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Manual page for shmop(2)

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.

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) is true, then the segment is attached to the first available suitably aligned address. When (shmflg&SHM_SHARE_MMU) 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 *.

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 and errno is set to indicate the error.

ERRORS

The shmat() function fails and does not attach the shared memory segment 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 fails and does not detach the shared memory segment if:

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

NOTES

The user must explicitly remove shared memory segments after the last reference to them has been removed.

Last modified 07/October/97