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BN_ZERO(3) Library Functions Manual BN_ZERO(3)

BN_zero, BN_one, BN_value_one, BN_set_word, BN_get_word
BIGNUM assignment operations

#include <openssl/bn.h>
BN_zero(BIGNUM *a);
BN_one(BIGNUM *a);
const BIGNUM *
BN_set_word(BIGNUM *a, BN_ULONG w);
BN_get_word(BIGNUM *a);

BN_ULONG is a macro that expands to an unsigned integral type optimized for the most efficient implementation on the local platform.
BN_zero(), BN_one(), and BN_set_word() set a to the values 0, 1 and w respectively. BN_zero() and BN_one() are macros.
BN_value_one() returns a BIGNUM constant of value 1. This constant is useful for comparisons and assignments.

BN_get_word() returns the value a, or a number with all bits set if a cannot be represented as a BN_ULONG.
BN_zero(), BN_one(), and BN_set_word() return 1 on success, 0 otherwise. BN_value_one() returns the constant.

BN_bn2bin(3), BN_new(3), BN_set_bit(3), BN_set_negative(3)

BN_zero(), BN_one(), BN_value_one(), and BN_set_word() first appeared in SSLeay 0.5.1. BN_get_word() first appeared in SSLeay 0.6.0. All these functions have been available since OpenBSD 2.4.

Someone might change the constant.
If the value of a BIGNUM is equal to a BN_ULONG with all bits set, the return value of BN_get_word() collides with return value used to indicate errors.
BN_ULONG should probably be a typedef rather than a macro.
March 27, 2018 OpenBSD-current