semaphore. If a semaphore is created in an anonymous memory region, any refer-
ence by a process sharing the same region by use of a (struct msemaphore *) pointer that resolves
to the same offset from the start of the region is interpreted as a reference to the same semaphore.
Any previous semaphore state stored in the
msemaphore structure will be ignored and overwritten.
Implementation Notes
In order to ensure that an
msemaphore structure is entirely contained in a single memory page, sem
must be at an address that is an exact multiple of
sizeof(struct msemaphore)
. The size of the
msemaphore structure is guaranteed to prevent semaphores that cross page boundaries given the above
restriction.
For a memory mapped file region, the system deallocates memory that corresponds to a range of the file
that has been truncated with
ftruncate() or truncate(). If a semaphore is located in memory so
deallocated, the effect is equivalent to an msem_remove()
on the semaphore.
RETURN VALUE
msem_init() returns the address of the initialized msemaphore structure; otherwise, it returns
NULL and sets errno to indicate the error.
ERRORS
msem_init() fails if any of the following conditions are encountered:
[EINVAL] sem points to an
msemaphore structure that is not located in a mapped region
created by mmap() and with read and write access, or initial_value is not valid.
[ENOMEM] A new semaphore could not be created.
[EFAULT] sem is an invalid pointer.
AUTHOR
msem_init() was developed by HP and OSF.
SEE ALSO
mmap(2), msem_lock(2), msem_remove(2), msem_unlock(2), mman(5).
STANDARDS CONFORMANCE
msem_init(): AES
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

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