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Hi !

My B program is a simple script, which reads a string from STDIN,
converts it to uppercase and prints it to STDOUT.

B.pl
#!/bin/perl
use strict ;
my $string=<STDIN>;
print STDOUT uc $string,"\n" ;

Now, I have A program, which listens on a TCP socket. Whenever string
come over socket, it should be put on STDIN buffer (yes, STDIN), forking
a new child, redirecting the parent SOCKET to child's STDOUT and exec
B.pl. (The same way Apache & CGI modules work). I was trying to use
select() but no luck.

A.pl
#!/bin/perl
use strict ;
....
SOCKET CREATION, LISTENING, GETTING LINE FROM A SOCKET INTO $LINE
....

TCP settings....
.......
if (my $pid=fork)
{
waitpid($pid,0); # Ok, here is parent
} else
{
my $string="This must be put in STDIN buffer" ; << Don't know how
my $old=select(SOCKET) ; << This is not working
exec("A.pl"); # and child
select $oldstdout ;
}

Any help would be appreciated

Glodalec <> wrote:
> Now, I have A program, which listens on a TCP socket. Whenever string
> come over socket, it should be put on STDIN buffer (yes, STDIN), forking
> a new child, redirecting the parent SOCKET to child's STDOUT and exec
> B.pl. (The same way Apache & CGI modules work). I was trying to use
> select() but no luck.

No, that's not how filehandles work at all. You can't push stuff back
onto them: if you want to pass stuff to your child you have to use a
pipe.

> A.pl
> #!/bin/perl
> use strict ;
> ...
> SOCKET CREATION, LISTENING, GETTING LINE FROM A SOCKET INTO $LINE
> ...
>
> TCP settings....
> ......
> if (my $pid=fork)

fork can fail, in which case it returns undef.

> {
> waitpid($pid,0); # Ok, here is parent
> } else
> {
> my $string="This must be put in STDIN buffer" ; << Don't know how
> my $old=select(SOCKET) ; << This is not working
> exec("A.pl"); # and child
> select $oldstdout ;
> }
>
> Any help would be appreciated

my $pid = open my $TOCHILD, '|-', 'A.pl'
or die "fork failed: $!";

print $TOCHILD "The child will read this on its STDIN";

See perldoc -f open.

Ben

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In article <bvoe9t$t31$>,
says...
>
> Glodalec <> wrote:
> > Now, I have A program, which listens on a TCP socket. Whenever string
> > come over socket, it should be put on STDIN buffer (yes, STDIN), forking
> > a new child, redirecting the parent SOCKET to child's STDOUT and exec
> > B.pl. (The same way Apache & CGI modules work). I was trying to use
> > select() but no luck.
>
> No, that's not how filehandles work at all. You can't push stuff back
> onto them: if you want to pass stuff to your child you have to use a
> pipe.
Had never used them (except on command line), will read about them.
>
> > A.pl
> > #!/bin/perl
> > use strict ;
> > ...
> > SOCKET CREATION, LISTENING, GETTING LINE FROM A SOCKET INTO $LINE
> > ...
> >
> > TCP settings....
> > ......
> > if (my $pid=fork)
>
> fork can fail, in which case it returns undef.
I know, this script has been arranged a lot when posting here.
>
> > {
> > waitpid($pid,0); # Ok, here is parent
> > } else
> > {
> > my $string="This must be put in STDIN buffer" ; << Don't know how
> > my $old=select(SOCKET) ; << This is not working
> > exec("A.pl"); # and child
> > select $oldstdout ;
> > }
> >
> > Any help would be appreciated
>
> my $pid = open my $TOCHILD, '|-', 'A.pl'
> or die "fork failed: $!";
>
> print $TOCHILD "The child will read this on its STDIN";
THanks for advice. How about a child exec, which should actually write
to parent's SESSION handler (got from accept()), instead of its own
STDOUT.?
>
> See perldoc -f open.
>
> Ben
>
>

Glodalec <> wrote:
> > print $TOCHILD "The child will read this on its STDIN";
> THanks for advice. How about a child exec, which should actually write
> to parent's SESSION handler (got from accept()), instead of its own
> STDOUT.?

It's probably easiest to do that manually, viz (untested):

accept my $NEWCONN, $LISTENER or die "accept failed: $!";

# or you'd probably be better off using IO::Socket::INET
my $NEWCONN = $LISTENER->accept or die "accept failed: $!";

pipe my ($FROMPARENT, $TOCHILD) or die "pipe failed: $!";

my $pid = fork;
defined $pid or die "fork failed: $!";
unless ($pid) {
open STDIN, '<&', $FROMPARENT or die "dup2 to STDIN failed: $!";
open STDOUT, '>&', $NEWCONN or die "dup2 to STDOUT failed: $!";
exec 'A.pl' or die "exec failed: $!";
}

close $FROMPARENT;
close $NEWCONN;

Obviously you want to put all this in a sub, so you can say

my $TOCHILD = accept_new_child($LISTENER);

If you get errors from the open '<&' syntax, you may need a newer
version of perl; or you can replace them with

open STDIN, '<&' . fileno $FROMPARENT or ...

Ben

--
Like all men in Babylon I have been a proconsul; like all, a slave ... During
one lunar year, I have been declared invisible; I shrieked and was not heard,
I stole my bread and was not decapitated.
~ ~ Jorge Luis Borges, 'The Babylon Lottery'