SHMAT(3) Library Functions Manual SHMAT(3)

shmat — XSI shared memory attach operation


#include <sys/shm.h>

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

The shmat () function operates on XSI shared memory (see XBD V1_chap03(7)). It is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in V2_chap02(3).

The shmat () function attaches the shared memory segment associated with the shared memory identifier specified by 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 - (( ) 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.

Upon successful completion, shmat () shall increment the value of 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 timestamp shall be set to the current time, as described in V2_chap02(3).

Otherwise, the shared memory segment shall not be attached, shmat () shall return SHM_FAILED, and shall be set to indicate the error.

The shmat () function shall fail if:

Operation permission is denied to the calling process; see .
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.
The number of shared memory segments attached to the calling process would exceed the system-imposed limit.
The available data space is not large enough to accommodate the shared memory segment.

None.

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 V2_chap02(3) can be easily modified to use the alternative interfaces.

The symbol SHM_FAILED is used for the failure return of shmat () for consistency with MAP_FAILED for mmap(3). 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.

None.

V2_chap02(3), V2_chap02(3), exec(3), exit(3), fork(3), shmctl(3), shmdt(3), shmget(3), shm_open(3), shm_unlink(3)

XBD V1_chap03(7), <sys/shm.h>

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

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.

The Open Group Corrigendum U021/13 is applied.

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.

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.

June 14, 2024 posix.fail