shmop.2 (2010 09)

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shmop(2) shmop(2)

NAME

shmat(), shmdt() - shared memory operations

SYNOPSIS

#include <sys/shm.h>

void *shmat(int shmid, void *shmaddr, int shmflg);

int shmdt(void *shmaddr);

DESCRIPTION

shmat() 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

& SHM_RDONLY) 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

shmat()

call, shmaddr must be speciﬁed as zero and the segment is attached at a location selected by the operat-

ing system. That location is identical in all processes accessing that shared memory object. Once the

operating system selects a location for a shared memory segment, the same location will be used across

any subsequent shmat() and shmdt() calls on the segment until it is removed by the

IPC_RMID

operation of shmctl(). See exceptions for MPAS processes below.

If this is not the ﬁrst

shmat() 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

shmat() calls for that segment. 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 segment,

shmat() fails and returns

SHM_FAILED regardless of what value is passed in shmaddr . See exceptions for MPAS processes below.

shmdt() detaches from the calling process’s data segment the shared memory segment located at the

address speciﬁed by shmaddr .

Exceptions for MPAS Processes

On Itanium(R)-based platforms, MPAS (Mostly Private Address Space) processes are not restricted to

passing in zero or a ﬁxed value to calls to

shmat(). MPAS processes may pass in other addresses. The

shmat() 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 implementation dependent. Failure will be indicated by a return value of

SHM_FAILED.

An MPAS process should specify

IPC_SHARE32 or IPC_GLOBAL ﬂags in the call to

shmat(). These

follow the same rules as such ﬂags passed to shmget (2).

See the Adaptive Address Space Whitepaper for details.

RETURN VALUE

shmat() returns the following values:

n Successful completion. n is the data segment start address of the attached shared memory seg-

ment.

SHM_FAILED

Failure. The shared memory segment is not attached. errno is set to indicate the error. The

symbol SHM_FAILED is deﬁned in the header <sys/shm.h>. No successful return from

shmat() will return the value SHM_FAILED.

shmdt() returns the following values:

0 Successful completion.

-1 Failure. errno is set to indicate the error.

ERRORS

shmat() fails, errno is set to one of the following values.

HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

Summary of content (2 pages)

PAGE 1
shmop(2) shmop(2) NAME shmat(), shmdt() - shared memory operations SYNOPSIS #include void *shmat(int shmid, void *shmaddr, int shmflg); int shmdt(void *shmaddr); DESCRIPTION shmat() 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 & SHM_RDONLY) is "true"; otherwise, it is attached for reading and writing.
PAGE 2
shmop(2) shmop(2) [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 IPC_RMID), or the calling process is already attached to shmid. [EINVAL] shmaddr is not zero and the machine does not permit nonzero values, or shmaddr is not equal to the current attach location for the shared memory segment.