@deftypefun {int} {gnutls_x509_crt_set_key_purpose_oid} (gnutls_x509_crt_t @var{cert}, const void * @var{oid}, unsigned int @var{critical})
@var{cert}: a certificate of type @code{gnutls_x509_crt_t} 

@var{oid}: a pointer to a null terminated string that holds the OID

@var{critical}: Whether this extension will be critical or not

This function will set the key purpose OIDs of the Certificate.
These are stored in the Extended Key Usage extension (2.5.29.37)
See the GNUTLS_KP_* definitions for human readable names.

Subsequent calls to this function will append OIDs to the OID list.

@strong{Returns:} On success, @code{GNUTLS_E_SUCCESS}  (0) is returned,
otherwise a negative error code is returned.
@end deftypefun