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gawk: Read Timeout

 
 4.11 Reading Input with a Timeout
 =================================
 
 This minor node describes a feature that is specific to 'gawk'.
 
    You may specify a timeout in milliseconds for reading input from the
 keyboard, a pipe, or two-way communication, including TCP/IP sockets.
 This can be done on a per-input, per-command, or per-connection basis,
 by setting a special element in the 'PROCINFO' array (Auto-set):
 
      PROCINFO["input_name", "READ_TIMEOUT"] = TIMEOUT IN MILLISECONDS
 
    When set, this causes 'gawk' to time out and return failure if no
 data is available to read within the specified timeout period.  For
 example, a TCP client can decide to give up on receiving any response
 from the server after a certain amount of time:
 
      Service = "/inet/tcp/0/localhost/daytime"
      PROCINFO[Service, "READ_TIMEOUT"] = 100
      if ((Service |& getline) > 0)
          print $0
      else if (ERRNO != "")
          print ERRNO
 
    Here is how to read interactively from the user(1) without waiting
 for more than five seconds:
 
      PROCINFO["/dev/stdin", "READ_TIMEOUT"] = 5000
      while ((getline < "/dev/stdin") > 0)
          print $0
 
    'gawk' terminates the read operation if input does not arrive after
 waiting for the timeout period, returns failure, and sets 'ERRNO' to an
 appropriate string value.  A negative or zero value for the timeout is
 the same as specifying no timeout at all.
 
    A timeout can also be set for reading from the keyboard in the
 implicit loop that reads input records and matches them against
 patterns, like so:
 
      $ gawk 'BEGIN { PROCINFO["-", "READ_TIMEOUT"] = 5000 }
      > { print "You entered: " $0 }'
      gawk
      -| You entered: gawk
 
    In this case, failure to respond within five seconds results in the
 following error message:
 
      error-> gawk: cmd. line:2: (FILENAME=- FNR=1) fatal: error reading input file `-': Connection timed out
 
    The timeout can be set or changed at any time, and will take effect
 on the next attempt to read from the input device.  In the following
 example, we start with a timeout value of one second, and progressively
 reduce it by one-tenth of a second until we wait indefinitely for the
 input to arrive:
 
      PROCINFO[Service, "READ_TIMEOUT"] = 1000
      while ((Service |& getline) > 0) {
          print $0
          PROCINFO[Service, "READ_TIMEOUT"] -= 100
      }
 
      NOTE: You should not assume that the read operation will block
      exactly after the tenth record has been printed.  It is possible
      that 'gawk' will read and buffer more than one record's worth of
      data the first time.  Because of this, changing the value of
      timeout like in the preceding example is not very useful.
 
    If the 'PROCINFO' element is not present and the 'GAWK_READ_TIMEOUT'
 environment variable exists, 'gawk' uses its value to initialize the
 timeout value.  The exclusive use of the environment variable to specify
 timeout has the disadvantage of not being able to control it on a
 per-command or per-connection basis.
 
    'gawk' considers a timeout event to be an error even though the
 attempt to read from the underlying device may succeed in a later
 attempt.  This is a limitation, and it also means that you cannot use
 this to multiplex input from two or more sources.  Retrying
 Input for a way to enable later I/O attempts to succeed.
 
    Assigning a timeout value prevents read operations from blocking
 indefinitely.  But bear in mind that there are other ways 'gawk' can
 stall waiting for an input device to be ready.  A network client can
 sometimes take a long time to establish a connection before it can start
 reading any data, or the attempt to open a FIFO special file for reading
 can block indefinitely until some other process opens it for writing.
 
    ---------- Footnotes ----------
 
    (1) This assumes that standard input is the keyboard.
 

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