How to Add New System Calls to Minix 3.1.8

There are some tutorials on the Internet. However, they are not up-to-date. Currently Minix has the latest version 3.1.8. It has some changes, say a virtual file system. So a  newer tutorial is necessary. This article also includes a part for adding system calls to servers that are not visible to users. For example, users cannot call the functions in MFS directly because there is a middleware layer, VFS, between them. In order to call some function in MFS, one need to add a system call to MFS and then call it from VFS. The steps of adding system calls to MFS is slightly different.

I had an OSDI class last semester, which required us to do some Minix kernel modification. I learned a lot from it.

Here are the steps to add a new system call to Minix 3.1.8.

Suppose we need to add a system call, setmgroup(), to Process Manager(PM).

  1. Find an unused entry in /usr/src/include/minix/callnr.h, add a line. For example

    #define SETMGROUP 70
  2. In /usr/src/servers/pm/table.c, find the corresponding entry (with same number) in the table and change its name to setmgroup
  3. Add

    _PROTOTYPE( int do_setmgroup, (void) );

    to /usr/src/servers/pm/proto.h.

  4. Add the function body of do_setmgroup() to /usr/src/servers/pm/misc.c.
  5. Go to /usr/src/servers/pm/, and


    to check if it can pass the compilation.

  6. create file mcastlib.h in /usr/include/ add the setmgroup(int, int) to this file, which includes _syscall()
  7. Do

     cd /usr/src/tools
     make hdboot
  8. Test it by using the system call.

The mcastlib.h is like this:

#include <lib.h>
#include <unistd.h>
#define setmgroup _setmgroup
PUBLIC int setmgroup ( int _groupid )
    message m;
    m.m1_i1 = _groupid;
    return( _syscall( PM_PROC_NR, SETMGROUP, &m) );

Add a System Call to MFS

Suppose we need to add a system call named listblocknum() to MFS.

  1. add an entry in /usr/src/include/minix/vfsif.h

    add this line:

    #define REQ_LISTBLOCKNUM (VFS_BASE + 33)

    Add 1 to the number of request

     #define NREQS 34
  2. find the entry in the table and change its name to fs_listblocknum in /usr/src/servers/mfs/table.c
  3. find the entry in the table and change its name to no_sys in /usr/src/servers/hgfs/table.c and other file systems like ext2, etc. If you already changed NREQS and still got the error “array size is negative”. Try to “$make includes” in /usr/src. Then try to make in hgfs again.
  4. add _PROTOTYPE( int fs_listblocknum, (void) ); to /usr/src/servers/mfs/proto.h
  5. add the function body of fs_listblocknum() to /usr/src/servers/mfs/misc.c
  6. Do

     $cd /usr/src/servers/mfs/
  7. go to /usr/src/tools,

     $make hdboot

After doing this, VFS can send message to MFS to call a function, just like USER can send a message to VFS to call a function. Refer to request.c in VFS to see how to call the functions in MFS.

Written on February 2, 2012