Ignore this: I found it in C:\Perl\lib\CORE\config.h:1274:#define VAL_O_NONBLOCK O_NONBLOCK

I read Net::Gen fails while using threads and decided to try and install Net::Gen (under win32). The attempt fell at the first hurdle with:

Gen.c
Gen.xs(718) : warning C4101: 'adata' : unreferenced local variable
Gen.xs(783) : error C2065: 'O_NONBLOCK' : undeclared identifier
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Unsurprising perhaps, but still something about this is bugging me. Basically, I cannot work out how this from Gen.xs(781):

BOOT:
#ifdef    VAL_O_NONBLOCK
    newXSconstUV("Net::Gen::VAL_O_NONBLOCK", VAL_O_NONBLOCK, file);
#else
    newmissing(missing, "Net::Gen::VAL_O_NONBLOCK", file);
#endif
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which becomes this in Gen.c(1079):

#line 782 "Gen.xs"
#ifdef    VAL_O_NONBLOCK
    newXSconstUV("Net::Gen::VAL_O_NONBLOCK", VAL_O_NONBLOCK, file);
#else
    newmissing(missing, "Net::Gen::VAL_O_NONBLOCK", file);
#endif
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becomes this is the post preprocessed code seen by the compiler (Gen.E(115644)):

#line 782 "Gen.xs"

    S_newXSconstUV(
           ((PerlInterpreter *)pthread_getspecific((*Perl_Gthr_key_ptr
+(0)))), 
           "Net::Gen::VAL_O_NONBLOCK", O_NONBLOCK, file
      );
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That is, how and where did VAL_O_NONBLOCK become translated to O_NONBLOCK?

And more importantly, how did you find it?