SELECT(3am)                GNU Awk Extension Modules               SELECT(3am)

       select - enable I/O multiplexing, non-blocking I/O, and signal trapping

       @load "select"

       result = kill(<pid>, <signal number or name>)
       string = select_signal(<signal number or name>, {default|ignore|trap}
       [, <override>])
       fd = input_fd(<file or command> [, <redirection type>])
       fd = output_fd(<file or command>, <redirection type>)
       result = set_non_blocking(<file or command or fd number> [,
       <redirection type>])
       result = select(<readfds>, <writefds>, <exceptfds> [, <timeout> [,

       The select extension adds six functions as follows:

       kill() This function calls kill(2) to send a signal to a process.  The
              second argument may be specified as an integer or as a standard
              signal name defined on this system.  The names are not case-
              sensitive, and the leading "SIG" is optional.  It returns the
              value returned by the kill system call.  If the return code is
              negative, it updates ERRNO.

              This function uses sigaction(2) to change signal actions.  The
              first signal argument may be specified as an integer or as a
              standard signal name defined on this system.  The names are not
              case-sensitive, and the leading "SIG" is optional.  The second
              argument must be a string equal to default, ignore, or trap.  If
              the previously installed handler is the default handler or
              ignore or our standard trapping handler, then the new value is
              installed.  If the previous handler is not recognized, then the
              new handler is not installed unless the 3rd override argument is
              present, is numeric, and is non-zero.  This function returns ""
              on error, otherwise a string describing the previously installed
              handler: default, ignore, trap, or unknown.  Any trapped signals
              will be reported synchronously in the results from the select

              Look up the file or command and return the associated input file
              descriptor value or -1 on error.  The file or command will be
              opened or started if gawk has not done so previously.  If the
              first argument is "", then it returns the fd for the currently
              open file corresponding to FILENAME.  Otherwise, the second
              argument is required and must have one of the following values:

              >  a file opened for output;

              >> a file opened for append;

              <  a file opened for input;

              |> a pipe used for output;

              |< a pipe used for input;

              |& a two-way coprocess.

              This is similar to input_fd but returns a file descriptor
              suitable for output.  Note that gawk may choose to use a
              different file descriptor for coprocess input and output.  For
              this function, two arguments are required.  On error, -1 is

              If the first argument is "", then the 2nd argument is not
              required, and the currently open file corresponding to FILENAME
              will be set to non-blocking by using fcntl(2) to turn on
              O_NONBLOCK.  Similarly, if the first argument is numeric, we
              simply set that file descriptor to be non-blocking.  Otherwise,
              we look up the <file or command> and <redirection type>.  This
              returns 0 on success or -1 on error.  If this argument is a two-
              way coprocess that defines different input and output file
              descriptors, we set the input side to be non-blocking.  For
              finer control, please use input_fd or output_fd to specify which
              file descriptor to configure for non-blocking behavior.  If the
              first argument is a name and not a file descriptor, and we are
              able to configure non-blocking mode successfully, and it is not
              an output-only file, we also automatically create the array
              entry PROCINFO [<file or command>, "RETRY"] so that I/O will
              automatically be retried for input from this file.

              This function returns -1 on error or the number of file
              descriptors that matched.  On return, the <signals> array, if
              supplied, contains a list of signals that were trapped since the
              last call.  The index is the signal number, and the value will
              be the symbolic signal name (e.g. "INT") if we are able to look
              it up.  If <timeout> is present and numeric, that is the maximum
              number of seconds to wait.  Otherwise, it will block
              indefinitely.  The <readfds>, <writefds>, and <exceptfds> arrays
              will have the <command> in the index, and the <command type> as
              the value.  This works the same way as the set_non_blocking
              function.  If the index value is numeric and the value is "", it
              will be treated as a file descriptor.

       One note regarding signal behavior: the extension uses sigaction to
       request that signal calls be restarted.  But on Linux, the "select" is
       always interrupted in any case.  So that's nice -- the signals get
       delivered quickly.  On Cygwin, I noticed that select does seem to
       restart, so the signal is not delivered to the application right away.

       Please refer to
       which are included in the distribution.

       kill(2), sigaction(2), select(2), fcntl(2)

       Andrew Schorr

       Copyright © 2012, 2013, Free Software Foundation, Inc.

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Free Software Foundation          Jan 15 2013                      SELECT(3am)