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

Hello,

I was wondering if anyone can explain the difference between the outputs of two following programs. The only difference between the programs is that 0 is assigned to $! in the second program.

Program 1:

stat("non_existing_file");
die "died at first stat" if -$!;
stat("non_existing_file");
die "died at second stat" if -$!;

[download]
Output: 
died at first stat at a.pl line 3.

Program 2:

$! = 0;
stat("non_existing_file");
die "died at first stat" if -$!;
stat("non_existing_file");
die "died at second stat" if -$!;

[download]

Output:

died at second stat at a.pl line 5.

If 0 is assigned to $! before the first stat, the first failed stat does not set the $!.

I also noticed that if instead of $!=0 we do a undef $!, the program behaves as the first one.

Thanks!

Replies are listed 'Best First'.
Re: Different behaviour of 'stat' function after assignment to $! (errno).
by Neighbour (Friar) on May 24, 2011 at 12:40 UTC
    If 0 is assigned to $! before the first stat, the first failed stat does not set the $!.
    But it does (as the numerous examples above show). The failed stat fills the Numeric Value of $!, and using $! in your code reads it (Devel::Peek::Dump shows the flags of the var not including pIOK, so the Integer Value is not used). However using -$! forces perl to use the Integer Value instead, which still contains your assigned value.(This was already pointed out in Re^3: Different behaviour of 'stat' function after assignment to $! (errno). by Anonymous Monk, but I thought I might explain it with a bit more words).
    This could be considered unwanted behaviour, and you could opt to patch perl to set or clear the IV on error.

    On the other hand, you could stop using -$! in your checks :)
[reply]
      It's a little deeper than that. 
      use Devel::Peek;
$! = 0.0;
Dump($!);
stat("non_existing_file");
Dump($!);
die "died at first stat" if -$!;
stat("non_existing_file");
Dump($!);
die "died at second stat" if -$!;

      [download]
      now yields 
      SV = PVMG(0x82c690) at 0x803478
  REFCNT = 1
  FLAGS = (GMG,SMG,pIOK,pNOK)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x804900
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x803460
    MG_LEN = 1
    MG_PTR = 0x8032d0 "!"
SV = PVMG(0x82c690) at 0x803478
  REFCNT = 1
  FLAGS = (GMG,SMG,pIOK,pNOK)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x804900
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x803460
    MG_LEN = 1
    MG_PTR = 0x8032d0 "!"
died at first stat at bug.pl line 6.

      [download]
      pNOK is now set, but the value hasn't been updated by stat itself. It's only when $! is referenced that the updated value materializes. I certainly wouldn't recommend using -$!, but perlvar says
      If used numerically, yields the current value of the C "errno" variable, or in other words, if a system or library call fails, it sets this variable. This means that the value of $! is meaningful only immediately after a failure:
      so one can be excused for doing numerical things. It also says
      You can assign a number to $! to set errno if, for instance, you want "$!" to return the string for error n, or you want to set the exit value for the die() operator.
      and having to assign a non-integer value to avoid surprises doesn't fall in my category of "do what I mean".
[reply]
[d/l]
[select]

        pNOK is now set, but the value hasn't been updated by stat itself. It's only when $! is referenced that the updated value materializes.

        Correct. It's set by SvGETMAGIC, which is called when the value of a magic SV is needed. The state of pNOK and the like should be completely ignored by -$! since negation should call SvGETMAGIC, but negation isn't calling SvGETMAGIC in the OP's version of Perl.

[reply]
[d/l]
        and having to assign a non-integer value to avoid surprises doesn't fall in my category of "do what I mean".

        :) Um, do not assign anything to $!, unless you're curious about what types of error messages your system can generate -- errno is there for the system to assign to

[reply]

      This was a toy example. -$! is returned from a perl callback for which it makes sense to return a negated errno back to C. Unfortunately, strange bugs started to pop up under FreeBSD, and this issue could be related to it.

      Thanks everyone for helping.

[reply]
Re: Different behaviour of 'stat' function after assignment to $! (errno).
by Anonymous Monk on May 24, 2011 at 10:53 UTC
    When you do funny things, funny things happen :) 
    $ rm 1
rm: cannot remove `1': No such file or directory

$ perl -e " stat 1; printf qq[%d %s %d\n], $!,$!,-$!;
2 No such file or directory -2

$ perl -e " $!=0; stat 1; printf qq[%d %s %d\n], $!,$!,-$!;
2 No such file or directory 0

$ perl -e " $!=2; stat 1; printf qq[%d %s %d\n], $!,$!,-$!;
2 No such file or directory -2

$ perl -e " $!=1; stat 1; printf qq[%d %s %d\n], $!,$!,-$!;
2 No such file or directory -1

    [download]
[reply]
[d/l]
      I can reproduce it, but not explain it. 
      use Devel::Peek;
$! = 0;
Dump($!);
stat("non_existing_file");
Dump($!);
die "died at first stat" if -$!;
stat("non_existing_file");
Dump($!);
die "died at second stat" if -$!;

      [download]
      yields 
      SV = PVMG(0x214d690) at 0x2124478
  REFCNT = 1
  FLAGS = (GMG,SMG,pIOK)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x2125900
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x2124460
    MG_LEN = 1
    MG_PTR = 0x21242d0 "!"
SV = PVMG(0x214d690) at 0x2124478
  REFCNT = 1
  FLAGS = (GMG,SMG,pIOK)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x2125900
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x2124460
    MG_LEN = 1
    MG_PTR = 0x21242d0 "!"
SV = PVMG(0x214d690) at 0x2124478
  REFCNT = 1
  FLAGS = (GMG,SMG,pNOK,pPOK)
  IV = 0
  NV = 2
  PV = 0x21243a0 "No such file or directory"\0
  CUR = 25
  LEN = 32
  MAGIC = 0x2125900
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x2124460
    MG_LEN = 1
    MG_PTR = 0x21242d0 "!"
died at second stat at bug.pl line 9.

      [download]
      Nothing "funny" has been done to $! before the second Dump, but it hasn't changed since before the stat.
[reply]
[d/l]
[select]
        Nothing "funny" has been done to $! before the second Dump, but it hasn't changed since before the stat.

        $! , errno, is supposed to be positive, so using -$! is something funny :)

[reply]
        The battle of IV versus NV, lol 
        $ perl -MDevel::Peek -e " Dump($!); $!=20; stat 1; printf qq[%d %s %d\
+n], $!,$!,-$!; Dump($!);  "
SV = PVMG(0x9b3adc) at 0x9ba5ac
  REFCNT = 1
  FLAGS = (GMG,SMG)
  IV = 0
  NV = 0
  PV = 0
  MAGIC = 0x9c6d9c
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x9ba59c
    MG_LEN = 1
    MG_PTR = 0x9c2b7c "!"
2 No such file or directory -20
SV = PVMG(0x9b3adc) at 0x9ba5ac
  REFCNT = 1
  FLAGS = (GMG,SMG,pNOK,pPOK)
  IV = 20
  NV = 2
  PV = 0x3ffed4 "No such file or directory"\0
  CUR = 25
  LEN = 28
  MAGIC = 0x9c6d9c
    MG_VIRTUAL = &PL_vtbl_sv
    MG_TYPE = PERL_MAGIC_sv(\0)
    MG_OBJ = 0x9ba59c
    MG_LEN = 1
    MG_PTR = 0x9c2b7c "!"

        [download]
[reply]
[d/l]
Re: Different behaviour of 'stat' function after assignment to $! (errno).
by ikegami (Patriarch) on May 24, 2011 at 15:41 UTC

    I can replicate the problem in Perl 5.12.2. I confirmed that stat returns false, so it's a bug. I'm guessing there's a call to SvGETMAGIC missing in negation.

    I cannot replicate the problem with blead, so it's already been fixed. Probably in 5.14.0.

[reply]
[d/l]
[select]
Re: Different behaviour of 'stat' function after assignment to $! (errno).
by anonymized user 468275 (Curate) on May 24, 2011 at 14:49 UTC
    $! is the extended O/S error. It isn't intended to be written to. Perl tends to be quite a permissive language. But if Perl lends you a loaded gun, is it Perl's fault if you aim at your foot and pull the trigger?

    One world, one people

[reply]

      Writing to errno both directly and via $! aren't new, and there's no need to avoid doing so. 

      $ perl -E'say $!=2'
No such file or directory

$ perl -E'say $!=3'
No such process

$ perl -E'say $!=4'
Interrupted system call

      [download]
[reply]
[d/l]
[select]
      Well, when -$! is not the same as 0-$! then I'd say that it's open to discussion. ;)
[reply]
        I was addressing assigning to it as being the main issue, but I wouldn't negate it either as a general rule, because it certainly doesn't have to be numeric.

        One world, one people

[reply]

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