sem_open — initialise and open a named semaphore
#include <semaphore.h>
sem_t
*sem_open( |
const char * | name, |
int | oflag) ; |
sem_t
*sem_open( |
const char * | name, |
int | oflag, | |
mode_t | mode, | |
unsigned int | value) ; |
sem_open
() creates a new
POSIX semaphore or opens an existing semaphore. The semaphore
is identified by name
. For details of the
construction of name
,
see sem_overview(7).
The oflag
argument
specifies flags that control the operation of the call. If
O_CREAT
is specified in
oflag
, then the
semaphore is created if it does not already exist. The owner
(user ID) of the semaphore is set to the effective user ID of
the calling process. The group ownership (group ID) is set to
the effective group ID of the calling process. If both
O_CREAT
and O_EXCL
are specified in oflag
, then an error is
returned if a semaphore with the given name
already exists.
If O_CREAT
is specified in
oflag
, then two
additional arguments must be supplied. The mode
argument specifies the
permissions to be placed on the new semaphore, as for
open(2). The permissions
settings are masked against the process umask. Both read and
write permission should be granted to each class of user that
will access the semaphore. The value
argument specifies the
initial value for the new semaphore. If O_CREAT
is specified, and a semaphore with
the given name
already exists, then mode
and value
are ignored.
On success, sem_open
()
returns the address of the new semaphore; this address is
used when calling other semaphore-related functions. On
error, sem_open
() returns
SEM_FAILED
, with errno
set to indicate the error.
The semaphore exists, but the caller does not have permission to open it.
Both O_CREAT
and
O_EXCL
were specified in
oflag
, but a
semaphore with this name
already exists.
value
was
greater than SEM_VALUE_MAX
.
The process already has the maximum number of files and open.
name
was too
long.
The system limit on the total number of open files has been reached.
The O_CREAT
flag was
not specified in oflag
, and no semaphore
with this name
exists.
Insufficient memory.
sem_close(3), sem_getvalue(3), sem_post(3), sem_unlink(3), sem_wait(3), sem_overview(7)
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