Info Catalog gawkinet: Gawk Special Files gawkinet: Using Networking gawkinet: Troubleshooting

gawkinet: TCP Connecting

 
 2.2 Establishing a TCP Connection
 =================================
 
 Let's observe a network connection at work.  Type in the following
 program and watch the output.  Within a second, it connects via TCP
 ('/inet/tcp') to the machine it is running on ('localhost') and asks the
 service 'daytime' on the machine what time it is:
 
      BEGIN {
        "/inet/tcp/0/localhost/daytime" |& getline
        print $0
        close("/inet/tcp/0/localhost/daytime")
      }
 
    Even experienced 'awk' users will find the second line strange in two
 respects:
 
    * A special file is used as a shell command that pipes its output
      into 'getline'.  One would rather expect to see the special file
      being read like any other file ('getline <
      "/inet/tcp/0/localhost/daytime")'.
 
    * The operator '|&' has not been part of any 'awk' implementation
      (until now).  It is actually the only extension of the 'awk'
      language needed (apart from the special files) to introduce network
      access.
 
    The '|&' operator was introduced in 'gawk' 3.1 in order to overcome
 the crucial restriction that access to files and pipes in 'awk' is
 always unidirectional.  It was formerly impossible to use both access
 modes on the same file or pipe.  Instead of changing the whole concept
 of file access, the '|&' operator behaves exactly like the usual pipe
 operator except for two additions:
 
    * Normal shell commands connected to their 'gawk' program with a '|&'
      pipe can be accessed bidirectionally.  The '|&' turns out to be a
      quite general, useful, and natural extension of 'awk'.
 
    * Pipes that consist of a special file name for network connections
      are not executed as shell commands.  Instead, they can be read and
      written to, just like a full-duplex network connection.
 
    In the earlier example, the '|&' operator tells 'getline' to read a
 line from the special file '/inet/tcp/0/localhost/daytime'.  We could
 also have printed a line into the special file.  But instead we just
 read a line with the time, printed it, and closed the connection.
 (While we could just let 'gawk' close the connection by finishing the
 program, in this Info file we are pedantic and always explicitly close
 the connections.)
 

Info Catalog gawkinet: Gawk Special Files gawkinet: Using Networking gawkinet: Troubleshooting