---
name: "__pmOverrideLastFd"
section: "3"
platform: "linux"
package: "pcp"
version: "7.2.2-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/__pmOverrideLastFd"
---

# \_\_pmOverrideLastFd(3)

IPC version infrastructure support

## NAME

**\_\_pmOverrideLastFd**, **\_\_pmPrintIPC**, **\_\_pmResetIPC** - IPC version infrastructure support

## C SYNOPSIS

**#include "pmapi.h"**\
 **#include "libpcp.h"**

**void \_\_pmOverrideLastFd(int** *fd***);**\
 **void \_\_pmPrintIPC(void);**\
 **void \_\_pmResetIPC(int** *fd***);**

**cc ... -lpcp**

## CAVEAT

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.

## DESCRIPTION

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 file descriptor.

The **\_\_pmOverrideLastFd** is an escape mechanism for use in those situations where the last PDU fetch did not go through the usual channels (i.e. \_\_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 (e.g. 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.

## DIAGNOSTICS

A negative return value from **\_\_pmOverrideLastFd** indicates that the file descriptor argument 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.

## SEE ALSO

[**PMAPI**(3)](/PMAPI/3.md)