Openssl: Querying extensions on X509 certificates

X509_get_ext_d2i(X509 *x, int nid, int *crit, int *idx)

X509* x509_cert = ...
// without X509_check_ca x509_cert->ex_kusage always returns 0 (no idea why)
int ca = X509_check_ca(x509_cert);
unsigned long usage = x509_cert->ex_kusage;

SC_PKCS15INIT_X509_DIGITAL_SIGNATURE     0x0080UL
SC_PKCS15INIT_X509_NON_REPUDIATION       0x0040UL
SC_PKCS15INIT_X509_KEY_ENCIPHERMENT      0x0020UL
SC_PKCS15INIT_X509_DATA_ENCIPHERMENT     0x0010UL
SC_PKCS15INIT_X509_KEY_AGREEMENT         0x0008UL
SC_PKCS15INIT_X509_KEY_CERT_SIGN         0x0004UL
SC_PKCS15INIT_X509_CRL_SIGN              0x0002UL

/* Handle key usage */
if((usage=X509_get_ext_d2i(x, NID_key_usage, NULL, NULL))) {
    if(usage->length > 0) {
        x->ex_kusage = usage->data[0];
        if(usage->length > 1)
        x->ex_kusage |= usage->data[1] << 8;
    } else x->ex_kusage = 0;
    x->ex_flags |= EXFLAG_KUSAGE;
    ASN1_BIT_STRING_free(usage);
}