shmop(2)shmop(2)NAMEshmat(), shmdt() - shared memory operations
SYNOPSISDESCRIPTION
attaches the shared memory segment associated with the shared memory
identifier specified by shmid to the data segment of the calling
process.
The segment is attached for reading if (shmflg is "true"; otherwise, it
is attached for reading and writing. It is not possible to attach a
segment for write only.
If the shared memory segment has never been attached to by any process
prior to the current call, shmaddr must be specified as zero and the
segment is attached at a location selected by the operating system.
That location is identical in all processes accessing that shared mem‐
ory object. Once the operating system selects a location for a shared
memory segment, the same location will be used across any subsequent
and calls on the segment until it is removed by the operation of See
exceptions for MPAS processes below.
If this is not the first call on the shared memory segment throughout
the system, shmaddr must either be zero or contain a nonzero address
that is identical to the one returned from previous calls for that seg‐
ment. Even if no processes are currently attached to the segment, as
long as the segment has been attached before, the same rule applies.
See exceptions for MPAS processes below.
If the calling process is already attached to the shared memory seg‐
ment, fails and returns regardless of what value is passed in shmaddr.
See exceptions for MPAS processes below.
detaches from the calling process's data segment the shared memory seg‐
ment located at the address specified by shmaddr.
Exceptions for MPAS Processes
On Itanium(R)-based platforms, MPAS (Mostly Private Address Space) pro‐
cesses are not restricted to passing in zero or a fixed value to calls
to MPAS processes may pass in other addresses. The call may fail or
succeed due to implementation dependent reasons. MGAS processes have
all the restrictions outlined above. In addition, an MGAS process may
not assume that it can attach at the address that an MPAS process can
use to attach to the same segment.
An MPAS process may be able to attach to the same shared memory segment
multiple times. Success or failure of such an operation is implementa‐
tion dependent. Failure will be indicated by a return value of
An MPAS process should specify or flags in the call to These follow the
same rules as such flags passed to shmget(2).
See the for details.
RETURN VALUE
returns the following values:
Successful completion.
n is the data segment start address of the attached
shared memory segment.
Failure.
The shared memory segment is not attached. is set to
indicate the error. The symbol is defined in the header
No successful return from will return the value
returns the following values:
Successful completion.
Failure.
is set to indicate the error.
ERRORS
If fails, is set to one of the following values.
[EACCES] Operation permission is denied to the calling
process.
[EINVAL] shmid is not a valid shared memory identifier,
(possibly because the shared memory segment was
already removed using shmctl(2) with or the call‐
ing process is already attached to shmid.
[EINVAL] shmaddr is not zero and the machine does not per‐
mit nonzero values, or shmaddr is not equal to
the current attach location for the shared memory
segment.
[ENOMEM] The available data space is not large enough to
accommodate the shared memory segment.
[EMFILE] The number of shared memory segments attached to
the calling process exceed the system-imposed
limit.
If fails, is set to one of the following values.
[EINVAL] shmaddr is not the data segment start address of
a shared memory segment.
EXAMPLES
The following call to attaches the shared memory segment to the
process. This example assumes the process has a valid shmid, which can
be obtained by calling shmget(2).
The following call to then detaches the shared memory segment.
SEE ALSOipcs(1), exec(2), exit(2), fork(2), ftok(3C), shmctl(2), shmget(2),
privileges(5).
in
STANDARDS CONFORMANCEshmop(2)
