@deftypefun {int} {gnutls_x509_crt_get_subject_key_id} (gnutls_x509_crt_t @var{cert}, void * @var{ret}, size_t * @var{ret_size}, unsigned int * @var{critical})
@var{cert}: should contain a @code{gnutls_x509_crt_t}  type

@var{ret}: The place where the identifier will be copied

@var{ret_size}: Holds the size of the result field.

@var{critical}: will be non-zero if the extension is marked as critical (may be null)

This function will return the X.509v3 certificate's subject key
identifier.  This is obtained by the X.509 Subject Key identifier
extension field (2.5.29.14).

@strong{Returns:} On success, @code{GNUTLS_E_SUCCESS}  (0) is returned, @code{GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE} 
if the extension is not present, otherwise a negative error value.
@end deftypefun