Global variables in header file

Question

I have a 2 modules   c files  and one  h header file   file1 c    include  lt stdio h gt   include  global h   int main         i   100      printf   d n  i       foo        return 0      file2 c   include  lt stdio h gt   include  global h   void foo         i   10      printf   d n  i       global h  int i  extern void foo     When I do gcc file1 c file2 c everything works fine and I get the expected output  Now  when I initialize variable  i  in the header file to say 0 and compile again I get a linker error    tmp cc0oj7yA o   bss 0x0   multiple definition of  i   tmp cckd7TTI o   bss 0x0   first defined here   If I just compile file1 c  removing call to foo    with the initialization in the header file i e  gcc file1 c  everything works fine  What is going on

User · Answer

The currently-accepted answer to this question is wrong. C11 6.9.2/2:

If a translation unit contains one or more tentative definitions for an identifier, and the translation unit contains no external definition for that identifier, then the behavior is exactly as if the translation unit contains a file scope declaration of that identifier, with the composite type as of the end of the translation unit, with an initializer equal to 0.

So the original code in the question behaves as if file1.c and file2.c each contained the line int i = 0; at the end, which causes undefined behaviour due to multiple external definitions (6.9/5).

Since this is a Semantic rule and not a Constraint, no diagnostic is required.

Here are two more questions about the same code with correct answers:

User · Answer

You should not define global variables in header files  You can declare them as extern in header file and define them in a  c source file    Note  In C  int i  is a tentative definition  it allocates storage for the variable    is a definition  if there is no other definition found for that variable in the translation unit

User · Answer

glglgl already explained why what you were trying to do was not working  Actually  if you are really aiming at defining a variable in a header  you can trick using some preprocessor directives   file1 c    include  lt stdio h gt    define DEFINE I  include  global h   int main         printf   d n  i       foo        return 0      file2 c    include  lt stdio h gt   include  global h   void foo         i   54      printf   d n  i       global h    ifdef DEFINE I int i   42   else extern int i   endif  void foo      In this situation  i is only defined in the compilation unit where you defined DEFINE I and is declared everywhere else  The linker does not complain   I have seen this a couple of times before where an enum was declared in a header  and just below was a definition of a char   containing the corresponding labels  I do understand why the author preferred to have that definition in the header instead of putting it into a specific source file  but I am not sure whether the implementation is so elegant

User · Answer

Dont define varibale in header file   do declaration in header file good practice      in your case it is working because multiple weak symbols    Read about weak and strong symbol     link  http   csapp cs cmu edu public ch7-preview pdf  This type of code create problem while porting

User · Answer

Don t initialize variables in headers  Put declaration in header and initialization in one of the c files   In the header   extern int i    In file2 c   int i 1

User · Answer

There are 3 scenarios  you describe    with 2  c files and with int i  in the header  With 2  c files and with int i 100  in the header  or any other value  that doesn t matter   With 1  c file and with int i 100  in the header    In each scenario  imagine the contents of the header file inserted into the  c file and this  c file compiled into a  o file and then these linked together   Then following happens    works fine because of the already mentioned  tentative definitions   every  o file contains one of them  so the linker says  ok   doesn t work  because both  o files contain a definition with a value  which collide  even if they have the same value  - there may be only one with any given name in all  o files which are linked together at a given time  works of course  because you have only one  o file and so no possibility for collision    IMHO a clean thing would be   to put either extern int i  or just int i  into the header file  and then to put the  real  definition of i  namely int i   100   into file1 c  In this case  this initialization gets used at the start of the program and the corresponding line in main   can be omitted   Besides  I hope the naming is only an example  please don t name any global variables as i in real programs

[c] Global variables in header file

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