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System Callsshmop(2)


NAME

 shmop, shmat, shmdt - shared memory operations

SYNOPSIS

 
#include <sys/types.h>
#include <sys/shm.h>
void *shmat(int shmid, const void *shmaddr, int shmflg);

Default

 
 int shmdt(char *shmaddr);

Standard conforming

 
 int shmdt(const void *shmaddr);

DESCRIPTION

 

The shmat() function attaches the shared memory segment associated with the shared memory identifier specified by shmid to the data segment of the calling process.

The permission required for a shared memory control operation is given as {token}, where token is the type of permission needed. The types of permission are interpreted as follows:

 
00400    READ by user
00200    WRITE by user
00040    READ by group
00020    WRITE by group
00004    READ by others
00002    WRITE by others

See the Shared Memory Operation Permissions section of intro(2) for more information.

When (shmflg&SHM_SHARE_MMU) is true, virtual memory resources in addition to shared memory itself are shared among processes that use the same shared memory.

When (shmflg&SHM_PAGEABLE) is true, virtual memory resources are shared and the dynamic shared memory (DISM) framework is created. The dynamic shared memory can be resized dynamically within the specified size in shmget(2). The DISM shared memory is pageable unless it is locked.

The shared memory segment is attached to the data segment of the calling process at the address specified based on one of the following criteria:

  • If shmaddr is equal to (void *) 0, the segment is attached to the first available address as selected by the system.
  • If shmaddr is equal to (void *) 0 and ( shmflg&SHM_SHARE_MMU) or (shmflg&SHM_PAGEABLE) is true, then the segment is attached to the first available suitably aligned address. When (shmflg&SHM_SHARE_MMU) or (shmflg&SHM_PAGEABLE) is set, however, the permission given by shmget() determines whether the segment is attached for reading or reading and writing.
  • If shmaddr is not equal to (void *) 0 and (shmflg&SHM_RND) is true, the segment is attached to the address given by (shmaddr - (shmaddr modulus SHMLBA)).
  • If shmaddr is not equal to (void *) 0 and (shmflg&SHM_RND) is false, the segment is attached to the address given by shmaddr.
  • The segment is attached for reading if (shmflg&SHM_RDONLY) is true {READ}, otherwise it is attached for reading and writing {READ/WRITE}.

The shmdt() function detaches from the calling process's data segment the shared memory segment located at the address specified by shmaddr. If the application is standard-conforming (see standards(5)), the shmaddr argument is of type const void *. Otherwise it is of type char *.

Shared memory segments must be explicitly removed after the last reference to them has been removed.

RETURN VALUES

 

Upon successful completion, shmat() returns the data segment start address of the attached shared memory segment; shmdt() returns 0. Otherwise, -1 is returned, the shared memory segment is not attached, and errno is set to indicate the error.

ERRORS

 

The shmat() function will fail if:

EACCES
Operation permission is denied to the calling process (see intro(2)).
EINVAL
The shmid argument is not a valid shared memory identifier.
EINVAL
The shmaddr argument is not equal to 0, and the value of (shmaddr - (shmaddr modulus SHMLBA)) is an illegal address.
EINVAL
The shmaddr argument is not equal to 0, is an illegal address, and (shmflg&SHM_RND) is false.
EINVAL
The shmaddr argument is not equal to 0, is not properly aligned, and (shmfg&SHM_SHARE_MMU) is true.
EINVAL
SHM_SHARE_MMU is not supported in certain architectures.
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 shmdt() function will fail if:

EINVAL
The shmaddr argument is not the data segment start address of a shared memory segment.

SEE ALSO

 

intro(2), exec(2), exit(2), fork(2), shmctl(2), shmget(2), standards(5)


SunOS 5.9Go To TopLast Changed 15 May 2000

 
      
      
Copyright 2002 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms.