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.TH SHMCTL "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
shmctl
\(em XSI shared memory control operations
.SH SYNOPSIS
.LP
.nf
#include <sys/shm.h>
.P
int shmctl(int \fIshmid\fP, int \fIcmd\fP, struct shmid_ds *\fIbuf\fP);
.fi
.SH DESCRIPTION
The
\fIshmctl\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
\fIshmctl\fR()
function provides a variety of shared memory control operations as
specified by
.IR cmd .
The following values for
.IR cmd
are available:
.IP IPC_STAT 12
Place the current value of each member of the
.BR shmid_ds
data structure associated with
.IR shmid
into the structure pointed to by
.IR buf .
The contents of the structure are defined in
.IR <sys/shm.h> .
.IP IPC_SET 12
Set the value of the following members of the
.BR shmid_ds
data structure associated with
.IR shmid
to the corresponding value found in the structure pointed to by
.IR buf :
.RS 12 
.sp
.RS 4
.nf
\fB
shm_perm.uid
shm_perm.gid
shm_perm.mode    \fRLow-order nine bits.\fP
.fi \fR
.P
.RE
.P
Also, the
.IR shm_ctime
timestamp shall be set to the current time, as described in
.IR "Section 2.7.1" ", " "IPC General Description".
.P
IPC_SET can only be executed by a process that has an effective user ID
equal to either that of a process with appropriate privileges or to the
value of
.IR shm_perm.cuid
or
.IR shm_perm.uid
in the
.BR shmid_ds
data structure associated with
.IR shmid .
.RE
.IP IPC_RMID 12
Remove the shared memory identifier specified by
.IR shmid
from the system and destroy the shared memory segment and
.BR shmid_ds
data structure associated with it. IPC_RMID can only be executed by a
process that has an effective user ID equal to either that of a process
with appropriate privileges or to the value of
.IR shm_perm.cuid
or
.IR shm_perm.uid
in the
.BR shmid_ds
data structure associated with
.IR shmid .
.SH "RETURN VALUE"
Upon successful completion,
\fIshmctl\fR()
shall return 0; otherwise, it shall return \(mi1 and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fIshmctl\fR()
function shall fail if:
.TP
.BR EACCES
The argument
.IR cmd
is equal to IPC_STAT and the calling process does not have read
permission; see
.IR "Section 2.7" ", " "XSI Interprocess Communication".
.TP
.BR EINVAL
The value of
.IR shmid
is not a valid shared memory identifier, or the value of
.IR cmd
is not a valid command.
.TP
.BR EPERM
The argument
.IR cmd
is equal to IPC_RMID or IPC_SET and the effective user ID of the
calling process is not equal to that of a process with appropriate
privileges and it is not equal to the value of
.IR shm_perm.cuid
or
.IR shm_perm.uid
in the data structure associated with
.IR shmid .
.br
.P
The
\fIshmctl\fR()
function may fail if:
.TP
.BR EOVERFLOW
The
.IR cmd
argument is IPC_STAT and the
.IR gid
or
.IR uid
value is too large to be stored in the structure pointed to by the
.IR buf
argument.
.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 "\fIshmat\fR\^(\|)",
.IR "\fIshmdt\fR\^(\|)",
.IR "\fIshmget\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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