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Copyright 1989 AT&T
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#include <sys/types.h> #include <sys/shm.h> void *shmat(int shmid, const void *shmaddr, int shmflg);
int shmdt(char *shmaddr);
int shmdt(const void *shmaddr);
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.
For shared memory segments created with the SHM_SHARE_MMU or SHM_PAGEABLE flags, the default protections cannot be changed so as to prevent a single process from affecting other processes sharing the same shared segment.
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.
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.
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. The shmaddr argument is not equal to 0, and the value of (shmaddr- (shmaddr modulus SHMLBA)) is an illegal address. The shmaddr argument is not equal to 0, is an illegal address, and (shmflg&SHM_RND) is false. The shmaddr argument is not equal to 0, is not properly aligned, and (shmfg&SHM_SHARE_MMU) is true. SHM_SHARE_MMU is not supported in certain architectures. Both (shmflg&SHM_SHARE_MMU) and (shmflg&SHM_PAGEABLE) are true. (shmflg&SHM_SHARE_MMU) is true and the shared memory segment specified by shmid() had previously been attached by a call to shmat() in which (shmflg&SHM_PAGEABLE) was true. (shmflg&SHM_PAGEABLE) is true and the shared memory segment specified by shmid() had previously been attached by a call to shmat() in which (shmflg&SHM_SHARE_MMU) was true.
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.
ENOMEM
(shmflg&SHM_SHARE_MMU) is true and attaching to the shared memory segment would exceed a limit or resource control on locked memory.
Using a fixed value for the shmaddr argument can adversely affect performance on certain platforms due to D-cache aliasing.
See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
| Interface Stability | Committed |
| MT-Level | Async-Signal-Safe |
| Standard | See standards(5). |
Intro(2), exec(2), exit(2), fork(2), shmctl(2), shmget(2), attributes(5), standards(5)