BN_ZERO(3) | Library Functions Manual | BN_ZERO(3)

# NAME

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

# SYNOPSIS

library “libcrypto”
`#include <openssl/bn.h>`

void
`BN_zero`(BIGNUM *a);

int
`BN_one`(BIGNUM *a);

const BIGNUM *
`BN_value_one`(void);

int
`BN_set_word`(BIGNUM *a, BN_ULONG w);

BN_ULONG
`BN_get_word`(const BIGNUM *a);

# DESCRIPTION

BN_ULONG is a macro that expands to an unsigned integral type optimized for the most efficient implementation on the local platform. It is unsigned long (= uint64_t) on `_LP64` platforms and unsigned int (= uint32_t) elsewhere.

`BN_zero`(), `BN_one`(), and `BN_set_word`() set a to the values 0, 1 and w respectively.

`BN_value_one`() returns a BIGNUM constant of value 1. This constant is useful for comparisons and assignments.

# RETURN VALUES

`BN_get_word`() returns the value a, or a number with all bits set if a cannot be represented as a BN_ULONG.

`BN_one`() and `BN_set_word`() return 1 on success, 0 otherwise. `BN_value_one`() returns the constant.

# SEE ALSO

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

# HISTORY

`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. These functions have been available since OpenBSD 2.4.

# BUGS

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.

June 14, 2025 | Debian