differences between <math.h> and gcc's <math.h>

[Martin Sebor]

Can someone help me understand why the simple program below compiles
fine when it #includes <math.h> but fails when it #includes the very
same header using an absolute pathname? The two translation unit are
in fact quite different -- the one that #includes <math.h> contains
in addition a bunch of C99 functions (e.g., modff). I don't see any
differences in the output from gcc -v other than the name of the temp
file. This is on SunOS 5.8/i86.

Thanks
Martin

$ cat t.cpp ; gcc t.cpp -H -c -DMATH_H="\""`gcc t.cpp -H -c -DMATH_H="<math.h>"
2>&1 | head -1 | sed -n "s/^[^/]*\(\/.*\)/\1/p"`"\""
#include MATH_H

void foo ()
{
    (void)&modff;
}

. /usr/local/gcc-3.0.1/lib/gcc-lib/i386-pc-solaris2.8/3.0.1/include/math.h
.. /usr/include/iso/math_iso.h
t.cpp: In function `void foo()':
t.cpp:5: `modff' undeclared (first use this function)
t.cpp:5: (Each undeclared identifier is reported only once for each function it 
   appears in.)