NAME

shmop, shmat, shmdt − shared memory operations

SYNOPSIS

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

void ∗shmat(int shmid, void ∗shmaddr, int shmflg);

int shmdt(void ∗shmaddr);

DESCRIPTION

shmat() 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:

 00400READ by user
00200WRITE by user
00040READ by group
00020WRITE by group
00004READ by others
00002WRITE 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 aligned address.  See NOTES for the alignment requirement. 

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}. 

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. 

shmdt() detaches from the calling process’s data segment the shared memory segment located at the address specified by shmaddr.

RETURN VALUES

Upon successful completion, the return value is as follows:

shmat() returns the data segment start address of the attached shared memory segment. 

shmdt() returns a value of 0. 

Otherwise, a value of −1 is returned and errno is set to indicate the error. 

ERRORS

shmat() fails and does not attach the shared memory segment if one or more of the following are true:

EACCES Operation permission is denied to the calling process (see intro(2)). 

EINVAL shmid is not a valid shared memory identifier. 

EINVAL shmaddr is not equal to zero, and the value of (shmaddr - (shmaddr modulus SHMLBA)) is an illegal address. 

EINVAL shmaddr is not equal to zero, (shmflg&SHM_RND) is false, and the value of shmaddr is an illegal address. 

EINVAL shmaddr is not equal to zero, (shmfg&SHM_SHARE_MMU) is true, and shmaddr is not aligned properly. 

EINVAL shmdt() fails and does not detach the shared memory segment if shmaddr is not the data segment start address of a shared memory segment. 

EINVAL SHM_SHARE_MMU has been used inconsistently. See BUGS below. 

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. 

SEE ALSO

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

NOTES

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

The alignment requirement, which varies on different machines, is determined by the mapping size of the virtual memory system. 

BUGS

For a shared memory segment, either every shmat() uses SHM_SHARE_MMU or none at all, never mixed. 

SunOS 5.2  —  Last change: 3 Mar 1993