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BN_CTX_START(3) Library Functions Manual BN_CTX_START(3)

BN_CTX_start, BN_CTX_get, BN_CTX_end
use temporary BIGNUM variables

#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);

These functions are used to obtain temporary BIGNUM variables from a BN_CTX (which can be created using BN_CTX_new(3)) in order to save the overhead of repeatedly creating and freeing BIGNUMs in functions that are called from inside a loop.
A function must call BN_CTX_start() first. Then, BN_CTX_get() may be called repeatedly to obtain temporary BIGNUMs. All BN_CTX_get() calls must be made before calling any other functions that use the ctx as an argument.
Finally, BN_CTX_end() must be called before returning from the function. When BN_CTX_end() is called, the BIGNUM pointers obtained from BN_CTX_get() become invalid.

BN_CTX_get() returns a pointer to the BIGNUM, or NULL on error. 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. In case of an error, an error code is set which can be obtained by ERR_get_error(3).

BN_CTX_new(3), BN_new(3)

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.
March 22, 2018 OpenBSD-current