NetBSD/lib/libc/sys/shmget.2

.\"	$NetBSD: shmget.2,v 1.10 2000/12/12 16:47:05 briggs Exp $	
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.\" Copyright (c) 1995 Frank van der Linden
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.Dd August 25, 1999
.Dt SHMGET 2
.Os
.Sh NAME
.Nm shmget
.Nd get shared memory segment
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.Fd #include <sys/shm.h>
.Ft int
.Fn shmget "key_t key" "size_t size" "int shmflg"
.Sh DESCRIPTION
.Fn shmget
returns the shared memory identifier associated with the key
.Fa key .
The
.Fa shmflg
should be a combination of the following flags:
.Bl -tag -offset indent -width IPC_NOWAIT
.It Dv IPC_CREAT
create if segment does not exist
.It Dv IPC_EXCL
fail if segment already exists
.It Dv IPC_NOWAIT
return error instead of waiting
.El
.Pp
and nine bits of access bits, based on:
.Bl -tag -offset indent -width IPC_R
.It Dv IPC_R
read permission
.It Dv IPC_W
write permission
.It Dv IPC_M
changing control info permission
.El
.Pp
A shared memory segment is created if either
.Fa key
is equal to
.Dv IPC_PRIVATE ,
or
.Fa key
does not have a shared memory segment identifier associated with it, and the
.Dv IPC_CREAT
bit is set in
.Fa shmflg.
.Pp
If a new shared memory segment is created, the data structure
associated with it (the
.Va shmid_ds
structure, see
.Xr shmctl 2 )
is initialized as follows:
.Bl -bullet
.It 
.Va shm_perm.cuid
and
.Va shm_perm.uid
are set to the effective uid of the calling process.
.It
.Va shm_perm.gid
and
.Va shm_perm.cgid
are set to the effective gid of the calling process.
.It
.Va shm_perm.mode
is set to the lower 9 bits of
.Fa shmflg .
.It
.Va shm_lpid ,
.Va shm_nattch ,
.Va shm_atime ,
and 
.Va shm_dtime
are set to 0.
.It
.Va shm_ctime
is set to the current time.
.It
.Va shm_segsz
is set to the value of
.Fa size .
.El
.Sh RETURN VALUES
Upon successful completion a positive shared memory segment identifier
is returned.
Otherwise, -1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EACCES
A shared memory segment is already associated with
.Fa key
and the caller has no permission to access it.
.It Bq Er EEXIST
Both
.Dv IPC_CREAT
and
.Dv IPC_EXCL
are set in
.Fa shmflg ,
and a shared memory segment is already associated with
.Fa key .
.It Bq Er ENOSPC
A new shared memory identifier could not be created because the system limit
for the number of shared memory identifiers has been reached.
.It Bq Er ENOENT
.Dv IPC_CREAT
is not set in
.Fa shmflg
and no shared memory segment associated with
.Fa key
was found.
.It Bq Er ENOMEM
There is not enough memory left to created a shared memory segment of the
requested size.
.El
.Sh SEE ALSO
.Xr ftok 3 ,
.Xr mmap 2 ,
.Xr shmat 2 ,
.Xr shmctl 2
.Sh STANDARDS
The
.Nm
system call conforms to
.St -xsh5 .
.Sh HISTORY
Shared memory segments appeared in the first release of
.At V .