System Calls shmop(2)
NAME
shmop, shmat, shmdt - shared memory operations
SYNOPSIS
#include
#include
void *shmat(int shmid, const void *shmaddr, int shmflg); 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 opera-
tion is given as {token}, where token is the type of permis-
sion needed. The types of permission are interpreted as fol-
lows: 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.SunOS 5.11 Last change: 10 Mar 2008 1
System Calls shmop(2)
When (shmflg&SHM_PAGEABLE) is true, virtual memory resources
are shared and the dynamic shared memory (DISM) framework iscreated. The dynamic shared memory can be resized dynami-
cally 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: o If shmaddr is equal to (void *) 0, the segment is attached to the first available address as selected by the system. o 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. o 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)). o If shmaddr is not equal to (void *) 0 and(shmflg&SHM_RND) is false, the segment is attached
to the address given by shmaddr. o 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
SunOS 5.11 Last change: 10 Mar 2008 2
System Calls shmop(2)
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 pro-
cess (see Intro(2)). EINVAL The shmid argument is not a valid shared memory identifier. The shmaddr argument is not equal to 0, and thevalue of (shmaddr- (shmaddr modulus SHMLBA)) is
an illegal address. The shmaddr argument is not equal to 0, is anillegal address, and (shmflg&SHM_RND) is false.
The shmaddr argument is not equal to 0, is notproperly aligned, and (shmfg&SHM_SHARE_MMU) is
true.SHM_SHARE_MMU is not supported in certain archi-
tectures.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 tothe calling process would exceed the system-
imposed limit. ENOMEM The available data space is not large enough to accommodate the shared memory segment.SunOS 5.11 Last change: 10 Mar 2008 3
System Calls shmop(2)
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. WARNINGS Using a fixed value for the shmaddr argument can adverselyaffect performance on certain platforms due to D-cache
aliasing.ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Committed ||_____________________________|_____________________________|
| MT-Level | Async-Signal-Safe |
|_____________________________|_____________________________|
| Standard | See standards(5). ||_____________________________|_____________________________|
SEE ALSO
Intro(2), exec(2), exit(2), fork(2), shmctl(2), shmget(2), attributes(5), standards(5)SunOS 5.11 Last change: 10 Mar 2008 4