.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\"
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.\" manual page may be incorrect or out-of-date.  The author(s) assume no
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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>
.\"
.TH SHMOP 2 "November 28, 1993" "Linux 0.99.13" "Linux Programmer's Manual" 
.SH NAME
shmop \- shared memory operations
.SH SYNOPSIS
.nf
.B
# include <sys/types.h>
.B
# include <sys/shm.h>
.fi
.sp
.BI "void *shmat ( int " shmid ,
.BI "const void *" shmaddr ,
.BI "int " shmflg " )"
.sp
.BI "int shmdt ( const void *" shmaddr ")"
.SH DESCRIPTION
The function
.B shmat
attaches the shared memory segment identified by
.B shmid
to the data segment of the calling process.
The attaching address is specified by
.I shmaddr
with one of the following criteria:
.IP
If
.I shmaddr
is
.BR 0 ,
the system tries to find an unmapped region in the range 1 \- 1.5G
starting from the upper value and coming down from there.
.IP
If
.I shmaddr
isn't
.B 0
and
.B SHM_RND
is asserted in
.IR shmflg ,
the attach occurs at address equal to the rounding down of
.I shmaddr
to a multiple of
.BR SHMLBA .
Otherwise
.I shmaddr
must be a page aligned address at which the attach occurs.
.PP
If
.B SHM_RDONLY
is asserted in
.IR shmflg ,
the segment is attached for reading and the process must have
read access permissions to the segment.
Otherwise the segment is attached for read and write
and the process must have read and write access permissions to the segment.
There is no notion of write-only shared memory segment.
.PP
The
.B brk
value of the calling process is not altered by the attach.
The segment will automatically 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
On a successful
.B shmat
call the system updates the members of the structure
.B shmid_ds
associated to the shared memory segment as follows:
.IP
.B shm_atime
is set to the current time.
.IP
.B shm_lpid
is set to the process-ID of the calling process.
.IP
.B shm_nattch
is incremented by one.
.PP
Note that the attach succeeds also if the shared memory segment is
marked as to be deleted.
.PP
The function
.B shmdt
detaches from the calling process's data segment the shared memory
segment located at the address specified by
.IR shmaddr .
The detaching shared memory segment must be one among the currently
attached ones (to the process's address space) with
.I shmaddr
equal to the value returned by the its attaching
.B shat
call.
.PP
On a successful
.B shmdt
call the system updates the members of the structure
.B shmid_ds
associated to the shared memory segment as follows:
.IP
.B shm_dtime
is set to the current time.
.IP
.B shm_lpid
is set to the process-ID of the calling process.
.IP
.B shm_nattch
is decremented by one.
If it becomes 0 and the segment is marked for deletion,
the segment is deleted.
.PP
The occupied region in the user space of the calling process is
unmapped.
.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"
On a failure both functions return
.B \-1
with
.B errno
indicating the error,
otherwise
.B shmat
returns the address of the attached shared memory segment, and
.B shmdt
returns
.BR 0 .
.SH ERRORS
When
.B shmat
fails, at return
.B errno
will be set to one among the following values:
.TP 11
.B EACCES
The calling process has no access permissions for the requested attach
type.
.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 failing attach at
.BR brk .
.TP
.B ENOMEM
Could not allocate memory for the descriptor or for the page tables.
.PP
The function
.B shmdt
can fails only if there is no shared memory segment attached at
.IR shmaddr ,
in such a case at return
.B errno
will be set to
.BR EINVAL .
.SH NOTES
On executing a
.BR fork (2)
system call, the child inherits all the attached shared memory segments.
.PP
The shared memory segments attached to a process executing an
.BR execve (2)
system call will not be attached to the resulting process.
.PP
The following is a system parameter affecting a
.B shmat
system call:
.TP 11
.B SHMLBA
Segment low boundary address multiple.
Must be page aligned.
For the current implementation the
.B SHMBLA
value is
.BR PAGE_SIZE .
.PP
The implementation has no intrinsic limit to the per process maximum
number of shared memory segments
.RB ( SHMSEG )
.SH "CONFORMING TO"
SVr4, SVID.  SVr4 documents an additional error condition EMFILE.
In SVID-v4 the type of the \fIshmaddr\fP argument was changed from
.B "char *"
into
.BR "const void *" ,
and the returned type of \fIshmat\fP() from
.B "char *"
into
.BR "void *" .
(Linux libc4 and libc5 have the
.B "char *"
prototypes; glibc2 has
.BR "void *" .)
.SH "SEE ALSO"
.BR ipc (5),
.BR shmctl (2),
.BR shmget (2)