12.5.3 Receiving X.25 Messages in Records

By default, each recv returns a complete X.25 message. To force recv to return data before a complete X.25 message has been assembled, issue the X25_RECORD_SIZE ioctl after the socket is created:

int s, record_size, error;
 /* Set record_size to n, where n is the number of
  * maximum size packets with more bit turned on that
  * will be received before the accumulated data is
  * returned in a recv call.
  */
 error = ioctl(s, X25_RECORD_SIZE, &record_size);

Here, record_size specifies the number of full (maximum size) packets with M-bit turned on that X.25 will receive before the accumulated data is returned to the user as a record (or message). Thus, the maximum record size seen by the user will be record_size times the maximum packet size for the virtual circuit. If a complete X.25 message comprises less than record_size packets, it will be returned to the user as in the normal case.

The X25_RECORD_SIZE ioctl is useful when complete X.25 messages are potentially very long, so that either they cannot be buffered in the socket receive buffers (limited by the high water mark), or it is too much of a performance bottleneck for the application to wait for the whole message to be assembled before processing it, or the application does not wish to dedicate very large buffers for receiving data. If record boundaries (that is, message boundaries) are important, this method must not be used. Rather, the X25_HEADER ioctl must be used, as indicated earlier, to obtain a header byte for each packet that indicates whether or not the packet is the last one in a record (that is, message).