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\" From: FreeBSD: src/lib/libc/gen/sem_open.3,v 1.12 2004/07/02 16:45:56 ru
\"
Dd May 4, 2018
Dt SEM_OPEN 3
Os
Sh NAME
Nm sem_open ,
Nm sem_close ,
Nm sem_unlink
Nd named semaphore operations
Sh LIBRARY
Lb librt
Sh SYNOPSIS
In semaphore.h
Ft "sem_t *"
Fn sem_open "const char *name" "int oflag" ...
Ft int
Fn sem_close "sem_t *sem"
Ft int
Fn sem_unlink "const char *name"
Sh DESCRIPTION
The
Fn sem_open
function creates or opens the named semaphore specified by
Fa name .
The returned semaphore may be used in subsequent calls to
Xr sem_getvalue 3 ,
Xr sem_timedwait 3 ,
Xr sem_trywait 3 ,
Xr sem_wait 3 ,
Xr sem_post 3 ,
and
Fn sem_close .
Pp
The following bits may be set in the
Fa oflag
argument:
Bl -tag -width ".Dv O_CREAT"
It Dv O_CREAT
Create the semaphore if it does not already exist.
Pp
The third argument to the call to
Fn sem_open
must be of type
Vt mode_t
and specifies the mode for the semaphore.
Only the
Dv S_IWUSR , S_IWGRP ,
and
Dv S_IWOTH
bits are examined;
it is not possible to grant only
Dq read
permission on a semaphore.
The mode is modified according to the process's file creation
mask; see
Xr umask 2 .
Pp
The fourth argument must be an
Vt "unsigned int"
and specifies the initial value for the semaphore,
and must be no greater than
Dv SEM_VALUE_MAX .
It Dv O_EXCL
Create the semaphore if it does not exist.
If the semaphore already exists,
Fn sem_open
will fail.
This flag is ignored unless
Dv O_CREAT
is also specified.
El
Pp
The
Fn sem_close
function closes a named semaphore that was opened by a call to
Fn sem_open .
Pp
The
Fn sem_unlink
function removes the semaphore named
Fa name .
Resources allocated to the semaphore are only deallocated when all
processes that have the semaphore open close it.
Sh RETURN VALUES
If successful,
the
Fn sem_open
function returns the address of the opened semaphore.
If the same
Fa name
argument is given to multiple calls to
Fn sem_open
by the same process without an intervening call to
Fn sem_close ,
the same address is returned each time.
If the semaphore cannot be opened,
Fn sem_open
returns
Dv SEM_FAILED
and the global variable
Va errno
is set to indicate the error.
Pp
Rv -std sem_close sem_unlink
Sh ERRORS
The
Fn sem_open
function will fail if:
Bl -tag -width Er
It Bq Er EACCES
The semaphore exists and the permissions specified by
Fa oflag
at the time it was created deny access to this process;
or the semaphore does not exist, but permission to create it is denied.
It Bq Er EEXIST
Dv O_CREAT
and
Dv O_EXCL
are set but the semaphore already exists.
It Bq Er EINTR
The call was interrupted by a signal.
It Bq Er EINVAL
The
Fa name
argument does not begin with a
Sq /
or contains more slashes.
This is implementation-specific behavior and allowed by
St -p1003.1-96 .
Or, the
Fa value
argument is greater than
Dv SEM_VALUE_MAX .
\"FreeBSD never returns EMFILE
\".It Bq Er EMFILE
\"Too many semaphores are in use by this process.
It Bq Er ENAMETOOLONG
The specified
Fa name
is longer than
Dv NAME_MAX ,
or longer than the implementing file system will allow.
It Bq Er ENFILE
The system limit on semaphores has been reached.
It Bq Er ENOENT
Dv O_CREAT
is not set and the named semaphore does not exist.
It Bq Er ENOSPC
There is not enough space to create the semaphore.
El
Pp
The
Fn sem_close
function will fail if:
Bl -tag -width Er
It Bq Er EINVAL
The
Fa sem
argument is not a valid semaphore.
El
Pp
The
Fn sem_unlink
function will fail if:
Bl -tag -width Er
It Bq Er EACCES
Permission is denied to unlink the semaphore.
It Bq Er ENAMETOOLONG
The specified
Fa name
is longer than
Dv NAME_MAX ,
or longer than the implementing file system will allow.
It Bq Er ENOENT
The named semaphore does not exist.
El
Sh SEE ALSO
Xr close 2 ,
Xr open 2 ,
Xr umask 2 ,
Xr unlink 2 ,
Xr sem_getvalue 3 ,
Xr sem_post 3 ,
Xr sem_trywait 3 ,
Xr sem_wait 3 ,
Xr sem 4
Sh STANDARDS
The
Fn sem_open ,
Fn sem_close ,
and
Fn sem_unlink
functions conform to
St -p1003.1-96 .
Sh HISTORY
Support for named semaphores first appeared in
Nx 2.0 .

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