Section: POSIX Programmer's Manual (3P)
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.
--- XSI shared memory attach operation
void *shmat(int shmid, const void *shmaddr, int shmflg);
function operates on XSI shared memory (see the Base Definitions volume of POSIX.1-2017,
, Shared Memory Object).
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
function attaches the shared memory segment associated with the shared
memory identifier specified by
to the address space of the calling process. The segment is attached
at the address specified by one of the following criteria:
is a null pointer, the segment is attached at the first available
address as selected by the system.
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
is the C-language remainder operator.
is not a null pointer and (shmflg &SHM_RND) is 0, the segment is
attached at the address given by
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,
shall increment the value of
in the data structure associated with the shared memory ID of the
attached shared memory segment and return the segment's start address.
timestamp shall be set to the current time, as described in
, IPC General Description.
Otherwise, the shared memory segment shall not be attached,
shall return (void *)-1, and
shall be set to indicate the error.
function shall fail if:
Operation permission is denied to the calling process; see
Section 2.7, XSI Interprocess Communication.
The value of
is not a valid shared memory identifier, the
is not a null pointer, and the value of
(shmaddr -((uintptr_t)shmaddr %SHMLBA))
is an illegal address for attaching shared memory; or the
is not a null pointer, (shmflg &SHM_RND) is 0, and the value of
is an illegal address for attaching shared memory.
The number of shared memory segments attached to the calling process
would exceed the system-imposed limit.
The available data space is not large enough to accommodate the shared
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
, XSI Interprocess Communication
can be easily modified to use the alternative interfaces.
, XSI Interprocess Communication,
The Base Definitions volume of POSIX.1-2017,
Section 3.346, Shared Memory Object,
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
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