/usr/include/lib.h defines or includes headers that define:

1. MM (This is defined as PM_PROC_NR and identifies the receiver process as the pm).
2. MYGETPID, etc., defines for the number of each system call.

   (These are not defined directly in lib.h, but lib.h includes minix/callnr.h where they are defined.

   So in general, you don't change lib.h, but you will need to change callnr.h when adding a new system call to Minix.

3. _syscall prototype (defined in lib.h)
4. message (This is defined in minix/ipc.h which is included by lib.h)