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.TH SHMGET "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.

.SH NAME
shmget
\(em get an XSI shared memory segment
.SH SYNOPSIS
.LP
.nf
#include <sys/shm.h>
.P
int shmget(key_t \fIkey\fP, size_t \fIsize\fP, int \fIshmflg\fP);
.fi
.SH DESCRIPTION
The
\fIshmget\fR()
function operates on XSI shared memory (see the Base Definitions volume of POSIX.1\(hy2008,
.IR "Section 3.342" ", " "Shared Memory Object").
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
.IR "Section 2.8" ", " "Realtime".
.P
The
\fIshmget\fR()
function shall return the shared memory identifier associated with
.IR key .
.P
A shared memory identifier, associated data structure, and shared
memory segment of at least
.IR size
bytes (see
.IR <sys/shm.h> )
are created for
.IR key
if one of the following is true:
.IP " *" 4
The argument
.IR key
is equal to IPC_PRIVATE.
.IP " *" 4
The argument
.IR key
does not already have a shared memory identifier associated with it and
(\fIshmflg\fP &IPC_CREAT) is non-zero.
.P
Upon creation, the data structure associated with the new shared memory
identifier shall be initialized as follows:
.IP " *" 4
The values of
.IR shm_perm.cuid ,
.IR shm_perm.uid ,
.IR shm_perm.cgid ,
and
.IR shm_perm.gid
are set to the effective user ID and effective group ID,
respectively, of the calling process.
.IP " *" 4
The low-order nine bits of
.IR shm_perm.mode
are set to the low-order nine bits of
.IR shmflg .
.IP " *" 4
The value of
.IR shm_segsz
is set to the value of
.IR size .
.IP " *" 4
The values of
.IR shm_lpid ,
.IR shm_nattch ,
.IR shm_atime ,
and
.IR shm_dtime
are set to 0.
.IP " *" 4
The value of
.IR shm_ctime
is set to the current time, as described in
.IR "Section 2.7.1" ", " "IPC General Description".
.P
When the shared memory segment is created, it shall be initialized
with all zero values.
.SH "RETURN VALUE"
Upon successful completion,
\fIshmget\fR()
shall return a non-negative integer, namely a shared memory identifier;
otherwise, it shall return \(mi1 and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fIshmget\fR()
function shall fail if:
.TP
.BR EACCES
A shared memory identifier exists for
.IR key
but operation permission as specified by the low-order nine bits of
.IR shmflg
would not be granted; see
.IR "Section 2.7" ", " "XSI Interprocess Communication".
.TP
.BR EEXIST
A shared memory identifier exists for the argument
.IR key
but (\fIshmflg\fR &IPC_CREAT) &&(\fIshmflg\fR &IPC_EXCL) is non-zero.
.TP
.BR EINVAL
A shared memory segment is to be created and the value of size is
less than the system-imposed minimum or greater than the
system-imposed maximum.
.TP
.BR EINVAL
No shared memory segment is to be created and a shared memory
segment exists for
.IR key
but the size of the segment associated with it is less than
.IR size .
.TP
.BR ENOENT
A shared memory identifier does not exist for the argument
.IR key
and (\fIshmflg\fP &IPC_CREAT) is 0.
.TP
.BR ENOMEM
A shared memory identifier and associated shared memory segment shall
be created, but the amount of available physical memory is not
sufficient to fill the request.
.TP
.BR ENOSPC
A shared memory identifier is to be created, but the system-imposed
limit on the maximum number of allowed shared memory identifiers
system-wide would be exceeded.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
The POSIX Realtime Extension defines alternative interfaces for interprocess
communication. Application developers who need to use IPC should
design their applications so that modules using the IPC routines
described in
.IR "Section 2.7" ", " "XSI Interprocess Communication"
can be easily modified to use the alternative interfaces.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.7" ", " "XSI Interprocess Communication",
.IR "Section 2.8" ", " "Realtime",
.IR "\fIftok\fR\^(\|)",
.IR "\fIshmat\fR\^(\|)",
.IR "\fIshmctl\fR\^(\|)",
.IR "\fIshmdt\fR\^(\|)",
.IR "\fIshm_open\fR\^(\|)",
.IR "\fIshm_unlink\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2008,
.IR "Section 3.342" ", " "Shared Memory Object",
.IR "\fB<sys_shm.h>\fP"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.unix.org/online.html .

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