NAME

shmat — XSI shared memory attach operation

SYNOPSIS

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

void *shmat(int shmid, const void *shmaddr, int shmflg);

DESCRIPTION

The shmat() function operates on XSI shared memory (see XBD 3.332 Shared Memory Object). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in 2.8 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 ((char *)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 (shmflg & SHM_RDONLY) 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. Also, the shm_atime timestamp shall be set to the current time, as described in 2.7.1 IPC General Description.

Otherwise, the shared memory segment shall not be attached, shmat() shall return SHM_FAILED, 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 2.7 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 ((char *)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.

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 2.7 XSI Interprocess Communication can be easily modified to use the alternative interfaces.

RATIONALE

The symbol SHM_FAILED is used for the failure return of shmat() for consistency with MAP_FAILED for mmap(). However, SHM_FAILED is required to have the same value as ((void *)(intptr_t)-1) to provide backwards compatibility for applications written to earlier versions of this standard, where the failure return was specified as (void *)-1. This means that implementations need to ensure that shmat() cannot return ((void *)(intptr_t)-1) on a successful call.

FUTURE DIRECTIONS

None.

SEE ALSO

2.7 XSI Interprocess Communication, 2.8 Realtime, exec, exit, fork, shmctl, shmdt, shmget, shm_open, shm_unlink

XBD 3.332 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.

Issue 8

Austin Group Defect 1237 is applied, changing "shmaddr - ..." to "(char *)shmaddr - ...".

Austin Group Defect 1238 is applied, changing the DESCRIPTION to avoid an ambiguous use of "it".

Austin Group Defect 1239 is applied, adding SHM_FAILED.