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.TH SEM_OPEN 3 2006-03-25 "Linux 2.6.16" "Linux Programmer's Manual"
.SH NAME
sem_open \- initialise and open a named semaphore
.SH SYNOPSIS
.nf
.B #include <semaphore.h>
.sp
.BI "sem_t *sem_open(const char *" name ", int " oflag );
.BI "sem_t *sem_open(const char *" name ", int " oflag ", "
.BI "                mode_t " mode ", unsigned int " value );
.fi
.SH DESCRIPTION
.BR sem_open ()
creates a new POSIX semaphore or opens an existing semaphore.
The semaphore is identified by
.IR name .
For details of the construction of 
.IR name , 
see
.BR sem_overview (7).

The
.I oflag
argument specifies flags that control the operation of the call.
If
.B O_CREAT
is specified in 
.IR 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.
.\" In reality the file system IDs are used on Linux.
If both
.B O_CREAT
and
.B O_EXCL
are specified in
.IR oflag ,
then an error is returned if a semaphore with the given
.I name
already exists.
.PP
If 
.B O_CREAT
is specified in 
.IR oflag ,
then two additional arguments must be supplied.
The
.I mode
argument specifies the permissions to be placed on the new semaphore,
as for
.BR 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 
.I value
argument specifies the initial value for the new semaphore.
If 
.B O_CREAT
is specified, and a semaphore with the given
.I name
already exists, then
.I mode
and
.I value
are ignored.
.SH RETURN VALUE
On success,
.BR sem_open ()
returns a the address of the new semaphore; 
this address is used when calling other semaphore-related functions.
On error, 
.BR sem_open ()
returns 
.BR SEM_FAILED ,
with
.I errno
set to indicate the error.
.SH ERRORS
.TP
.B EACCESS
The semaphore exists, but the caller does not have permission to 
open it.
.TP
.B EEXIST
Both
.B O_CREAT
and
.B O_EXCL
were specified in 
.IR oflag ,
but a semaphore with this 
.I name
already exists.
.TP
.B EINVAL
.I value 
was greater than
.BR SEM_VALUE_MAX .
.TP
.B EMFILE
The process already has the maximum number of files and open.
.TP
.B ENAMETOOLONG
.IR name
was too long.
.TP
.B ENFILE
The system limit on the total number of open files has been reached.
.TP
.B ENOENT
The
.B O_CREAT
flag was not specified in 
.IR oflag ,
and no semaphore with this 
.I name
exists.
.TP
.B ENOMEM
Insufficient memory.
.SH CONFORMING TO
POSIX.1-2001.
.SH "SEE ALSO"
.BR sem_close (3),
.BR sem_getvalue (3),
.BR sem_post (3),
.BR sem_unlink (3),
.BR sem_wait (3),
.BR sem_overview (7)