The struct is a declaration of a type. You cannot mix it with the initialization. This is how it could be:

In foo.h:

#ifndef FOO_H
#define FOO_H

struct Foo{
  int randInt;
};

extern struct Foo foo;
struct Foo *initFoo(struct Foo *foo_ptr);
#endif

In foo.c:

#include "foo.h"

struct Foo foo;
struct Foo *initFoo(struct Foo *foo_ptr) {
  if (!foo_ptr)
    return NULL;
  foo_ptr->randInt = random(0, 101);
  return foo_ptr;
}

random() is a function and therefore its return value cannot be used as static initializer.

The struct is a declaration of a type. You cannot mix it with the initialization. This is how it could be:

In foo.h:

#ifndef FOO_H
#define FOO_H

struct Foo{
  int randInt;
};

extern struct Foo foo;
struct Foo *initFoo(struct Foo *foo_ptr);
#endif

In foo.c:

#include "foo.h"

struct Foo foo;
struct Foo *initFoo(struct Foo *foo_ptr) {
  if (!foo_ptr)
    return NULL;
  foo_ptr->randInt = random(0, 101);
  return foo_ptr;
}

random() is a function and therefore its return value cannot be used as static initializer. Unless -std=c++11 or -std=gnu++11 are enabled.