BN_CTX_START(3)

💅 BN_CTX_START(3) — aka the temporary BIGNUM plug 🎀

okay so before we even start: this is a page from the Library Functions Manual, bestie. three little functions that work as a team and honestly? they ate. 🫶

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✨ NAME ✨

BN_CTX_start, BN_CTX_get, BN_CTX_end — use temporary BIGNUM variables 🧚

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✨ SYNOPSIS ✨

the library is “libcrypto”, and you gotta pull in the header first, no cap:

#include <openssl/bn.h>

then the three besties, in their natural habitat:

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 here's the vibe. these functions let you grab temporary BIGNUM variables out of a BN_CTX — and yes, you can make yourself one of those with BN_CTX_new(3). why bother? because it saves you the whole overhead of creating and freeing BIGNUMs over and over again in functions that get called from inside a loop. it's giving efficiency. it's giving "I'm not allocating in a loop like a clown." 💀

the ritual goes like this:

1. a function must call BN_CTX_start() first. non-negotiable, periodt.
2. then BN_CTX_get() can be called repeatedly to obtain temporary BIGNUMs. 🎀
3. all BN_CTX_get() calls must be made before calling any other functions that use the ctx as an argument. like, all of them. get them out of the way first, iykyk.
4. finally, BN_CTX_end() must be called before returning from the function. no sneaky early returns leaving it hanging.

⚠️ big warning energy: when BN_CTX_end() is called, the BIGNUM pointers you got from BN_CTX_get() become invalid. literally do not touch them after that. they're gone. 🪦

oh and if ctx is NULL, no action occurs. nothing happens. she simply does not participate. 🙈

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⭐ RETURN VALUES ⭐

BN_CTX_get() returns a pointer to the BIGNUM, or NULL on error.

and here's the cute part: 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 vibe check at the end and you're done, no need to check every single one. 👑

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)
• BN_new(3)

go read those too, bestie, they're the supporting cast 💜

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👑 HISTORY 👑

BN_CTX_start(), BN_CTX_get(), and BN_CTX_end() first appeared in OpenSSL 0.9.5 and have been available since OpenBSD 2.7. absolute icons, older than most of us, still slaying. 🔥

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and that's the whole page, mwah 💋 — brought to you by Debian, dated June 8, 2025. go forth and hoard your temporaries responsibly. 🌈✨