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 reference 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.
STANDARDS CONFORMANCE
msem_init(): AES
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