c - Can #ifndef ignore method or variable duplications? -


consider code.

#ifndef foo_h #define foo_h //code #endif

code can following cases

// case 1:  #define foo 0 
// case 2: void foo_method(){}; 
// case 3: int foo;

foo.h included in many c files. when compile case 1 without errors, other cases throw errors of duplication.

why when foo.h not concatenated c files except 1 while compiling?

about case 2:
should declare function signature, not body. has nothing pre-processor commands.

in header file (decleration only)

#if <condition> void foo(); #endif

in c file 

#if <condition> void foo(){    //body }  #endif

about case 3:
it's similar case 2, addition should declare variables in header file if extern , there no need declare them in header file otherwise. if are declared extern, need declared in c file without extern keyword:

in header file:

#if <condition> extern int bar; #endif

in c file:

#if <condition> int bar; #endif

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