linux - Perl anonymous pipe no output -

-

question

why nothing printed when using anonymous pipe, unless print actual data pipe ?

example

use strict; use warnings;  $child_process_id = 0; $vmstat_command = 'vmstat 7|'; $child_process_id = open(vmstat, $vmstat_command) || die "error when executing \"$vmstat_command\": $!";   while (<vmstat>) {     print "hi" ; } close vmstat or die "bad command: $! $?";

appears hang

use strict; use warnings;  $child_process_id = 0; $vmstat_command = 'vmstat 7|'; $child_process_id = open(vmstat, $vmstat_command) || die "error when executing \"$vmstat_command\": $!";   while (<vmstat>) {     print "hi" . $_ ; #                ^^^ added } close vmstat or die "bad command: $! $?";

prints 

hiprocs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------ hi r  b   swpd   free   buff  cache   si      bi    bo   in   cs sy id wa st hi 1  0      0 7264836 144200 307076    0    0     0     1    0   14  0  0 100  0  0

etc...

expected behaviour

would print hi every line of output of vmstat first example.

versions

perl, v5.10.0 gnu bash, version 3.2.51

misc

it appears hang when using chomp before printing line (which thought removes newlines).

i feel i'm missing fundamental how pipe read , processed not find similar question. if there 1 dupe , i'll have @ it.

any further information needed ask.

alter 

print "hi";

to 

print "hi\n";

and "works"

the reason fails output line buffered default

setting $| flush buffer straight away

if set nonzero, forces flush right away , after every write or print on selected output channel. default 0 (regardless of whether channel buffered system or not; "$|" tells whether you've asked perl explicitly flush after each write). stdout typically line buffered if output terminal , block buffered otherwise. setting variable useful when outputting pipe or socket, such when running perl program under rsh , want see output it's happening. has no effect on input buffering. see getc entry in perlfunc manpage that. (mnemonic: when want pipes piping hot.)


Comments

Post a Comment

Popular posts from this blog

Hatching array of circles in AutoCAD using c# -

-
i beginner in autocad plugins .i have created array of circle using following code. now, have hatch circles in array .how that??how hatch circles @ once? moreover, how can return hatch method , use hatch objects.sorry language.thanks in advance. [assembly: commandclass(typeof(beamsection.class1))] namespace beamsection { public class class1 { //class1 obj = new class1(); [commandmethod("beamatsupport")] public void addlightweightpolyline() { // current document , database document acdoc = application.documentmanager.mdiactivedocument; database accurdb = acdoc.database; promptpointresult ppr = acdoc.editor.getpoint("\nselect starting point "); var ucs = acdoc.editor.currentusercoordinatesystem; point3d startingpt = ppr.value.transformby(ucs); list<double> radius = new list<double>(); radius.add(0.5);
Read more

ios - UITEXTFIELD InputView Uipicker not working in swift -

-
hello trying pop uipickerview view programmatically when user clicks on textfield. have tried doesn't doing anything. isn't working. nothing happening when click textfield class userprofiletableviewcontroller: uitableviewcontroller,uitextfielddelegate,uipickerviewdelegate,uipickerviewdatasource { var itempicker: uipickerview! = uipickerview() @iboutlet weak var gendertxtfield: uitextfield! var gender = ["male","female"] override func viewdidload() { super.viewdidload() gendertxtfield.delegate = self itempicker!.delegate = self itempicker!.datasource = self itempicker!.backgroundcolor = uicolor.blackcolor() self.gendertxtfield.inputview = itempicker } func numberofcomponentsinpickerview(pickerview: uipickerview) -> int{ return 1 } // returns # of rows in each component.. func pickerview(pickerview: uipickerview, numberofrowsincomponent component:
Read more

Python Pig Latin Translator -

-
so have been working on @ codecademy.com while trying learn python. don't know if blind , can't see why won't work or what. here's code won't let me use. function print variable if had typed in , print "empty" if didn't type anything. print "welcome english pig latin translator!" original = raw_input("what's name?") if len(original) > 0 print original else print "empty" if len(original) > 0 else the above lines contains syntax errors. try: if len(original) > 0: else: note added colons, ( : ).
Read more