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Free Software Foundation
Jan 15 2013


Select I/O multiplexing library for gawk


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> [, <signals>]])


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 function.
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 returned.
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.


Andrew Schorr


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