path: root/man2/shmop.2

.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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.\" Modified Sun Nov 28 17:06:19 1993, Rik Faith (faith@cs.unc.edu)
.\"          with material from Luigi P. Bai (lpb@softint.com)
.\" Portions Copyright 1993 Luigi P. Bai
.\" Modified Tue Oct 22 22:04:23 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified, 5 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified, 19 Sep 2002, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Added SHM_REMAP flag description
.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added notes on capability requirements
.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Language and formatting clean-ups
.\"	Changed wording and placement of sentence regarding attachment
.\"		of segments marked for destruction
.\"
.\" FIXME . Add an example program to this page.
.\"
.TH SHMOP 2 2014-07-08 "Linux" "Linux Programmer's Manual"
.SH NAME
shmat, shmdt \- System V shared memory operations
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/shm.h>

.BI "void *shmat(int " shmid ", const void *" shmaddr ", int " shmflg );

.BI "int shmdt(const void *" shmaddr );
.fi
.SH DESCRIPTION
.SS shmat()
.BR shmat ()
attaches the System\ V shared memory segment identified by
.I shmid
to the address space of the calling process.
The attaching address is specified by
.I shmaddr
with one of the following criteria:
.IP * 3
If
.I shmaddr
is NULL,
the system chooses a suitable (unused) address at which to attach
the segment.
.IP *
If
.I shmaddr
isn't NULL
and
.B SHM_RND
is specified in
.IR shmflg ,
the attach occurs at the address equal to
.I shmaddr
rounded down to the nearest multiple of
.BR SHMLBA .
.IP *
Otherwise,
.I shmaddr
must be a page-aligned address at which the attach occurs.
.PP
In addition to
.BR SHM_RND ,
the following flags may be specified in the
.I shmflg
bit-mask argument:
.TP
.BR SHM_EXEC " (Linux-specific; since Linux 2.6.9)"
Allow the contents of the segment to be executed.
The caller must have execute permission on the segment.
.TP
.BR SHM_RDONLY
Attach the segment for read-only access.
The process must have read permission for the segment.
If this flag is not specified,
the segment is attached for read and write access,
and the process must have read and write permission for the segment.
There is no notion of a write-only shared memory segment.
.TP
.BR SHM_REMAP " (Linux-specific)"
This flag specifies
that the mapping of the segment should replace
any existing mapping in the range starting at
.I shmaddr
and continuing for the size of the segment.
(Normally, an
.B EINVAL
error would result if a mapping already exists in this address range.)
In this case,
.I shmaddr
must not be NULL.
.PP
The
.BR brk (2)
value of the calling process is not altered by the attach.
The segment will automatically be detached at process exit.
The same segment may be attached as a read and as a read-write
one, and more than once, in the process's address space.
.PP
A successful
.BR shmat ()
call updates the members of the
.I shmid_ds
structure (see
.BR shmctl (2))
associated with the shared memory segment as follows:
.IP
.I shm_atime
is set to the current time.
.IP
.I shm_lpid
is set to the process-ID of the calling process.
.IP
.I shm_nattch
is incremented by one.
.\"
.SS shmdt()
.BR shmdt ()
detaches the shared memory segment located at the address specified by
.I shmaddr
from the address space of the calling process.
The to-be-detached segment must be currently
attached with
.I shmaddr
equal to the value returned by the attaching
.BR shmat ()
call.
.PP
On a successful
.BR shmdt ()
call, the system updates the members of the
.I shmid_ds
structure associated with the shared memory segment as follows:
.IP
.I shm_dtime
is set to the current time.
.IP
.I shm_lpid
is set to the process-ID of the calling process.
.IP
.I shm_nattch
is decremented by one.
If it becomes 0 and the segment is marked for deletion,
the segment is deleted.
.SH RETURN VALUE
On success,
.BR shmat ()
returns the address of the attached shared memory segment; on error,
.I (void\ *)\ \-1
is returned, and
.I errno
is set to indicate the cause of the error.

On success,
.BR shmdt ()
returns 0; on error \-1 is returned, and
.I errno
is set to indicate the cause of the error.
.SH ERRORS
When
.BR shmat ()
fails,
.I errno
is set to one of the following:
.TP
.B EACCES
The calling process does not have the required permissions for
the requested attach type, and does not have the
.B CAP_IPC_OWNER
capability.
.TP
.B EIDRM
\fIshmid\fP points to a removed identifier.
.TP
.B EINVAL
Invalid
.I shmid
value, unaligned (i.e., not page-aligned and \fBSHM_RND\fP was not
specified) or invalid
.I shmaddr
value, or can't attach segment at
.IR shmaddr ,
or
.B SHM_REMAP
was specified and
.I shmaddr
was NULL.
.TP
.B ENOMEM
Could not allocate memory for the descriptor or for the page tables.
.PP
When
.BR shmdt ()
fails,
.I errno
is set as follows:
.TP
.B EINVAL
There is no shared memory segment attached at
.IR shmaddr ;
or,
.\" The following since 2.6.17-rc1:
.I shmaddr
is not aligned on a page boundary.
.SH CONFORMING TO
SVr4, POSIX.1-2001.
.\" SVr4 documents an additional error condition EMFILE.

In SVID 3 (or perhaps earlier),
the type of the \fIshmaddr\fP argument was changed from
.I "char\ *"
into
.IR "const void\ *" ,
and the returned type of
.BR shmat ()
from
.I "char\ *"
into
.IR "void\ *" .
(Linux libc4 and libc5 have the
.I "char\ *"
prototypes; glibc2 has
.IR "void\ *" .)
.SH NOTES
.PP
After a
.BR fork (2),
the child inherits the attached shared memory segments.

After an
.BR execve (2),
all attached shared memory segments are detached from the process.

Upon
.BR _exit (2),
all attached shared memory segments are detached from the process.

Using
.BR shmat ()
with
.I shmaddr
equal to NULL
is the preferred, portable way of attaching a shared memory segment.
Be aware that the shared memory segment attached in this way
may be attached at different addresses in different processes.
Therefore, any pointers maintained within the shared memory must be
made relative (typically to the starting address of the segment),
rather than absolute.
.PP
On Linux, it is possible to attach a shared memory segment even if it
is already marked to be deleted.
However, POSIX.1-2001 does not specify this behavior and
many other implementations do not support it.
.LP
The following system parameter affects
.BR shmat ():
.TP
.\" FIXME A good explanation of the rationale for the existence
.\" of SHMLBA would be useful here
.B SHMLBA
Segment low boundary address multiple.
When explicitly specifying an attach address in a call to
.BR shmat (),
the caller should ensure that the address is a multiple of this value.
This is necessary on some architectures,
in order either to ensure good CPU cache performance or to ensure that
different attaches of the same segment have consistent views
within the CPU cache.
.B SHMLBA
is normally some multiple of the system page size
(on many Linux architectures, it is the same as the system page size).
.PP
The implementation places no intrinsic per-process limit on the
number of shared memory segments
.RB ( SHMSEG ).
.SH SEE ALSO
.BR brk (2),
.BR mmap (2),
.BR shmctl (2),
.BR shmget (2),
.BR capabilities (7),
.BR shm_overview (7),
.BR svipc (7)