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.
The shmat() function operates on XSI shared memory (see the Base
Definitions volume of POSIX.1‐2008, Section 3.342, Shared MemoryObject). It is unspecified whether this function interoperates with
the realtime interprocess communication facilities defined in Section2.8, Realtime.
The shmat() function attaches the shared memory segment associated
with the shared memory identifier specified by shmid to the address
space of the calling process. The segment is attached at the address
specified by one of the following criteria:
* If shmaddr is a null pointer, the segment is attached at the
first available address as selected by the system.
* If shmaddr is not a null pointer and (shmflg &SHM_RND) is non-
zero, the segment is attached at the address given by (shmaddr
−((uintptr_t)shmaddr %SHMLBA)). The character '%' is the C-
language remainder operator.
* If shmaddr is not a null pointer and (shmflg &SHM_RND) is 0, the
segment is attached at the address given by shmaddr.
* The segment is attached for reading if (shmflg &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.
Upon successful completion, shmat() shall increment the value of
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 shm_atime timestamp shall be set to the current
time, as described in Section 2.7.1, IPC General Description.
Otherwise, the shared memory segment shall not be attached, shmat()
shall return −1, and errno shall be set to indicate the error.
The shmat() function shall fail if:
EACCES Operation permission is denied to the calling process; see
Section 2.7, XSI Interprocess Communication.
EINVAL The value of shmid is not a valid shared memory identifier,
the shmaddr is not a null pointer, and the value of (shmaddr
−((uintptr_t)shmaddr %SHMLBA)) is an illegal address for
attaching shared memory; or the shmaddr is not a null pointer,
(shmflg &SHM_RND) is 0, and the value of shmaddr is an illegal
address for attaching shared memory.
EMFILE The number of shared memory segments attached to the calling
process would exceed the system-imposed limit.
ENOMEM The available data space is not large enough to accommodate
the shared memory segment.
The following sections are informative.
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 Section 2.7, XSI Interprocess Communication can
be easily modified to use the alternative interfaces.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
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
IEEE/The Open Group 2013 SHMAT(3P)