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