HP X.25/9000 Programmer's Guide

Chapter 4 63

Sending and Receiving Data

Setting Buffer Thresholds and Sizes

X.25 allows developers to ﬁne-tune socket behavior by specifying certain

characteristics such as when a socket allows information to be written to

its buffers (is “writable”). This is done by setting the threshold values

that control the size of the three socket buffers: outbound message, send,

and receive.

Setting the Write Buffer Threshold

The write buffer threshold is used to determine if there is enough buffer

space available to send another message without blocking the socket. If

the buffer space available is greater than or equal to the write threshold,

the socket will indicate writable when a select() call is issued. The

ioctl(X25_WR_WTHRESHOLD) call is used to set the write threshold

value.

The value speciﬁed with ioctl(X25_WR_THRESHOLD) affects send(),

write(), and select(). Whenever a send() or write() call is issued,

the amount of space remaining in the send socket buffer is checked (that

is, the amount of data in the outbound queue is subtracted from the size

of the send socket’s buffer). This check is done using network memory

units and is therefore subject to round-off error. If this remaining space

is insufﬁcient to accept another message (the remaining space is less

than the write threshold size), the free space size for the send socket is

forced to zero and remains that way until enough data is moved to the

X.25 interface. If another send() or write() call is issued during this

time, the call is blocked (for nonblocking I/O, EWOULDBLOCK is

returned).

The ioctl(X25_WR_WTHRESHOLD) call and its parameters are described

below.

Syntax for ioctl (X25_WR_WTHRESHOLD)

The syntax for the ioctl(X25_WR_WTHRESHOLD) system call and its

parameters are described below.

#include <x25/x25ioctls.h>

#include <x25/x25str.h>

int err;

int sd, thresh;

36960-90061.bk Page 63 Friday, November 10, 2000 3:42 PM