dl.kudish has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I am new to perlXS. I am using perl XS(http://perldoc.perl.org/perlguts.html). and writing perl wrapper over my C++ code. I want to return a value of type int64_t from C++ to perl. Please see my code snippet. But it is truncating the value.(of course while compiling itself I am getting this Warning: Conversion of 64 bit type value to "long" causes truncation). In documentation I do not find any information on how to send int64_t (or long) values. Please tell me what i can do here. Thanks in advance D. L. Kumar 
SV* get()
{
   int64_t val = ..;
   if (...){
       return &PL_sv_undef;
   }
   return newSViv(val);
}

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Re: Returning int64_t value from C++ to Perl
by BrowserUk (Patriarch) on Sep 12, 2008 at 10:46 UTC

    Two possibilities.

    1. Convert the I64 to a double and return it as an NV: 
      double i64d ( ) {
    __int64 x = 123435;
    return (double)x;
}

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    2. Return the 8 bytes packed into a string and unpack them in Perl: 
      SV* i64p( ) {
    union {
        __int64 i;
        char s[8];
    } x;
    x.i = 123456789;
    return newSVpv( x.s, 8 );
}

...

## Note: The unpacking will vary with endianness of the platform.
my( @i64 ) = unpack 'VV', i64p();
print $i64[ 1 ] * 2**32 + $i64[ 0 ];

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    Either way, if you are using a 32-bit Perl, you are going to be restricted to integers < ~2**52, so the latter method would only really be useful if you are going pass the I64 (back) to another piece of i64 capable code.

    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: Returning int64_t value from C++ to Perl
by salva (Canon) on Sep 12, 2008 at 10:54 UTC
    You can:
    • Use a perl compiled with 64bit IVs.
    • Store the number as an NV, on most CPU architectures, NVs (=C doubles) can represent integers of 48 bits (49 including the sign). For instance, if you are representing file offsets, that means 256 Terabytes. 
      SV *get() {
  ...
  return newSVnv((NV)val);
}

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      (note that newSVnv() will generate an IV or UV instead of and NV if the data fits inside)
    • Use and external module as Math::Int64 or Math::GMP
    • Return the int64 as a string, converted to decimal or packed.
    Adding a C API to Math::Int64 (I am the author ;-) is in my TODO list. update: DONE!
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[d/l]
      Hi, Thanks a lot. Now I am using external module Math::Int64. Thanks D. L. Kumar
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Re: Returning int64_t value from C++ to Perl
by Anonymous Monk on Sep 12, 2008 at 10:27 UTC
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