SHMAT(P) POSIX Programmer's Manual SHMAT(P)
shmat - XSI shared memory attach operation
void *shmat(int shmid, const void *shmaddr, int shmflg);
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
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 charac-
ter '%' 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.
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 XSI Interprocess Commu-
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 ille-
gal 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 attach-
ing 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 seg-
The following sections are informative.
The POSIX Realtime Extension defines alternative interfaces for interprocess communica-
tion. 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.
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>
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SHMAT(P)