.\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993
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.\" 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>
.\"
.TH SHMGET 2 "November 28, 1993" "Linux 0.99.11" "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 to the value of the argument
.IR key .
A new shared memory segment, with size equal to the round up of
.I size
to a multiple of
.BR PAGE_SIZE ,
is created if
.I key
has value
.B IPC_PRIVATE
or
.I key
isn't
.BR IPC_PRIVATE ,
no shared memory segment is associated to
.IR key ,
and
.B IPC_CREAT
is asserted in
.I shmflg
(i.e.
.IB shmflg &IPC_CREAT
isn't zero).
The presence in
.PP
.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, check to see if 
the user has permission to receive the \fIshmid\fP associated with the
segment, and ensure the segment is not marked for destruction.
.TP
.B IPC_EXCL
used with \fBIPC_CREAT\fP to ensure failure if the segment 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.
.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:
.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
Furthermore, while creating,
the system call initializes the system shared memory segment 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 EINVAL
is returned if \fBSHMMIN\fP > \fIsize\fP or
\fIsize\fP > \fBSHMMAX\fP, or \fIsize\fP is greater than size of segment.
.TP
.B EEXIST
is returned if 
.B IPC_CREAT | IPC_EXCL
was specified and the segment exists.
.TP
.B EIDRM
is returned if the segment is marked as destroyed, or was removed.
.TP
.B ENOSPC
is returned if all possible shared memory id's have been taken
.BR "" ( SHMMNI )
or if allocating a segment of the requested
.I size
would cause the system to exceed the system-wide limit on shared memory
.BR "" ( SHMALL ).
.TP
.B ENOENT
is returned if no segment exists for the given \fIkey\fP, and
.B IPC_CREAT
was not specified.
.TP
.B EACCES
is returned if the user does not have permission to access the 
shared memory segment.
.TP
.B ENOMEM
is returned if no memory could be allocated for segment overhead.
.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).
.PP
The implementation has no specific limits for the per process maximum
number of shared memory segments
.RB ( SHMSEG ).
.SH BUGS
Use of
.B IPC_PRIVATE
doesn't inhibit to other processes the access to the allocated
shared memory segment.
.PP
As for the files, there is currently no intrinsic way for a process to ensure
exclusive access to a shared memory segment.
Asserting both
.B IPC_CREAT
and
.B IPC_EXCL
in
.I shmflg
only ensures (on success) that a new shared memory segment will be created,
it doesn't imply exclusive access to the segment.
.SH "CONFORMING TO"
SVr4, SVID.  SVr4 documents an additional error condition EEXIST.
Neither SVr4 nor SVID documents an EIDRM condition. 
.SH "SEE ALSO"
.BR ftok (3),
.BR ipc (5),
.BR shmctl (2),
.BR shmat (2),
.BR shmdt (2)