man-pages/man2/shmctl.2

.\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993
.\" Portions Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
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.\" Modified 1993-07-28, Rik Faith <faith@cs.unc.edu>
.\" Modified 1993-11-28, Giorgio Ciucci <giorgio@crcc.it>
.\" Modified 1997-01-31, Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2001-02-18, Andries Brouwer <aeb@cwi.nl>
.\" Modified 2002-01-05, 2004-05-27, 2004-06-17,
.\"    Michael Kerrisk <mtk-manpages@gmx.net>
.\" Modified 2004-10-11, aeb
.\"
.TH SHMCTL 2 2004-06-17 "Linux 2.6.7" "Linux Programmer's Manual"
.SH NAME
shmctl \- shared memory control
.SH SYNOPSIS
.ad l
.B #include <sys/ipc.h>
.sp
.B #include <sys/shm.h>
.sp
.BI "int shmctl(int " shmid ", int " cmd ", struct shmid_ds *" buf );
.ad b
.SH DESCRIPTION
\fBshmctl()\fP
allows the user to receive information on a shared memory segment,
set the owner, group, and permissions of a shared memory segment,
or destroy a segment. The information about the segment identified by
\fIshmid\fP is returned in a \fIshmid_ds\fP
structure:
.PP
.in +4n
.nf
struct shmid_ds {
    struct ipc_perm shm_perm;  /* operation perms */
    int shm_segsz;             /* size of segment (bytes) */
    time_t shm_atime;          /* last attach time */
    time_t shm_dtime;          /* last detach time */
    time_t shm_ctime;          /* last change time */
    unsigned short shm_cpid;   /* pid of creator */
    unsigned short shm_lpid;   /* pid of last operator */
    short shm_nattch;          /* no. of current attaches */
    ...
};
.fi
.in -4n
.PP
The highlighted fields in the member \fIshm_perm\fP can be set:
.PP
.in +4n
.nf
struct ipc_perm {
    key_t  key;
    \fBushort uid\fP;   /* \fBowner\fP euid and egid */
    \fBushort gid\fP;
    ushort cuid;  /* creator euid and egid */
    ushort cgid;
    \fBushort mode\fP;  /* lower 9 bits of access modes */
    ushort seq;   /* sequence number */
};
.fi
.in -4n
.PP
The following \fIcmds\fP are available:
.br
.TP 12
.B IPC_STAT
is used to copy the information about the shared memory segment into
the buffer \fIbuf\fP. The user must have \fBread\fP access to the
shared memory segment.
.TP
.B IPC_SET
is used to apply the changes the user has made to the \fIuid\fP, \fIgid\fP,
or \fImode\fP members of the \fIshm_perms\fP field.  Only the lowest 9 bits
of \fImode\fP are used.  The
.I shm_ctime
member is also updated.  The user must be the owner or creator, or be
privileged.
.TP
.B IPC_RMID
is used to mark the segment as destroyed. It will actually be destroyed
after the last detach.  (I.e., when the
.I shm_nattch
member of the associated structure
.I shmid_ds
is zero.)  The user must be the owner or creator, or be privileged.
.PP
The user \fImust\fP ensure that a segment is eventually destroyed; otherwise
its pages that were faulted in will remain in memory or swap.
.SS "Linux additions"
A privileged user can prevent or allow swapping of a shared
memory segment with the following \fIcmds\fP:
.br
.TP 12
.B SHM_LOCK
prevents swapping of a shared memory segment. The user must fault in
any pages that are required to be present after locking is enabled.
.TP
.B SHM_UNLOCK
allows the shared memory segment to be swapped out.
.PP
The
.BR IPC_INFO ,
.BR SHM_STAT
and
.B SHM_INFO
control calls are used by the
.BR ipcs (8)
program to provide information on allocated resources.
.SH "RETURN VALUE"
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP 11
.B EACCES
\fBIPC_STAT\fP or \fBSHM_STAT\fP is requested and
\fIshm_perm.modes\fP does not allow read access for
.IR shmid ,
and the calling process does not have the
.BR CAP_IPC_OWNER
capability.
.TP
.B EFAULT
The argument
.I cmd
has value
.B IPC_SET
or
.B IPC_STAT
but the address pointed to by
.I buf
isn't accessible.
.TP
.B EIDRM
\fIshmid\fP points to a removed identifier.
.TP
.B EINVAL
\fIshmid\fP is not a valid identifier, or \fIcmd\fP
is not a valid command.
.TP
.B EOVERFLOW
\fBIPC_STAT\fP is attempted, and the gid or uid value
is too large to be stored in the structure pointed to by
.IR buf . 
.TP
.B EPERM
\fBIPC_SET\fP or \fBIPC_RMID\fP is attempted, and the
effective user ID of the calling process is not that of the creator
(found in
.IR shm_perm.cuid ),
or the owner
(found in
.IR shm_perm.uid ),
and the process was not privileged (Linux: did not have the
.B CAP_SYS_ADMIN
capability).

Or,
.B SHM_LOCK
or
.B SHM_UNLOCK
was specified, but the process was not privileged
(Linux: did not have the
.B CAP_IPC_LOCK
capability).
.SH NOTE
Various fields in a \fIstruct shmid_ds\fP were shorts under Linux 2.2
and have become longs under Linux 2.4. To take advantage of this,
a recompilation under glibc-2.1.91 or later should suffice.
(The kernel distinguishes old and new calls by an IPC_64 flag in
.IR cmd .)
.SH "CONFORMING TO"
SVr4, SVID.  SVr4 documents additional error conditions EINVAL,
ENOENT, ENOSPC, ENOMEM, EEXIST.  Neither SVr4 nor SVID documents
an EIDRM error condition.
.SH "SEE ALSO"
.BR shmget (2),
.BR shmop (2),
.BR capabilities (7)