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. 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 a 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. $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.


  • sharvari

    i was trying to call MFS from VFS. It requires the endpoint no. to send message from vfs to mfs. i tried giving endpoint no. of MFS i.e. 7. but it is giving error that 7 is non existant endpoint. what can be the possible mistake??