12.7.3.1 Calling Side

To send fast select data, fast_select_type must be set to the proper value (with the X25_SET_FACILITY ioctl) before connect is called (see the section "12.7.1 Facility Specification and Negotiation"of this chapter for more information). Using the CONN_DB structure, a calling DTE can specify a user data field up to 102 bytes (including the optional protocol identifier). If 102 bytes of call user data are not enough for the current fast select message, use the X25_WR_USER_DATA ioctl before calling connect to pass the additional user data. The user data specified in connect will precede this additional user data. To write user data:

typedef struct user_data_db_s {
 u_char      datalen;
 u_char       data[MAX_USER_DATA];
 } USER_DATA_DB;
 int s, error;
 USER_DATA_DB user_data;
 error = ioctl(s, X25_WR_USER_DATA, &user_data)

Here, MAX_USER_DATA is 124.

If connect returns -1 and errno is EFASTDATA, the remote side has cleared the call by sending a Clear Indication packet with up to 32 bytes (1980) or 128 bytes (1984) of user data. At this time, the user can read the user data in the Clear Indication packet with calls to the X25_RD_USER_DATA ioctl until the returned datalen in USER_DATA_DB structure is 0 or less than MAX_USER_DATA, then close the socket with close.

To read user data:

USER_DATA_DB user_data;
 int s, error;
 error = ioctl(s, X25_RD_USER_DATA, &user_data);

If connect returns 0, it indicates that the connection has been set up successfully. If the connection is over an interface that supports 1984 X.25, the remote user may have sent user data in the Call Accepted packet. (This will happen only if the initiator of the connection has specified fast select with no restriction on response.) Thus the initiating user must repeatedly read any user data using the X25_RD_USER_DATA ioctl until the returned length in the USER_DATA_DB structure is less than MAX_USER_DATA.

When a call is cleared after being connected, the Clear Indication packet may contain user data if the interface supports 1984 X.25 and fast select is in effect for that call. Either the initiator of the connection or the responder can send user data in the Clear Request packet. Thus when a call with fast select is cleared by the remote user, user data must be read in the same way as for the other cases.

For 1980 X.25 interfaces, if the connection was accepted by the remote user, the Call Accepted and Clear Request packets will not have any user data; the only time that the Clear Request can have user data is when a fast select call is cleared immediately (this is detectable by means of the EFASTDATA error return).