The no_sys function in pm/utility.c:

  PUBLIC int no_sys()
  {
      /* A system call number not implemented by PM has been requested. */
      printf("PM: in no_sys, call nr %d from %d\n", call_nr, who_e);
      return(ENOSYS);
  }

PUBLIC and PRIVATE are preprocessor macros defined in

  /usr/src/include/minix/const.h

    #define PUBLIC   
    #define PRIVATE  static

If a function is defined in a file such as utility.c, and has the static modifier, it is not visible to the linker.

So it can't be called from any other files.

Consequently for static functions, each file can have its own 'private' version of a function.