BN_hex2bn() failed but ERR_get_error() returned no error?

[Clark J. Wang]

Hi list:

I'm new to OpenSSL and have a problem when trying BIGNUM usage. The function
ERR_get_error() returned 0 when BN_hex2bn() failed:

#include <openssl/bn.h>
#include <openssl/err.h>
#include <stdlib.h>

int
main()
{
    int r;
    char * dec, * hex = "xFFFF";
    char errmsg[256];
    BIGNUM * b1 = NULL;
    unsigned long err;

    ERR_load_crypto_strings();

    r = BN_hex2bn(&b1, hex);
    if (! r) {
        err = ERR_get_error();
        ERR_error_string_n(err, errmsg, sizeof errmsg);
        fprintf(stderr, "BN_hex2bn(): %s\n", errmsg);
        exit(1);
    }

    dec = BN_bn2dec(b1);
    printf("0x%s = %s\n", hex, dec);

    OPENSSL_free(dec);

    return 0;
}

When running, it outputs like this:

BN_hex2bn(): error:00000000:lib(0):func(0):reason(0)

So how can I get the BN_hex2bn() error? Anybody can help?

Thanks.