Adding a syscall to Minix 2

Getting Minix 2

Older Minix versions are available from the Minix 3 official website. If using a VM I would recommend the HD image from the Minix 2 Quick and Dirty edition. This is what I used for this blog post.

Running in VirtualBox

If using the Quick and Dirty edition HDD image from the link above, first convert it to VDI format:

VBoxManage convertdd minix-2.0-hd-64MB.img minix-2.0-hd-64MB.vdi  --format VDI

Create a Linux 2.4 32-bit VM with 64mb of RAM. Make sure that the storage controller is IDE.

Building the kernel

cd /usr/src/tools
make

This should build the kernel and out bootable kernel file named image. Copy it into the directory containing the kernel:

cp image /minix/custom

Now reboot Minix with the new kernel image. Perform a clean shutdown of the OS:

shutdown

From the Supervisor, set our custom kernel image as the kernel and boot back into Minix:

name=/minix/custom
boot

Adding a syscall to the memory manager server

/usr/include/minix/callnr.h

increment NCALLS and allocate an number for the new syscall:

#define HELLOWORLD 78

/usr/src/mm/main.c

PUBLIC void do_helloworld()
{
    message msg;
    msg = mm_in;
    _taskcall(SYSTASK, SYS_HELLO, &msg);
}

/usr/src/mm/proto.h

_PROTOTYPE(void do_helloworld, (void));

/usr/src/mm/table.c

do_helloworld, /* 78 = HELLOWORLD */

/usr/src/fs/table.c

no_sys , /* 78 = HELLOWORLD */

/usr/include/minix/com.h

#define SYS_HELLOWORLD 22 

/usr/src/kernel/system.c

case SYS_HELLOWORLD: r = do_helloworld(&m); break;

PRIVATE int do_helloworld(m_ptr)
register message *m_ptr;
{
    printf("Hello, World\n");
    return(OK);
}

Calling the syscall from user land

#include <lib.h>

int main(int argc, char** argv)
{
    int ret;
    message msg;
    ret = _syscall(MM, HELLOWORLD, &msg);
    return ret;
}

Build this program with cc -o hello hello.c. Running ./hello should output Hello, World from the new syscall.