BN_zero.3ssl man page

 #include <openssl/bn.h>

 int 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(BIGNUM *a);

BN_ULONG is a macro that will be an unsigned integral type optimied 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 use in comparisons and assignment.

BN_get_word() returns a, if it can be represented as a BN_ULONG.

BN_get_word() returns the value a, or 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.

If a BIGNUM is equal to the value of all-bits-set, it will collide with the error condition returned by BN_get_word() which uses that as an error value.

BN_ULONG should probably be a typedef.

bn(3), BN_bn2bin(3)

BN_zero(), BN_one() and BN_set_word() are available in all versions of SSLeay and OpenSSL. BN_value_one() and BN_get_word() were added in SSLeay 0.8.

BN_value_one() was changed to return a true const BIGNUM * in OpenSSL 0.9.7.

bn.3ssl(3), BN_bn2bin.3ssl(3).