__XYZ__ what it means?

[gaurava]

Hi!,
Can you tell me what is the significance of using "_" or "__" while declaring any function or #define?
I see a lot of #define or typedef structs with these things.
eg:
typedef struct xyz {
....
....
} __ABC__ efg.
or,
_func(); // in declarations usually,but when called,the "_" is dropped.

Regards,
Kumar Gaurava.

 

[gonzilla]

Hi,

Nothing too special with it. All they are really there for (as far as I have heard) is to make sure that there are no naming conflicts in case the definitions or functions are already defined in some header file you are including. So, suppose you put a define like

#ifndef STANDARD
#define STANDARD
....
#endif

And you include somebody elses header file and they had

#ifndef STANDARD
#define STANDARD
...
#endif

Well, you can see there will be a problem since it will define STANDARD on the first one it saw and never hit your other code. By using

#ifndef __STANDARD__
#define __STANDARD__
...
#endif

You make it less likely that someone else has already defined it somewhere else.

-Tyler
-Tyler