BN_CTX_START(3ossl)

📦
version 15.1 · 2 versions
, in freebsd 15.1

💅 BN_CTX_START — bestie's guide to temporary BIGNUMs 💅

okay so this is the 3ossl manual page for BN_CTX_start, BN_CTX_get, and BN_CTX_end, and honestly? it's giving "small functions, big savings." let's get into it 🫶

✨ NAME ✨

BN_CTX_start, BN_CTX_get, BN_CTX_end — the trio you call to get yourself some temporary BIGNUM variables. no cap, that's literally what they do 💖

🌸 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);

look at them. three little icons. start, get, end. the whole character arc in three lines 😭

🦋 DESCRIPTION 🦋

so here's the vibe: these functions are how you obtain temporary BIGNUM variables out of a BN_CTX (and yes, that BN_CTX can absolutely be one you made with BN_CTX_new(3)). why bother? because it saves you the overhead of repeatedly creating and freeing BIGNUMs inside functions that get called from inside a loop. and bestie, that overhead adds up — it's giving "why am i doing the same allocation 10,000 times" 💀

the rules of the ritual, in order, no skipping:

  1. a function must call BN_CTX_start() first. first. before anything else. periodt.
  2. then BN_CTX_get() may be called repeatedly to obtain temporary BIGNUMs. slay. get as many as you need.
  3. all BN_CTX_get() calls must be made before calling any other functions that use the ctx as an argument. this is the part people mess up, so like… don't 🙈

and finally, BN_CTX_end() must be called before returning from the function. required. not optional. if ctx is NULL, nothing is done — cute, it just shrugs and moves on.

⚠️ okay this part is important: when BN_CTX_end() is called, the BIGNUM pointers you got from BN_CTX_get() become invalid. like, gone. do not touch them after. that's undefined-behavior-flavored heartbreak 💔

🎀 RETURN VALUES 🎀

  • BN_CTX_start() and BN_CTX_end() return no values. they're void. they simply exist. main character energy with nothing to prove ✨
  • BN_CTX_get() returns a pointer to the BIGNUM, or NULL on error.

and here's the convenient bit: once BN_CTX_get() has failed, the subsequent calls will return NULL as well. so it is sufficient to check the return value of the last BN_CTX_get() call — one check, whole chain covered, we love efficiency 💅

in case of an error, an error code is set, which you can obtain with ERR_get_error(3). go get your error, bestie, you earned it 🍓

🔥 SEE ALSO 🔥

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>.

aaaand that's the page!! 🌈 version 3.5.6, dated 2026-04-07, from the 3ossl manual — go forth and manage your temporary BIGNUMs responsibly, bestie, and remember: start → get get get → end. no cap. 💜

📦 where this page comes from