what's thie meaning for "#ifndef __C51__"

I don't know the difference between definition and declaration but compiler does not allow me to declare a variable in H file
HUH?

I do it all over, having #defines such as

#ifdef main
#define U8 unsigned char
#else
#define U8 extern unsigned char
#endif

U8 hello;

I don't know the difference between definition and declaration but compiler does not allow me to declare a variable in H file
HUH?

I do it all over, having #defines such as

#ifdef main
#define U8 unsigned char
#else
#define U8 extern unsigned char
#endif

U8 hello;

Erik,

Placing the defining declaration in a header file is precisely what the other replies to this thread have explained is bad practice. Strangely enough, it makes code very difficult to maintain.

Stefan

Standard Pascal does not allow for separate compilation at all, so the issue of modules and name spaces does not arise. Real-world useful Pascal implementations (say, Borland's Turbo Pascal) added their own language mechanisms to support separate compilation, none of which are necessarily compatible. So it's hard to say anything meaningful about what "Pascal" does with namespaces.

I favor the practice of using a .h file as a "declarations" file only. Put in the internal #include guards. Some compilers will track the #include nesting and avoid re-including a file anyway, but it's best to be sure. Putting in the actual definitions (space-reserving, as opposed to extern references) tends to lead to trouble.

There are occasional other header file tricks, usually redefining a macro in two different ways to produce two different bits of C syntax when included from different contexts. (See, for example, Bob Horeck's comment and use of TxtItem in the thread http://www.keil.com/forum/docs/thread2785.asp. The authors of the OSE RTOS are also fond of this trick for implementing their configuration files.) This sort of thing is, in my opinion, best kept well separated from your "normal" header files to avoid confusion.

There's use of the #include mechanism to emulate module export / linkage mechanisms that ought to have been built into the language proper, which is a near-universal C programmer convention; and then there's use of the preprocessor just as a text macro processor not necessarily having anything to do with C.