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From: brennan@whidbey.com (Mike Brennan) Newsgroups: comp.lang.awk Subject: Re: Learn the SECRET to Attract Women Easily Date: 4 Aug 1997 17:34:46 GMT Message-ID: <5s53rm$eca@news.whidbey.com> On 3 Aug 1997 13:17:43 GMT, Want More Dates??? <tracy78@kilgrona.com> wrote: >Learn the SECRET to Attract Women Easily > >The SCENT(tm) Pheromone Sex Attractant For Men to Attract Women The scent of awk programmers is a lot more attractive to women than the scent of perl programmers. -- Mike Brennan
It is often useful to be able to
send data to a separate program for
processing and then read the result. This can always be
done with temporary files:
# write the data for processing
tempfile = ("/tmp/mydata." PROCINFO["pid"])
while (not done with data)
print data | ("subprogram > " tempfile)
close("subprogram > " tempfile)
# read the results, remove tempfile when done
while ((getline newdata < tempfile) > 0)
process newdata appropriately
close(tempfile)
system("rm " tempfile)
This works, but not elegantly.
Starting with version 3.1 of gawk, it is possible to
open a two-way pipe to another process. The second process is
termed a coprocess, since it runs in parallel with gawk.
The two-way connection is created using the new |& operator
(borrowed from the Korn shell, ksh):1
do {
print data |& "subprogram"
"subprogram" |& getline results
} while (data left to process)
close("subprogram")
The first time an I/O operation is executed using the |&
operator, gawk creates a two-way pipeline to a child process
that runs the other program. Output created with print
or printf is written to the program's standard input, and
output from the program's standard output can be read by the gawk
program using getline.
As is the case with processes started by |, the subprogram
can be any program, or pipeline of programs, that can be started by
the shell.
There are some cautionary items to be aware of:
gawk currently stands, the coprocess's
standard error goes to the same place that the parent gawk's
standard error goes. It is not possible to read the child's
standard error separately.
</itemizedlist>
gawk automatically
flushes all output down the pipe to the child process.
However, if the coprocess does not flush its output,
gawk may hang when doing a getline in order to read
the coprocess's results. This could lead to a situation
known as deadlock, where each process is waiting for the
other one to do something.
It is possible to close just one end of the two-way pipe to
a coprocess, by supplying a second argument to the close
function of either "to" or "from"
(see Closing Input and Output Redirections).
These strings tell gawk to close the end of the pipe
that sends data to the process or the end that reads from it,
respectively.
This is particularly necessary in order to use
the system sort utility as part of a coprocess;
sort must read all of its input
data before it can produce any output.
The sort program does not receive an end-of-file indication
until gawk closes the write end of the pipe.
When you have finished writing data to the sort
utility, you can close the "to" end of the pipe, and
then start reading sorted data via getline.
For example:
BEGIN {
command = "LC_ALL=C sort"
n = split("abcdefghijklmnopqrstuvwxyz", a, "")
for (i = n; i > 0; i--)
print a[i] |& command
close(command, "to")
while ((command |& getline line) > 0)
print "got", line
close(command)
}
This program writes the letters of the alphabet in reverse order, one
per line, down the two-way pipe to sort. It then closes the
write end of the pipe, so that sort receives an end-of-file
indication. This causes sort to sort the data and write the
sorted data back to the gawk program. Once all of the data
has been read, gawk terminates the coprocess and exits.
As a side note, the assignment LC_ALL=C in the sort
command ensures traditional Unix (ASCII) sorting from sort.