This documentation is intended for internal Performance Co-Pilot
(PCP) developer use.
These interfaces are not part of the PCP APIs that are guaranteed
to remain fixed across releases, and they may not work, or may
provide different semantics at some point in the future.
IPC channels throughout the distributed PCP framework are
affected by the PCP 2.0 (and later) PDU changes. These functions
are the interface to the libpcp IPC connection management global
data. This data consists of a hash table of __pmIPC structures
(indexed by file descriptor) and a cached, most-recently-used
Newly created IPC channels must be registered with the hash table
using __pmAddIPC, such that the PDU sending and decoding routines
can determine whether they need to perform any PDU version
translations or not, for backward compatibility with previous the
PCP 1.x IPC protocol.
__pmLookupIPC and __pmFdLookupIPC both provide handles to the
__pmIPC structure associated with the given file descriptor, as
previously established by a call to __pmAddIPC. The difference
between the two is that one allows an explicit file descriptor
lookup, and the other uses the cached, most-recently-used file
descriptor. So __pmLookupIPC actually calls __pmFdLookupIPC
using this cached file descriptor as the argument. The
justification for having both is that in some places it is not
possible to use __pmFdLookupIPC (which is preferred), since at
that particular level of the PMAPI a file descriptor is not
available (see the __pmDecodeError code for an example).
The __pmOverrideLastFd is an escape mechanism for use in those
situations where the last PDU fetch did not go through the usual
channels (ie. __pmGetPDU), so as to ensure that the cached file
descriptor is the correct file descriptor for the PDU which is
currently being processed. This will typically be used for
archive PDU processing or where version information is not
available for a given file descriptor (eg. immediately prior to a
PDU version exchange).
__pmPrintIPC is a useful debugging routine for displaying a table
mapping all currently registered file descriptors to their
associated PDU version numbers. Unused entries in this table
should display the value zero in the version column.
__pmResetIPC resets the version information associated with the
given file descriptor to some known (invalid) number. Subsequent
lookups on this file descriptor will return an UNKNOWN_VERSION
embedded within the __pmIPC structure.
A negative return value from __pmLookupIPC indicates that the
requested file descriptor is not registered in the hash table.
This typically indicates closure of an IPC channel, so PM_ERR_IPC
is returned if this is the case.
This page is part of the PCP (Performance Co-Pilot) project.
Information about the project can be found at
⟨http://www.pcp.io/⟩. If you have a bug report for this manual
page, send it to email@example.com. This page was obtained from the
project's upstream Git repository
⟨https://github.com/performancecopilot/pcp.git⟩ on 2023-06-23.
(At that time, the date of the most recent commit that was found
in the repository was 2023-06-21.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
Performance Co-Pilot PCP PMLOOKUPIPC(3)