V2615. MISRA. A compatible declaration should be visible when an object or function with external linkage is defined.

This diagnostic rule is based on the MISRA (Motor Industry Software Reliability Association) manual for software development.

This rule only applies to C. Don't define objects and/or functions with external linkage without prior declaration.

The point of this rule is to avoid "manual" use of external entities in places where they are needed. You should declare external objects and functions in a header file.

For example, the following code fragment defines an external variable and function without a prior declaration:

int foo;
void bar(void) {
  // ....
}

You should put declarations in a header file. It is also worth including the header file in the compiled one:

// file.h
extern int foo;
extern void bar(void);

// file.c
#include "file.h"
int foo;
void bar(void) {
  // ....
}

You do not need a prior declaration of objects and functions with internal linkage:

static int baz;
static void qux(void) {
  // ....
}

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