'\" et
.TH SHMAT "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
shmat
\(em XSI shared memory attach operation
.SH SYNOPSIS
.LP
.nf
#include <sys/shm.h>
.P
void *shmat(int \fIshmid\fP, const void *\fIshmaddr\fP, int \fIshmflg\fP);
.fi
.SH DESCRIPTION
The
\fIshmat\fR()
function operates on XSI shared memory (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.346" ", " "Shared Memory Object").
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
.IR "Section 2.8" ", " "Realtime".
.P
The
\fIshmat\fR()
function attaches the shared memory segment associated with the shared
memory identifier specified by
.IR shmid
to the address space of the calling process. The segment is attached
at the address specified by one of the following criteria:
.IP " *" 4
If
.IR shmaddr
is a null pointer, the segment is attached at the first available
address as selected by the system.
.IP " *" 4
If
.IR shmaddr
is not a null pointer and (\fIshmflg\fP &SHM_RND)
is non-zero, the segment is attached at the address given by
(\fIshmaddr\fP \-((\fIuintptr_t\fP)\fIshmaddr\fP %SHMLBA)). The
character
.BR '%' 
is the C-language remainder operator.
.IP " *" 4
If
.IR shmaddr
is not a null pointer and (\fIshmflg\fP &SHM_RND) is 0, the segment is
attached at the address given by
.IR shmaddr .
.IP " *" 4
The segment is attached for reading if (\fIshmflg\fP &SHM_RDONLY)
is non-zero and the calling process has read permission; otherwise, if
it is 0 and the calling process has read and write permission, the
segment is attached for reading and writing.
.SH "RETURN VALUE"
Upon successful completion,
\fIshmat\fR()
shall increment the value of
.IR shm_nattch
in the data structure associated with the shared memory ID of the
attached shared memory segment and return the segment's start address.
Also, the
.IR shm_atime
timestamp shall be set to the current time, as described in
.IR "Section 2.7.1" ", " "IPC General Description".
.P
Otherwise, the shared memory segment shall not be attached,
\fIshmat\fR()
shall return (\fBvoid *\fP)\-1, and
.IR errno
shall be set to indicate the error.
.SH ERRORS
The
\fIshmat\fR()
function shall fail if:
.TP
.BR EACCES
Operation permission is denied to the calling process; see
.IR "Section 2.7" ", " "XSI Interprocess Communication".
.TP
.BR EINVAL
The value of
.IR shmid
is not a valid shared memory identifier, the
.IR shmaddr
is not a null pointer, and the value of
(\fIshmaddr\fP \-((\fIuintptr_t\fP)\fIshmaddr\fP %SHMLBA))
is an illegal address for attaching shared memory; or the
.IR shmaddr
is not a null pointer, (\fIshmflg\fP &SHM_RND) is 0, and the value of
.IR shmaddr
is an illegal address for attaching shared memory.
.TP
.BR EMFILE
The number of shared memory segments attached to the calling process
would exceed the system-imposed limit.
.TP
.BR ENOMEM
The available data space is not large enough to accommodate the shared
memory segment.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
None.
.SH "APPLICATION USAGE"
The POSIX Realtime Extension defines alternative interfaces for interprocess
communication. Application developers who need to use IPC should
design their applications so that modules using the IPC routines
described in
.IR "Section 2.7" ", " "XSI Interprocess Communication"
can be easily modified to use the alternative interfaces.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.7" ", " "XSI Interprocess Communication",
.IR "Section 2.8" ", " "Realtime",
.IR "\fIexec\fR\^",
.IR "\fIexit\fR\^(\|)",
.IR "\fIfork\fR\^(\|)",
.IR "\fIshmctl\fR\^(\|)",
.IR "\fIshmdt\fR\^(\|)",
.IR "\fIshmget\fR\^(\|)",
.IR "\fIshm_open\fR\^(\|)",
.IR "\fIshm_unlink\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.346" ", " "Shared Memory Object",
.IR "\fB<sys_shm.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .