man-pages/man2/shmget.2

.\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\" 
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\" 
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" Modified Wed Jul 28 10:57:35 1993, Rik Faith <faith@cs.unc.edu>
.\" Modified Sun Nov 28 16:43:30 1993, Rik Faith <faith@cs.unc.edu>
.\"          with material from Giorgio Ciucci <giorgio@crcc.it>
.\" Portions Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
.\" Modified Tue Oct 22 22:03:17 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified, 8 Jan 2003, Michael Kerrisk, <mtk-manpages@gmx.net>
.\"	Removed EIDRM from errors - that can't happen...
.\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
.\"     Added notes on capability requirements
.\"
.TH SHMGET 2 2004-06-17 "Linux 2.6.7" "Linux Programmer's Manual"
.SH NAME
shmget \- allocates a shared memory segment
.SH SYNOPSIS
.ad l
.B #include <sys/ipc.h>
.sp
.B #include <sys/shm.h>
.sp
.BI "int shmget(key_t " key ", int " size ", int " shmflg );
.ad b
.SH DESCRIPTION
.B shmget()
returns the identifier of the shared memory segment
associated with the value of the argument
.IR key .
A new shared memory segment, with size equal to the value of
.I size
rounded up to a multiple of
.BR PAGE_SIZE ,
is created if
.I key
has the value
.B IPC_PRIVATE
or
.I key
isn't
.BR IPC_PRIVATE ,
no shared memory segment corresponding to
.IR key
exists, and
.B IPC_CREAT
is asserted in
.I shmflg
(i.e.
.IB shmflg &IPC_CREAT
isn't zero).
.PP
The value
.I shmflg
is composed of:
.TP 12
.B IPC_CREAT
to create a new segment. If this flag is not used, then 
.B shmget()
will find the segment associated with \fIkey\fP and check to see if 
the user has permission to access the segment.
.TP
.B IPC_EXCL
used with \fBIPC_CREAT\fP to ensure failure if the segment already exists. 
.TP
.B mode_flags (lowest 9 bits)
specifying the permissions granted to the owner, group, and world.
Presently, the execute permissions are not used by the system.
.\" FIXME -- document SHM_HUGETLB
.PP
If a new segment is created, the access permissions from
.I shmflg
are copied into the 
.I shm_perm
member of the 
.I shmid_ds
structure that defines the segment.
The \fIshmid_ds\fP structure has the following form:
.PP
.in +0.5i
.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 -0.5i
.PP
.in +0.5i
.nf
struct ipc_perm {
    key_t  key;
    ushort uid;   /* owner euid and egid */
    ushort gid;
    ushort cuid;  /* creator euid and egid */
    ushort cgid;
    ushort mode;  /* lower 9 bits of \fIshmflg\fP */
    ushort seq;   /* sequence number */
};
.fi
.PP
When creating a new shared memory segment,
the system call initializes the \fIshmid_ds\fP data structure
.B shmid_ds
as follows:
.IP
.B shm_perm.cuid
and
.B shm_perm.uid
are set to the effective user\-ID of the calling process.
.IP
.B shm_perm.cgid
and
.B shm_perm.gid
are set to the effective group\-ID of the calling process.
.IP
The lowest order 9 bits of
.B shm_perm.mode
are set to the lowest order 9 bit of
.IR shmflg .
.IP
.B shm_segsz
is set to the value of
.IR size .
.IP
.BR shm_lpid ,
.BR shm_nattch ,
.B shm_atime
and
.B shm_dtime
are set to
.BR 0 .
.IP
.B shm_ctime
is set to the current time.
.PP
If the shared memory segment already exists, the access permissions are
verified, and a check is made to see if it is marked for destruction.
.PP
.SH "SYSTEM CALLS"
.TP
.B fork()
After a
.B fork()
the child inherits the attached shared memory segments.
.TP
.B exec()
After an
.B exec()
all attached shared memory segments are detached (not destroyed).
.TP
.B exit()
Upon
.B exit()
all attached shared memory segments are detached (not destroyed).
.PP
.SH "RETURN VALUE"
A valid segment identifier,
.IR shmid ,
is returned on success, \-1 on error.
.SH ERRORS
On failure,
.B errno
is set to one of the following:
.TP 12
.B EACCES
The user does not have permission to access the 
shared memory segment, and does not have the
.B CAP_IPC_OWNER
capability.
.TP
.B EEXIST
.B IPC_CREAT | IPC_EXCL
was specified and the segment exists.
.TP
.\" FIXME -- SHM_HUGETLB requires CAP_IPC_LOCK, or the error EPERM
.\" results
.B EINVAL
A new segment was to be created and \fIsize\fP < \fBSHMMIN\fP
or \fIsize\fP > \fBSHMMAX\fP, or no new segment was to be created,
a segment with given key existed, but \fIsize\fP is greater than the size
of that segment.
.TP
.B ENFILE
.\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp()
The system limit on the total number of open files has been reached.
.TP
.B ENOENT
No segment exists for the given \fIkey\fP, and
.B IPC_CREAT
was not specified.
.TP
.B ENOMEM
No memory could be allocated for segment overhead.
.TP
.B ENOSPC
All possible shared memory id's have been taken
.RB ( SHMMNI ),
or allocating a segment of the requested
.I size
would cause the system to exceed the system-wide limit on shared memory
.RB ( SHMALL ).
.SH NOTES
.B IPC_PRIVATE
isn't a flag field but a
.B key_t
type.
If this special value is used for
.IR key ,
the system call ignores everything but the lowest order 9 bits of
.I shmflg
and creates a new shared memory segment (on success).
.PP
The followings are limits on shared memory segment resources affecting a
.B shmget
call:
.TP 11
.B SHMALL
System wide maximum of shared memory pages: policy dependent.
.TP
.B SHMMAX
Maximum size in bytes for a shared memory segment: implementation
dependent (currently 4M).
.TP
.B SHMMIN
Minimum size in bytes for a shared memory segment: implementation
dependent (currently 1 byte, though
.B PAGE_SIZE
is the effective minimum size).
.TP
.B SHMMNI
System wide maximum number of shared memory segments: implementation
dependent (currently 4096, was 128 before Linux 2.3.99).
.PP
The implementation has no specific limits for the per process maximum
number of shared memory segments
.RB ( SHMSEG ).
.SH BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW
would more clearly show its function.
.SH "CONFORMING TO"
SVr4, SVID.  SVr4 documents an additional error condition EEXIST.
Until version 2.3.30 Linux would return EIDRM for a
.B shmget
on a shared memory segment scheduled for deletion.
.SH "SEE ALSO"
.BR shmat (2),
.BR shmctl (2),
.BR shmdt (2),
.BR ftok (3),
.BR ipc (5),
.BR capabilities (7)