PMLOOKUPIPC(3)

💖 PMLOOKUPIPC(3) — IPC version infrastructure support 💖

okay bestie, gather round, because we're about to talk about __pmOverrideLastFd, __pmPrintIPC, and __pmResetIPC — the little trio that keeps libpcp's IPC connection-management brain from short-circuiting. this one's a Library Functions Manual moment, so you already know it's gonna be a little nerdy. but it's giving essential. 🧚

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✨ C SYNOPSIS ✨

#include "pmapi.h"
#include "libpcp.h"

void __pmOverrideLastFd(int fd);
void __pmPrintIPC(void);
void __pmResetIPC(int fd);

cc ... -lpcp

that's it. that's the whole vibe. three little functions and a compile line, no notes. 💅

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🚨 CAVEAT 🚨

real talk, bestie: 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 stay fixed across releases. lowkey they might not even work, or they might hand you completely different semantics at some point in the future. so like… do not build your whole personality on these. 🙈

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🌸 DESCRIPTION 🌸

so, the IPC channels all throughout the distributed PCP framework are affected by the PCP 2.0 (and later) PDU changes. that's just the lore. these functions are the interface to the libpcp IPC connection management global data, and that data is made of two things:

• a hash table of __pmIPC structures, indexed by file descriptor 👑
• a cached, most-recently-used file descriptor ✨

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🦋 __pmOverrideLastFd — the escape hatch

__pmOverrideLastFd is an escape mechanism, and it's for those situations where the last PDU fetch did not go through the usual channels (i.e. __pmGetPDU). the whole point is making sure the cached file descriptor is the correct file descriptor for the PDU that's currently being processed. no cap.

typically you'd reach for this in archive PDU processing, or anywhere version information isn't available for a given file descriptor — like, for example, immediately prior to a PDU version exchange. it's giving "trust me, I know which fd this is." 🫶

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🎀 __pmPrintIPC — the debugging diva

__pmPrintIPC is a genuinely useful debugging routine, and what it does is display a table mapping all the currently registered file descriptors to their associated PDU version numbers. iconic behaviour.

and listen: unused entries in this table should display the value zero in the version column. so if you see a zero there, that's correct, don't panic. 😌

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💜 __pmResetIPC — the reset button

__pmResetIPC resets the version information associated with the given file descriptor to some known (invalid) number. and then? subsequent lookups on this file descriptor will return an UNKNOWN_VERSION embedded within the __pmIPC structure. so it's very much a "we don't know her anymore" situation, structurally speaking. 💀

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🔥 DIAGNOSTICS 🔥

if __pmOverrideLastFd hands you a negative return value, that means the file descriptor argument is not registered in the hash table. periodt.

and what does that usually mean? typically it means an IPC channel got closed. so in that case, PM_ERR_IPC is returned. keep an eye out, bestie. ⭐

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🌈 SEE ALSO 🌈

• PMAPI(3)

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okay that's the whole page, mwah. this masterpiece of a manual came to you from Performance Co-Pilot and the lovely folks at PCP — so go forth, respect the internals, and remember these are internal-use interfaces, not a public promise. stay slay, stay registered in the hash table, and don't forget to vibe check your file descriptors. 💖✨🎀