BN_CTX_START(3SSL)

✨ NAME ✨

BN_CTX_start, BN_CTX_get, BN_CTX_end — the lil trio that hands you temporary BIGNUM variables (yes bestie, this is the 3SSL page, we're in OpenSSL land now 💖)

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🌸 SYNOPSIS 🌸

#include <openssl/bn.h>

void BN_CTX_start(BN_CTX *ctx);

BIGNUM *BN_CTX_get(BN_CTX *ctx);

void BN_CTX_end(BN_CTX *ctx);

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🎀 DESCRIPTION 🎀

okay so picture this: you're writing a function that gets called over and over inside a loop, and every single time it keeps creating and freeing BIGNUMs like it's got money to burn 💸. That overhead? Not the vibe. These functions let you grab temporary BIGNUM variables out of a BN_CTX (which you can make with BN_CTX_new(3)) so you can just… stop doing that. Save the overhead, keep the bag.

The rules of the ritual go like this:

1. A function must call BN_CTX_start() first. No start, no temp BIGNUMs, periodt.
2. Then you can call BN_CTX_get() over and over to scoop up temporary BIGNUMs. ✨
3. Every single BN_CTX_get() call has to happen before you call any other functions that use ctx as an argument. Like, all of them. First. No exceptions, bestie.
4. Finally, BN_CTX_end() must be called before you return from the function. If ctx is NULL, nothing happens — it just shrugs and does nothing. 💅

⚠️ Heads up: when BN_CTX_end() is called, the BIGNUM pointers you got from BN_CTX_get() become invalid. They're gone. Ghosted. Do not touch them after this. 😭

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💜 RETURN VALUES 💜

• BN_CTX_start() and BN_CTX_end() — return no values, they're void, they give you nothing back and that's fine, they're doing their best 🫶
• BN_CTX_get() — returns a pointer to the BIGNUM, or NULL on error.

ngl the error handling here is actually iconic: once BN_CTX_get() has failed, every subsequent call will also return NULL, so you literally only need to check the return value of the last BN_CTX_get() call. One check. That's it. Main character efficiency. 👑

In case of an error, an error code is set, and you can grab it with ERR_get_error(3).

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🦋 SEE ALSO 🦋

• BN_CTX_new(3)

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👑 COPYRIGHT 👑

Copyright 2000-2016 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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okay that's the whole page, you're officially fluent in temporary BIGNUM management 💖 this was version 4.0.3, dated 2026-09-29, and yes this is the BN_CTX_START(3SSL) manual page — go forth and stop leaking BIGNUMs in your loops, bestie, I'm so proud of you 🎀✨🌈