(gawk.info.gz) Close Files And Pipes

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 5.8 Closing Input and Output Redirections
 =========================================
 
 If the same file name or the same shell command is used with `getline'
 more than once during the execution of an `awk' program (
 Getline), the file is opened (or the command is executed) the first
 time only.  At that time, the first record of input is read from that
 file or command.  The next time the same file or command is used with
 `getline', another record is read from it, and so on.
 
    Similarly, when a file or pipe is opened for output, `awk' remembers
 the file name or command associated with it, and subsequent writes to
 the same file or command are appended to the previous writes.  The file
 or pipe stays open until `awk' exits.
 
    This implies that special steps are necessary in order to read the
 same file again from the beginning, or to rerun a shell command (rather
 than reading more output from the same command).  The `close()' function
 makes these things possible:
 
      close(FILENAME)
 
 or:
 
      close(COMMAND)
 
    The argument FILENAME or COMMAND can be any expression.  Its value
 must _exactly_ match the string that was used to open the file or start
 the command (spaces and other "irrelevant" characters included). For
 example, if you open a pipe with this:
 
      "sort -r names" | getline foo
 
 then you must close it with this:
 
      close("sort -r names")
 
    Once this function call is executed, the next `getline' from that
 file or command, or the next `print' or `printf' to that file or
 command, reopens the file or reruns the command.  Because the
 expression that you use to close a file or pipeline must exactly match
 the expression used to open the file or run the command, it is good
 practice to use a variable to store the file name or command.  The
 previous example becomes the following:
 
      sortcom = "sort -r names"
      sortcom | getline foo
      ...
      close(sortcom)
 
 This helps avoid hard-to-find typographical errors in your `awk'
 programs.  Here are some of the reasons for closing an output file:
 
    * To write a file and read it back later on in the same `awk'
      program.  Close the file after writing it, then begin reading it
      with `getline'.
 
    * To write numerous files, successively, in the same `awk' program.
      If the files aren't closed, eventually `awk' may exceed a system
      limit on the number of open files in one process.  It is best to
      close each one when the program has finished writing it.
 
    * To make a command finish.  When output is redirected through a
      pipe, the command reading the pipe normally continues to try to
      read input as long as the pipe is open.  Often this means the
      command cannot really do its work until the pipe is closed.  For
      example, if output is redirected to the `mail' program, the
      message is not actually sent until the pipe is closed.
 
    * To run the same program a second time, with the same arguments.
      This is not the same thing as giving more input to the first run!
 
      For example, suppose a program pipes output to the `mail' program.
      If it outputs several lines redirected to this pipe without closing
      it, they make a single message of several lines.  By contrast, if
      the program closes the pipe after each line of output, then each
      line makes a separate message.
 
    If you use more files than the system allows you to have open,
 `gawk' attempts to multiplex the available open files among your data
 files.  `gawk''s ability to do this depends upon the facilities of your
 operating system, so it may not always work.  It is therefore both good
 practice and good portability advice to always use `close()' on your
 files when you are done with them.  In fact, if you are using a lot of
 pipes, it is essential that you close commands when done. For example,
 consider something like this:
 
      {
          ...
          command = ("grep " $1 " /some/file | my_prog -q " $3)
          while ((command | getline) > 0) {
              PROCESS OUTPUT OF command
          }
          # need close(command) here
      }
 
    This example creates a new pipeline based on data in _each_ record.
 Without the call to `close()' indicated in the comment, `awk' creates
 child processes to run the commands, until it eventually runs out of
 file descriptors for more pipelines.
 
    Even though each command has finished (as indicated by the
 end-of-file return status from `getline'), the child process is not
 terminated;(1) more importantly, the file descriptor for the pipe is
 not closed and released until `close()' is called or `awk' exits.
 
    `close()' will silently do nothing if given an argument that does
 not represent a file, pipe or coprocess that was opened with a
 redirection.
 
    Note also that `close(FILENAME)' has no "magic" effects on the
 implicit loop that reads through the files named on the command line.
 It is, more likely, a close of a file that was never opened, so `awk'
 silently does nothing.
 
    When using the `|&' operator to communicate with a coprocess, it is
 occasionally useful to be able to close one end of the two-way pipe
 without closing the other.  This is done by supplying a second argument
 to `close()'.  As in any other call to `close()', the first argument is
 the name of the command or special file used to start the coprocess.
 The second argument should be a string, with either of the values
 `"to"' or `"from"'.  Case does not matter.  As this is an advanced
 feature, a more complete discussion is delayed until  Two-way
 I/O, which discusses it in more detail and gives an example.
 
 Advanced Notes: Using `close()''s Return Value
 ----------------------------------------------
 
 In many versions of Unix `awk', the `close()' function is actually a
 statement.  It is a syntax error to try and use the return value from
 `close()': (d.c.)
 
      command = "..."
      command | getline info
      retval = close(command)  # syntax error in many Unix awks
 
    `gawk' treats `close()' as a function.  The return value is -1 if
 the argument names something that was never opened with a redirection,
 or if there is a system problem closing the file or process.  In these
 cases, `gawk' sets the built-in variable `ERRNO' to a string describing
 the problem.
 
    In `gawk', when closing a pipe or coprocess (input or output), the
 return value is the exit status of the command.(2) Otherwise, it is the
 return value from the system's `close()' or `fclose()' C functions when
 closing input or output files, respectively.  This value is zero if the
 close succeeds, or -1 if it fails.
 
    The POSIX standard is very vague; it says that `close()' returns
 zero on success and nonzero otherwise.  In general, different
 implementations vary in what they report when closing pipes; thus the
 return value cannot be used portably.  (d.c.)  In POSIX mode (
 Options), `gawk' just returns zero when closing a pipe.
 
    ---------- Footnotes ----------
 
    (1) The technical terminology is rather morbid.  The finished child
 is called a "zombie," and cleaning up after it is referred to as
 "reaping."
 
    (2) This is a full 16-bit value as returned by the `wait()' system
 call. See the system manual pages for information on how to decode this
 value.
 

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