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Re^5: Using Inline::C in a package

by BrowserUk (Pope)
on Sep 05, 2013 at 12:30 UTC ( #1052557=note: print w/ replies, xml ) Need Help??


in reply to Re^4: Using Inline::C in a package
in thread Using Inline::C in a package

That seems to be all that's needed - for any HANDLEs I've passed around, at least.

Yes. I've done that too. The problem that arose was that if I did anything to the value in Perl that caused perl to look at it as anything other than an unsigned integer, and by the time that value got back to the C code it could be corrupted.

From memory, the first time was a quick debug statement something like:

print 'Before call: '. $handle;

Notice the accidental '.' instead of ','. That caused the handle to be converted to the string representation of itself, and by the time it got back to the c code and was converted back from string to number, it was corrupted.

I forget the exact details -- it was a while ago -- but from memory, the (32-bit) unsigned value was greater 2**31-1, so the default string representation took on a minus sign. When converting back it went through a signed integer before being cast to an unsigned integer and that screwed it.

Another problem is portability. In 32-bit, void* is a 32-bit unsigned integer; On 64-bit, a 64-bit value.

And cross-platform, aren't unsigned long different sizes on 64-bit *nix and Windows? (Or is that the long long type.) See this from wikipedia:

Many 64-bit compilers today use the LP64 model (including Solaris, AIX, HP-UX, Linux, OS X, BSD, and IBM z/OS native compilers). Microsoft's Visual C++ compiler uses the LLP64 model. The disadvantage of the LP64 model is that storing a long into an int may overflow. On the other hand, casting a pointer to a long will work. In the LLP model, the reverse is true. These are not problems which affect fully standard-compliant code, but code is often written with implicit assumptions about the widths of integer types.

And then there is the (somewhat cockeyed) reasoning MS use the "opaque" void * for things that are not pointers.

In general, I've found it best to (try to) keep values that originate from C/system libraries, well clear of anywhere Perl can try and manipulate them. Because whilst it works 99% of the time; tracking down the 1% is hell on earth :)


With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.


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Re^6: Using Inline::C in a package
by syphilis (Canon) on Sep 05, 2013 at 13:01 UTC
    if I did anything to the value in Perl that caused perl to look at it as anything other than an unsigned integer, and by the time that value got back to the C code it could be corrupted

    Yes, that would not be nice :-(

    aren't unsigned long different sizes on 64-bit *nix and Windows?

    I think that's generally (perhaps universally) so. I do know that the long type on my 64-bit Ubuntu box is 64 bits, but 32-bits on 64-bit Windows.
    This is not so much of a consideration as regards Windows HANDLEs specifically, but is of relevance in general.

    Cheers,
    Rob
      This is not so much of a consideration as regards Windows HANDLEs specifically ...

      Of course. Silly me :)


      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.

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