SHMAT(P) POSIX Programmer's Manual SHMAT(P)NAMEshmat - XSI shared memory attach operation
SYNOPSIS
#include <sys/shm.h>
void *shmat(int shmid, const void *shmaddr, int shmflg);
DESCRIPTION
The shmat() function operates on XSI shared memory (see the Base Defi‐
nitions volume of IEEE Std 1003.1-2001, Section 3.340, Shared Memory
Object). It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in 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 seg‐
ment is attached for reading and writing.
RETURN VALUE
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.
Otherwise, the shared memory segment shall not be attached, shmat()
shall return -1, and errno shall be set to indicate the error.
ERRORS
The shmat() function shall fail if:
EACCES Operation permission is denied to the calling process; see 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 attach‐
ing 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.
EXAMPLES
None.
APPLICATION USAGE
The POSIX Realtime Extension defines alternative interfaces for inter‐
process communication. Application developers who need to use IPC
should design their applications so that modules using the IPC routines
described in XSI Interprocess Communication can be easily modified to
use the alternative interfaces.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
XSI Interprocess Communication , Realtime , exec() , exit() , fork() ,
shmctl() , shmdt() , shmget() , shm_open() , shm_unlink() , the Base
Definitions volume of IEEE Std 1003.1-2001, <sys/shm.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
IEEE/The Open Group 2003 SHMAT(P)
