10.3 Using gawk for Network Programming

EMISTERED: A host is a host from coast to coast,
and no-one can talk to host that's close,
unless the host that isn't close
is busy hung or dead.

In addition to being able to open a two-way pipeline to a coprocess on the same system (see Two-way I/O), it is possible to make a two-way connection to another process on another system across an IP networking connection.

You can think of this as just a very long two-way pipeline to a coprocess. The way gawk decides that you want to use TCP/IP networking is by recognizing special file names that begin with `/inet/'.

The full syntax of the special file name is /inet/protocol/local-port/remote-host/remote-port. The components are:

protocol

The protocol to use over IP. This must be either `tcp', `udp', or `raw', for a TCP, UDP, or raw IP connection, respectively. The use of TCP is recommended for most applications.

Caution: The use of raw sockets is not currently supported in version 3.1 of gawk.

local-port

The local TCP or UDP port number to use. Use a port number of `0' when you want the system to pick a port. This is what you should do when writing a TCP or UDP client. You may also use a well-known service name, such as `smtp' or `http', in which case gawk attempts to determine the predefined port number using the C getservbyname function.

remote-host

The IP address or fully-qualified domain name of the Internet host to which you want to connect.

remote-port

The TCP or UDP port number to use on the given remote-host. Again, use `0' if you don't care, or else a well-known service name.

Consider the following very simple example:

     BEGIN {
       Service = "/inet/tcp/0/localhost/daytime"
       Service |& getline
       print $0
       close(Service)
     }

This program reads the current date and time from the local system's TCP `daytime' server. It then prints the results and closes the connection.

Because this topic is extensive, the use of gawk for TCP/IP programming is documented separately. See TCP/IP Internetworking with gawk, which comes as part of the gawk distribution, for a much more complete introduction and discussion, as well as extensive examples.