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 SHMSHAREMU or
SHMPAGEABLE 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&SHMSHAREMU) is true, virtual memory
resources in addition to shared memory itself are shared
among processes that use the same shared memory.
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System Calls shmop(2)
When (shmflg&SHMPAGEABLE) is true, virtual memory resources
are shared and the dynamic shared memory (DISM) framework is
created. 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&SHMSHAREMU) or (shmflg&SHMPAGEABLE) is
true, then the segment is attached to the first
available suitably aligned address. When
(shmflg&SHMSHAREMU) or (shmflg&SHMPAGEABLE) 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&SHMRND) 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&SHMRND) is false, the segment is attached
to the address given by shmaddr.
o The segment is attached for reading if
(shmflg&SHMRDONLY) 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
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System Calls shmop(2)
segment is not attached, and errno is set to indicate the
error.
ERORS
The shmat() function will fail if:
EACES 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 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&SHMRND) is false.
The shmaddr argument is not equal to 0, is not
properly aligned, and (shmfg&SHMSHAREMU) is
true.
SHMSHAREMU is not supported in certain archi-
tectures.
Both (shmflg&SHMSHAREMU) and
(shmflg&SHMPAGEABLE) are true.
(shmflg&SHMSHAREMU) is true and the shared
memory segment specified by shmid() had previously
been attached by a call to shmat() in which
(shmflg&SHMPAGEABLE) was true.
(shmflg&SHMPAGEABLE) is true and the shared
memory segment specified by shmid() had previously
been attached by a call to shmat() in which
(shmflg&SHMSHAREMU) 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.
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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&SHMSHAREMU) 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 adversely
affect performance on certain platforms due to D-cache
aliasing.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE 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)
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