int libssh2_session_last_error(LIBSSH2_SESSION *session, char **errmsg, int *errmsg_len, int want_buf);
session - Session instance as returned by libssh2_session_init_ex(3)
errmsg - If not NULL, is populated by reference with the human readable form of the most recent error message.
errmsg_len - If not NULL, is populated by reference with the length of errmsg. (The string is NUL-terminated, so the length is only useful as an optimization, to avoid calling strlen.)
want_buf - If set to a non-zero value, "ownership" of the errmsg buffer will be given to the calling scope. If necessary, the errmsg buffer will be duplicated.
Determine the most recent error condition and its cause.
Numeric error code corresponding to the the Error Code constants.