Beefy Boxes and Bandwidth Generously Provided by pair Networks
Perl-Sensitive Sunglasses

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??

The problem presumably is the following: the way you're connecting STDOUT to the pipe is changing the file descriptor number so that it's no longer 1. However, file descriptor 1 is what your exec'ed child process is assuming to be stdout (i.e. the default).

printf STDERR "before: fileno(STDOUT)=%d\n", fileno(STDOUT); open $old_stdout, ">&STDOUT" or die "open: $!"; close( STDOUT ); pipe( $smash_stdout, STDOUT ); printf STDERR "after: fileno(STDOUT)=%d\n", fileno(STDOUT);


before: fileno(STDOUT)=1 after: fileno(STDOUT)=6

Note that fileno changed from 1 to 6.  In other words, you now have fileno 6 connected to the pipe (something an exec'ed child would know nothing about...)

Something like this might work better

printf STDERR "before: fileno(STDOUT)=%d\n", fileno(STDOUT); open $old_stdout, ">&STDOUT" or die "open: $!"; # _don't_ explicitly close STDOUT here local(*RH, *WH); pipe RH, WH; open STDOUT, ">&WH" or die "open: $!"; printf STDERR "after: fileno(STDOUT)=%d\n", fileno(STDOUT);

because this way, STDOUT keeps being associated with file descriptor 1:

before: fileno(STDOUT)=1 after: fileno(STDOUT)=1

Update: a simple demo:

open $old_stdout, ">&STDOUT" or die "open: $!"; local(*RH, *WH); pipe RH, WH; open STDOUT, ">&WH" or die "open: $!"; # write something to the pipe print "foo"; # and have some fork/exec'ed process write to the pipe (via stdout) system('echo bar'); close WH; my $out = <RH>; print STDERR "got from pipe: $out"; # $out is "foobar\n"

In reply to Re: Forks, Pipes and Exec (file descriptors) by almut
in thread Forks, Pipes and Exec by diabelek

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Are you posting in the right place? Check out Where do I post X? to know for sure.
  • Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
    <code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>
  • Snippets of code should be wrapped in <code> tags not <pre> tags. In fact, <pre> tags should generally be avoided. If they must be used, extreme care should be taken to ensure that their contents do not have long lines (<70 chars), in order to prevent horizontal scrolling (and possible janitor intervention).
  • Want more info? How to link or How to display code and escape characters are good places to start.
Log In?

What's my password?
Create A New User
Domain Nodelet?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others meditating upon the Monastery: (1)
As of 2023-06-01 23:32 GMT
Find Nodes?
    Voting Booth?

    No recent polls found