#!/usr/bin/perl

use Inline C => <<'EOC';

void foo() {
    printf("something being sent to stdout\n");
    fflush(stdout);
}

EOC

my $output;
{
    # save original STDOUT
    open my $saved_stdout, ">&STDOUT";

    # create a pipe, which we'll use to read from our own STDOUT
    local(*RH, *WH);
    pipe RH, WH;

    # connect the writing side of the pipe to STDOUT, with
    # STDOUT being (and remaining) fileno 1 (!)
    open STDOUT, ">&WH" or die "open: $!";

    # debug: verify that fileno really is 1
    printf STDERR "fileno(STDOUT): %d\n", fileno(STDOUT);
    
    # call the C/XS function whose stdout we want to capture
    foo();
    
    # close WH to avoid buffering issues (pipes are buffered)
    close WH;
    # read output (one line)
    $output = <RH>;
    close RH;

    # restore original STDOUT
    open STDOUT, ">&", $saved_stdout or die "open: $!";
}
print "output of foo(): $output";

##</code><code>##

fileno(STDOUT): 1
output of foo(): something being sent to stdout