Rule Definition
When defining a static variable in a header file, a new instance of the variable is created for each file including the header file. This is often surprising as people often expect to have only one instance of the variable. This leads to errors that are very difficult to track/understand.

Remediation
Do not define the variable in header as static but define it as static in a source file (ex: *.c, *.cpp) and as extern in files using it instead. In the particular case of "static const variable=value;", just remove the static, it is redundant with the "const" qualifier.

Violation Code Sample

FILE: f.h

static int myvar = 0;

FILE: f2.cpp

#include "f.h"
void g() {
   myvar = 1;
}

FILE: f1.cpp

#include "f.h"
void g();
int main(int argc, char* argv[]) {
  g();
  printf("myvar value is %d\n", myvar); /* We expect 1 to be displayed, but it is not the case.. */
  return 0;
}

Fixed Code Sample

FILE: f.h

extern int myvar;

FILE: f2.cpp

#include "f.h"
void g() {
   myvar = 1;
}

FILE: f1.cpp

#include "f.h"
int myvar = 0;
void g();
int main(int argc, char* argv[]) {
   g();
   printf("myvar value is %d\n", myvar); /* Display 1 as expected. */
   return 0;
}