BN_CTX_START(3ossl)

💖 BN_CTX_START 💖

okay bestie, welcome to the BN_CTX_start(3ossl) page — it's part of the OpenSSL manuals and it's genuinely so useful once it clicks. we're talking temporary BIGNUM variables and how to not be wasteful about it. slay. ✨

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🌸 NAME 🌸

BN_CTX_start, BN_CTX_get, BN_CTX_end — use temporary BIGNUM variables 🎀

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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 🦋

so the whole vibe here is: these functions hand you temporary BIGNUM variables out of a BN_CTX (and you make one of those with BN_CTX_new(3)). why tho? because it saves you the overhead of constantly creating and freeing BIGNUMs inside functions that get called from inside a loop. it's giving efficiency. it's giving main character energy. 💅

the order of operations is not negotiable, bestie:

1. a function must call BN_CTX_start() first.
2. then BN_CTX_get() can be called repeatedly to grab temporary BIGNUMs.
3. all your BN_CTX_get() calls have to happen before you call any other functions that use the ctx as an argument.
4. finally, BN_CTX_end() must be called before returning from the function.

also if ctx is NULL, nothing is done. literally nothing. no cap. 🙈

and here's the part you cannot skip: once BN_CTX_end() is called, the BIGNUM pointers you got from BN_CTX_get() become invalid. gone. do not touch them. 💀

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⭐ RETURN VALUES ⭐

BN_CTX_start() and BN_CTX_end() return no values. that's it, that's the whole story. 🫶

BN_CTX_get() returns a pointer to the BIGNUM, or NULL on error. and here's the cute little shortcut: once BN_CTX_get() has failed, the subsequent calls will return NULL as well — so it's sufficient to check the return value of the last BN_CTX_get() call. 👑

in case of an error, an error code is set, which can be obtained by 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 bestie, you ate it 💖 this was BN_CTX_start(3ossl) from OpenSSL 3.5.9, dated 2026-09-29 — go forth and free your BIGNUMs responsibly, mwah 🎀🌸✨