void BN_CTX_start(BN_CTX *ctx);
BIGNUM *BN_CTX_get(BN_CTX *ctx);
void BN_CTX_end(BN_CTX *ctx);
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).