BN_CTX_START(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

💅 BN_CTX_START 💅

okay so this is the 3ossl manual page for OpenSSL's BN_CTX_start, BN_CTX_get and BN_CTX_end — the little trio that lets you borrow temp BIGNUMs and slay the whole "malloc, free, malloc, free" loop situation. bestie, let's get into it 🫶

✨ NAME ✨

BN_CTX_start, BN_CTX_get, BN_CTX_end — use temporary BIGNUM variables 🎀

🌸 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);

like, that's the whole API. three functions. iconic behaviour honestly 👑

🦋 DESCRIPTION 🦋

so these functions are how you get temporary BIGNUM variables out of a BN_CTX (which you can make with BN_CTX_new(3)), and the whole point is ✨efficiency✨ — you're dodging the overhead of repeatedly creating and freeing BIGNUMs inside functions that get called from inside a loop. no cap, that's the vibe.

the order is not negotiable, bestie:

  1. a function must call BN_CTX_start() first. periodt. 💅
  2. then you can call BN_CTX_get() over and over to grab your temporary BIGNUMs.
  3. but — and this is the part people mess up 😭 — all your BN_CTX_get() calls have to happen before you call any other functions that use the ctx as an argument.

finally, BN_CTX_end() must be called before you return from the function. yes, must. fr fr. 🙈

little detail that matters: if ctx is NULL, nothing is done. nothing happens. we simply do not.

and heads up 🚨 — once BN_CTX_end() is called, the BIGNUM pointers you got from BN_CTX_get() become invalid. like, do not touch them. they're gone. main character exit, no goodbye. 💀

🌈 RETURN VALUES 🌈

  • BN_CTX_start() and BN_CTX_end() return no values. void kings. 🫶
  • BN_CTX_get() returns a pointer to the BIGNUM, or NULL on error.

and here's the cute part that saves you a million checks: once BN_CTX_get() has failed, the subsequent calls will return NULL as well — so it's sufficient to just check the return value of the last BN_CTX_get() call. one check, whole chain covered. we love a shortcut ⭐

in case of an error, an error code is set, which you can grab with ERR_get_error(3). 🔥

🧚 SEE ALSO 🧚

BN_CTX_new(3)

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>.

and that's the page, bestie 🍓 this one's from OpenSSL 3.5.0, dated 2026-09-25 — go forth and manage your BN_CTX responsibly, mwah 💜✨

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