The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group
A newer edition of this document exists here

NAME

shmat - XSI shared memory attach operation

SYNOPSIS

[XSI] [Option Start] #include <sys/shm.h>

void *shmat(int
shmid, const void *shmaddr, int shmflg); [Option End]

DESCRIPTION

The shmat() function operates on XSI shared memory (see XBD 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:

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. Also, the shm_atime timestamp shall be set to the current time, as described in IPC General Description.

Otherwise, the shared memory segment shall not be attached, shmat() shall return (void *)-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

XBD Shared Memory Object, <sys/shm.h>

CHANGE HISTORY

First released in Issue 2. Derived from Issue 2 of the SVID.

Issue 5

Moved from SHARED MEMORY to BASE.

The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to a new APPLICATION USAGE section.

Issue 6

The Open Group Corrigendum U021/13 is applied.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0572 [345] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0327 [522] is applied.

End of informative text.

 

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