SHMOP(2) System Calls SHMOP(2)


NAME


shmop, shmat, shmdt - shared memory operations

SYNOPSIS


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

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 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:

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 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 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.


WARNINGS


Using a fixed value for the shmaddr argument can adversely affect
performance on certain platforms due to D-cache aliasing.

ATTRIBUTES


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). |
+--------------------+-------------------+

SEE ALSO


Intro(2), exec(2), exit(2), fork(2), shmctl(2), shmget(2), attributes(5),
standards(5)


March 10, 2008 SHMOP(2)