xref: /netbsd-src/lib/librt/sem_open.3 (revision 6237a794782646be1682f8910d468d1a3efdde41)

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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 .