OPENSSL_INSTRUMENT_BUS(3SSL)

💅 OPENSSL_INSTRUMENT_BUS — the 3SSL girlies' guide 💅

okay so bestie, you're reading OPENSSL_instrument_bus(3SSL), and I need you to sit down for this one because it's giving very specific vintage tech energy. ✨

🌸 NAME 🌸

OPENSSL_instrument_bus, OPENSSL_instrument_bus2 — instrument references to memory bus ✨ (yes literally, we're measuring the memory bus, it's so niche and I'm obsessed)

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🎀 SYNOPSIS 🎀

okay first thing's first, and I need you to hear me on this one:

🚨 THE FOLLOWING FUNCTIONS HAVE BEEN DEPRECATED SINCE OpenSSL 3.6, AND WILL BE REMOVED IN A FUTURE RELEASE. THEY SHOULD NO LONGER BE USED. 🚨

like, she's iconic but she's on her way out, no cap. do not build new stuff on her. periodt.

#ifdef OPENSSL_CPUID_OBJ
size_t OPENSSL_instrument_bus(unsigned int *vector, size_t num);
size_t OPENSSL_instrument_bus2(unsigned int *vector, size_t num, size_t max);
#endif

note the #ifdef OPENSSL_CPUID_OBJ guard bestie, it's doing heavy lifting keeping those declarations tucked away unless that macro is defined. 💜

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🦋 DESCRIPTION 🦋

so here's the tea. ☕ it was empirically found — like, someone actually sat there and measured — that the timings of references to primary memory are subject to irregular, apparently non-deterministic variations. translation: memory access timing is chaotic, messy, and honestly kind of a vibe. 🌈

the subroutines in question instrument these references for purposes of gathering randomness for a random number generator. so we're turning the universe's inherent chaos into entropy. main character energy fr fr. 👑

here's the clever part: in order to make it bus-bound, a 'flush cache line' instruction is used between probes. and in addition, probes are added to vector elements in atomic or interlocked manner — which should contribute additional noise on multi-processor systems. 🔥 so the multiprocessor chaos is feeding the randomness. slay.

and this is the important bit — this also means that vector[num] should be zeroed upon invocation (if you want to retrieve actual probe values). so like, zero it yourself bestie, she will not do it for you. 💀

• OPENSSL_instrument_bus() — performs num probes and records the number of oscillator cycles every probe took. straightforward, she's simple, I respect it. 🫶
• OPENSSL_instrument_bus2() — on the other hand, this one accumulates consecutive probes with the same value. i.e. in a way it records duration of periods when probe values appeared deterministic. she's the vibes-based one. The subroutine performs at most max probes in attempt to fill the vector[num], with a max value of 0 meaning "as many as it takes." 💅 (yes, 0 = go off, unlimited, iconic behavior)

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⭐ RETURN VALUES ⭐

a return value of 0 indicates that the CPU is not capable of performing the benchmark — either because the oscillator counter or 'flush cache line' is not available on the current platform. 😭 so 0 = your CPU said "not today."

for reference, on x86 the 'flush cache line' was introduced with the SSE2 extensions. iykyk. 🧚

otherwise, the number of recorded values is returned. ✨

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👑 COPYRIGHT 👑

Copyright 2011-2025 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

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and that's the whole page bestie, we ate that. 🍓 this was OpenSSL 4.0.3, dated 2026-09-29, from the OPENSSL_INSTRUMENT_BUS(3SSL) manual. go zero your vectors and stay hydrated. 💖🌸💜