Doesn't this work for you?:

Well - it doesn't work for me with 32-bit (gcc-5.3.0) builds of perl.
I was looking at 32-bit perl-5.24.0.: 
C:\_32>perl -V:archname
archname='MSWin32-x86-multi-thread-64int';

C:\_32>perl -le "print $]";
5.024000

C:\_32>perl -le "$d = unpack 'd', pack 'Q', 0x7ff0000000000001;print $
+d; printf '%f', $d;"
NaN
NaN

[download]
That output doesn't tell us anything about the type of NaN we have. However, POSIX::issignaling tells us that $d is not signalling, and printf '%x\n', unpack 'Q', pack 'd', $d; outputs 7ff8000000000001.

Things look ok for my 64-bit (gcc-5.3.0) builds of perl.
With 64-bit perl-5.24.0 the output is the same as just given.
But this time POSIX::issignaling tells us that $d is signalling and printf '%x\n', unpack 'Q', pack 'd', $d; outputs 7ff0000000000001.

A regards your Inline::C script, the output is as expected given the above results.
32-bit perl-5.24.0 ouptuts: 
NaN
7ff8000000000001

[download]
64-bit perl-5.24.0 outputs: 
NaN
7ff0000000000001

[download]
When it comes to Strawberry Perls 5.24.0 (which were built using gcc-4.9.2), I find that both 32-bit and 64-bit are buggy. Your I::C script outputs: 
NaN
7ff8000000000001

[download]
for both.

I'll try to investigate the problem a little better before I submit any bug reports. It may be that I can tie the behaviour to bugs in the mingw-w64 ports of gcc.

Cheers,
Rob

In reply to Re^5: unintentional conversion of signaling NaN to quiet NaN by syphilis
in thread unintentional conversion of signaling NaN to quiet NaN by pryrt

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