what is the purpose of __ASSEMBLY__?

http://blog.csdn.net/phenix_yw2/article/details/5961350

Slade Maurer 
Mon Jan 3 17:44:46 GMT 2005

 

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Let's keep in mind that we are discussing header files specifically.
__ASSEMBLY__ allows them to be shared between C and assembly code so all
the useful data is in the same place -- as Ho Lee said.

The CPP does process typedef and #defines as Rainer Weikusat states.
However, __ASSEMBLY__ is still required to solve problems with symbols
that gas can't understand but cc1 can.

For example, this section of code from a header file uses it to solve a
problem:

#ifdef __ASSEMBLY__
#define __PAGE_OFFSET  (0xB0000000)
#else
#define __PAGE_OFFSET  (0xB0000000UL)
#endif /* __ASSEMBLY__ */

Now, what is it doing? Well, cleary the developer thought gas would
choke on casting the constant as an unsigned long for cc1 and so uses
the __ASSEMBLY__ directive to allow __PAGE_OFFSET to be handled by gas
and by cc1. More precisely, CPP would use __ASSEMBLY__ to pick the
correct __PAGE_OFFSET token. Then, it would substitute occurances in the
file(s) it is pre-processing before they are passed to the assembler or
compiler. The assembler sees 0xB0000000 and the C compiler sees
0xB0000000UL so they are both happy!

Hope this helps.

-Slade