.\" of segments marked for destruction
.\"
.\" FIXME . Add an example program to this page.
-.\" FIXME Linux 2.6.9 added SHM_EXEC, which should be documented
-.TH SHMOP 2 2013-02-12 "Linux" "Linux Programmer's Manual"
+.\"
+.TH SHMOP 2 2014-07-08 "Linux" "Linux Programmer's Manual"
.SH NAME
-shmat, shmdt \- shared memory operations
+shmat, shmdt \- System V shared memory operations
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.BI "int shmdt(const void *" shmaddr );
.fi
.SH DESCRIPTION
+.SS shmat()
.BR shmat ()
-attaches the shared memory segment identified by
+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:
-.LP
+.IP * 3
If
.I shmaddr
is NULL,
the system chooses a suitable (unused) address at which to attach
the segment.
-.LP
+.IP *
If
.I shmaddr
isn't NULL
.I shmaddr
rounded down to the nearest multiple of
.BR SHMLBA .
-Otherwise
+.IP *
+Otherwise,
.I shmaddr
must be a page-aligned address at which the attach occurs.
.PP
-If
-.B SHM_RDONLY
-is specified in
-.IR shmflg ,
-the segment is attached for reading and the process must have
-read permission for the segment.
-Otherwise the segment is attached for read and write
+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.
-.PP
-The (Linux-specific)
-.B SHM_REMAP
-flag may be specified in
-.I shmflg
-to indicate that the mapping of the segment should replace
+.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
+(Normally, an
.B EINVAL
error would result if a mapping already exists in this address range.)
In this case,
.IP
.I shm_nattch
is incremented by one.
-.PP
+.\"
+.SS shmdt()
.BR shmdt ()
detaches the shared memory segment located at the address specified by
.I shmaddr
.PP
On a successful
.BR shmdt ()
-call the system updates the members of the
+call, the system updates the members of the
.I shmid_ds
structure associated with the shared memory segment as follows:
.IP
is decremented by one.
If it becomes 0 and the segment is marked for deletion,
the segment is deleted.
-.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.
.SH RETURN VALUE
-On success
+On success,
.BR shmat ()
-returns the address of the attached shared memory segment; on error
+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
+On success,
.BR shmdt ()
returns 0; on error \-1 is returned, and
.I errno
SVr4, POSIX.1-2001.
.\" SVr4 documents an additional error condition EMFILE.
-In SVID 3 (or perhaps earlier)
+In SVID 3 (or perhaps earlier),
the type of the \fIshmaddr\fP argument was changed from
-.I "char *"
+.I "char\ *"
into
-.IR "const void *" ,
+.IR "const void\ *" ,
and the returned type of
.BR shmat ()
from
-.I "char *"
+.I "char\ *"
into
-.IR "void *" .
-(Linux libc4 and libc5 have the
-.I "char *"
-prototypes; glibc2 has
-.IR "void *" .)
+.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
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.
-Must be page aligned.
-For the current implementation the
+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
-value is
-.BR PAGE_SIZE .
-.\" FIXME That last sentence isn't true for all Linux
-.\" architectures (i.e., SHMLBA != PAGE_SIZE for some architectures)
-.\" -- MTK, Nov 04
+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 limit on the per-process maximum
+The implementation places no intrinsic per-process limit on the
number of shared memory segments
.RB ( SHMSEG ).
.SH SEE ALSO
.BR capabilities (7),
.BR shm_overview (7),
.BR svipc (7)
+.SH COLOPHON
+This page is part of release 3.79 of the Linux
+.I man-pages
+project.
+A description of the project,
+information about reporting bugs,
+and the latest version of this page,
+can be found at
+\%http://www.kernel.org/doc/man\-pages/.
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