Variable declaration in a header file

Question

In case I have a variable that may be used in several sources - is it a good practice to declare it in a header  or is it better to declare it in a  c file and use extern in other files

User · Answer

What about this solution    ifndef VERSION H  define VERSION H  static const char SVER      14 2 1   static const char AVER      1 1 0 0     else  extern static const char SVER    extern static const char AVER      endif   VERSION H      The only draw back I see is that the include guard doesn t save you if you include it twice in the same file

User · Answer

The key is to keep the declarations of the variable in the header file and source file the same   I use this trick  ------sample c------  define sample c  include sample h   rest of sample  c   ------sample h------  ifdef sample c  define EXTERN  else  define EXTERN extern  endif  EXTERN int x    Sample c is only compiled once and it defines the variables   Any file that includes sample h is only given the  extern  of the variable  it does allocate space for that variable   When you change the type of x  it will change for everybody   You won t need to remember to change it in the source file and the header file

User · Answer

You can  should  declare it as extern in a header file  and define it in exactly 1  c file    Note that that  c file should also use the header and that the standard pattern looks like      file h extern int x      declaration     file c  include  file h  int x   1        definition and re-declaration

User · Answer

If you declare it like  int x    in a header file which is then included in multiple places  you ll end up with multiple instances of x  and potentially compile or link problems    The correct way to approach this is to have the header file say   extern int x     declared in foo c      and then in foo c you can say   int x     exported in foo h      THen you can include your header file in as many places as you like

User · Answer

You should declare the variable in a header file   extern int x    and then define it in one C file   int x    In C  the difference between a definition and a declaration is that the definition reserves space for the variable  whereas the declaration merely introduces the variable into the symbol table  and will cause the linker to go looking for it when it comes to link time

[c] Variable declaration in a header file

Examples related to c

Examples related to header