Generating a header file to be included by the C/XS code seems to be the more portable approach

That's probably sort of what I'm doing - and my initial post oversimplified things.
See this Scalar-List-Utils bug report for a better picture.

The real issue is that MY_FORMAT needs to be defined to "%.17" (if NV is 64 bit), to be defined to "%.21" (if NV is 80 bit), to be defined to "%.36" (if NV is 128 bit).
It's quite easy during the Makefile.PL processing to determine which it needs to be - and surely the simplest thing to do is to just have the Makefile.PL define MY_FORMAT to the correct string.
But you're right ... and jcb's suggestion (along with several variations thereof) is not working.

At the moment, the Makefile.PL is doing: 
....
if($Config{nvsize} == 8)   { $nvtype = 'LU_NV_IS_64_BIT'  } # double o
+r 8-byte long double
elsif(length(sqrt 2) > 25) { $nvtype = 'LU_NV_IS_128_BIT' } # IEEE lon
+g double or __float128
else                       { $nvtype = 'LU_NV_IS_80_BIT'  } # extended
+ precision long double
$defines .= " -D$nvtype";
....
WriteMakefile (
 ....
 DEFINE => $defines,
 ....
);

[download]
And the XS file is rewritten to redefine the symbol to the appropriate value: 
#if   defined(LU_NV_IS_64_BIT)
#define MY_FORMAT "%.17"
#elif defined(LU_NV_IS_80_BIT)
#define MY_FORMAT "%.21"
#else
#define MY_FORMAT "%.36"
#endif

[download]
It's working fine .... but I feel that I should be able to define MY_FORMAT directly within the Makefile.PL and avoid having to add that preprocessing to the XS file ??

Cheers,
Rob

In reply to Re^3: Defining an XS symbol in the Makefile.PL by syphilis
in thread Defining an XS symbol in the Makefile.PL by syphilis

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