shmat - shared memory attach operation

Syntax

#include <sys/shm.h>

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

Description

shmat() attaches the shared memory segment designated by the shared memory identifier
shmid to the data segment of the calling process. The location at which the segment is
attached is determined by the following criteria:

• If shmaddr is equal to 0, the segment is attached at the first free address found by the
  system.

• If shmaddr and (shmflg & SHM_RND) are not equal to 0, the segment is attached at the
  address given by (shmaddr -((ptrdiff_t) shmaddr % SHMLBA)).
  (The character % is the C-language remainder operator.)

• If shmaddr is not equal to 0 and (shmflg & SHM_RND) is equal to 0, the segment is attached
  at the address specified with shmaddr.

• If (shmflg & SHM_RDONLY) is not equal to 0 and the calling process has read permission,
  the segment is attached for reading.

• If (shmflg & SHM_RDONLY) is not equal to 0 and the calling process has read and write
  permission, the segment is attached for reading and writing.

The following symbolic names are defined in the header file sys/shm.h:

Name	Description
SHMLBA	Multiple of the address of the lower segment boundary
SHM_RDONLY	Attach only for reading
SHM_RND	Round up attachment address

Return val.

Start address of the data segment for the shared memory area

if successful. The value of shm_nattach is incremented in the data
structure associated with the shared memory ID.

 

-1

if an error occurs. The shared memory segment is not attached. errno is
set to indicate the error.

Errors

shmat() will fail if: 

 

EACCES

The calling process is denied the access permissions required for the
operation.

 

EINVAL

The value of shmid is not a valid shared memory ID, or
the value of shmaddr is not equal to 0 and the value of
(shmaddr -((ptrdiff_t) shmaddr % SHMLBA)) is an invalid address for
attaching shared memory, or
the value of shmaddr is not equal to 0, (shmflg & SHM_RND) is equal to 0 and
the value of shmaddr is an invalid address for attaching shared memory.

 

EMFILE

The number of attached shared memory segments for the calling process
would exceed the system-imposed limit.

 

ENOMEM

The available data space is not large enough to accommodate the shared
memory segment.

Notes

The IEEE 1003.4 Standards Committee is developing alternative interfaces for interprocess
communication. Application developers who need to use interprocess communication (IPC)
should design their applications so that modules using the IPC routines described here can
be easily modified at a later date. 

See also

exec, exit(), fork(), shmctl(), shmdt(), shmget(), sys/shm.h, section “Interprocess communication”.