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Re^2: Different behaviour of 'stat' function after assignment to $! (errno).

by jpl (Monk)
on May 24, 2011 at 13:20 UTC ( #906494=note: print w/ replies, xml ) Need Help??


in reply to Re: Different behaviour of 'stat' function after assignment to $! (errno).
in thread Different behaviour of 'stat' function after assignment to $! (errno).

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


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Re^3: Different behaviour of 'stat' function after assignment to $! (errno).
by ikegami (Pope) on May 24, 2011 at 15:56 UTC

    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.

Re^3: Different behaviour of 'stat' function after assignment to $! (errno).
by Anonymous Monk on May 24, 2011 at 13:31 UTC
    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

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