Thread: external variables

I am trying to use external variables in a fairly simple program. but on build time I am reciving an warnigs and errors

• warnig:benign redefinition of type
• error:intializer is not a constant

what I have done is use a constant foo let us say. declared foo in const.h

Code:

extern int foo;

then in const.c have said

Code:

 
      int foo;
      int m=3;
foo = 140 *m;

reciving the error.. I have no clue y it don't work .. any help is good cheers