SHMAT
Section: POSIX Programmer's Manual (P)
Updated: 2003
NAME
shmat - 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 Definitions 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 segment 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 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.
EXAMPLES
None.
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 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 .
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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