filehandle question

warpod has asked for the wisdom of the Perl Monks concerning the following question:

What is wrong with this code?

==== test. pl ======
#!/usr/bin/perl
use strict;
use mmm;

local* F;
mmm::fopen(F, ">tst");
print F "hi";
mmm::fclose F;

==== mmm.pm ======
#!/usr/bin/perl
use strict;
package mmm;
BEGIN {}
sub fopen(*;$)
{
  local *FH = $_[0];
  return CORE::open(FH, $_[1]);
}
sub fclose(*)
{
  local *FH = $_[0];
  return CORE::close(FH);
}
END { }
1;
[download]

when executed it says:
print() on unopened filehandle F at test.pl line 7.
but if I move fopen and fclose into test.pl, then this code

local* F;
fopen(F, ">tst");
print F "hi";
fclose F;
[download]

works just fine. What is wrong in package?

Comment on filehandle question Select or Download Code

Replies are listed 'Best First'.
Re: filehandle question by BrowserUk (Patriarch) on Oct 11, 2004 at 13:47 UTC
You can't pass a glob by bareword. Try this: `#!/usr/bin/perl use strict; use mmm; local* F; mmm::fopen( F, ">tst"); print F "hi"; mmm::fclose F;` [download] Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail "Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon	[reply] [d/l]
Re: filehandle question by revdiablo (Prior) on Oct 11, 2004 at 17:00 UTC
It may be beside the point, but -- assuming you have a new enough perl -- I would recommend avoiding bareword filehandles, and switching to autovivifying lexicals: `use strict; use warnings; { package mmm; sub fopen { CORE::open $_[0], $_[1]; } sub fclose { CORE::close $_[0]; } } mmm::fopen(my $fh, ">test"); print $fh "Foo, bar.\n"; mmm::fclose($fh);` [download]	[reply] [d/l]
Re: filehandle question by hv (Prior) on Oct 11, 2004 at 14:30 UTC
If you define fopen in the same package as you use it it works fine. When passing the bareword to the mmm package, however, it gets interpreted as `mmm::F` instead of `main::F`, so the file has been opened but in the wrong glob. Given the prototype, this is arguably a bug - the bareword 'F' should be coerced to a glob in the caller's context. However there is no guarantee it is easily fixable - I suspect that it is currently passed as a simple string, and not turned into a glob reference until the actual assignment in `mmm::fopen()`. Hugo	[reply] [d/l] [select]
Re: filehandle question by bobf (Monsignor) on Oct 11, 2004 at 16:53 UTC
As soon as I read your error message I thought, "how do you know you opened F in the first place?" Then I read the rest of your question and the other responses and realized that the globbing issue was the primary problem. Nonetheless, you should always check the return value of `open` to make sure it succeeded. If `open` fails, you will get the same error (`print() on unopened filehandle F`) when you try to print to the filehandle later. open, perlopentut HTH	[reply] [d/l] [select]
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