HP X.25/9000 Programmer's Guide

Chapter 5 71

Receiving and Transmitting Out-of-band Information

Building a Signal Handler

A signal handler is a routine that is called when a signal arrives. The

signal handler executes in the process address space, and all global data

items are available to the signal handler. The process is halted while the

signal handler executes. When the signal handler returns, the process

resumes execution at the point where it was halted.

A signal handler should carry out the following four steps each time it is

executed:

Step 1. Get the cause for the signal being sent.

To get the out-of-band event that caused the signal handler to be

executed in the ﬁrst place, use the recv() system call with the

MSG_OOB

ﬂag set. recv(MSG_OOB) returns a buffer. The ﬁrst byte in the buffer

contains the number of bytes in the event (range: 3 to 34 bytes). The

second byte contains the event code (described below), and the rest of the

buffer contains the event data, if any.

The recv(MSG_OOB) call is nonblocking. If the recv() call returns 0,

then no out-of-band events were queued. If the call returns a negative

value, an error occurred. A value greater than 0 indicates the size of the

buffer being returned.

Step 2. Process the event which caused the signal to be sent.

After the cause of the event has been received, the signal handler should

process the event. Usually out-of-band events have an effect on the state

of the VC. For example, the arrival of a CLEAR INDICATION packet

makes the VC unusable. This information should be made known to the

main program. Typically, this is done with globally-deﬁned state

variables. The state variables can be tested by the main program, or

examined only after a system call returns with an error. Many of the

state changes of a VC are made known to the process through

errno

when a system call is executed.

The actual strategy used to pass information to the main program is up

to the application designer.

Possible out-of-band events and the appropriate actions for each are

described below.

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