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Add method to VFS in Minix.

Published by Matheus (X-warrior) Bratfisch on August 11, 2010

Good Night,

First of all, what is VFS? VFS means Virtual File System. It is a layer that exists in a lot of OS and in Minix too, this layer takes care of the communication with the File System. It is a Master’s thesis in Computer Science from Balázs Gerófi and the name is “Design and implementation of the MINIX Virtual File system“.

But why to communicate with VFS server? Or why to add something on it? If you want to some tasks you need to do a SYSCALL to it. But sometimes the VFS server don’t do what you want. So you must add and change it to do it.

How to do this?

Edit “/usr/src/include/minix/callnr.h”. After this line “#define GETPGRP 63″ add this:
[code lang=”c”]
#define TESTCALL 64
[/code]

Then you need to change the VFS server and add what you want to do when it is called. Edit “/usr/src/servers/vfs/table.c”. You should replace the line “no_sys, /* 64 = unused */” with:
[code lang=”c”]
do_TESTCALL, /* 64 = unused */
[/code]

What happens? The method do_TESTCALL will be called when a syscall with number 64 is done.

Change the “/usr/src/servers/vfs/proto.h”. Add this:
[code lang=”c”]
/* do_TESTCALL EXAMPLE */
_PROTOTYPE( int do_TESTCALL, (void) );
[/code]

Now you have to create the method in “/usr/src/servers/vfs/read.c” (I did in this file and it worked). Add this:

[code lang=”c”]
/*===========================================================================*
*Trabalho: do_ver_contig *
*===========================================================================*/
PUBLIC int do_TESTCALL() {
message m;
printf(“Number received %d”, m_in.fd);
}
[/code]

But how to execute? How this works? How to compile?
Acess “/usr/src/tools/” and execute:

$ make libraries

(This process can take a long time)

In “/usr/src/tools” execute:

$ make hdboot

Reboot.

How to test and see if it is working? Create in your “/root” a file with this:
[code lang=”c”]
#include
#include
#include #include

int main(int argc, char *argv[]) {
int fd = 2;
message m;
m.m1_i1 = fd;
printf(“Number: %d”, fd);
_syscall(FS, 64, &m);
}
[/code]

Compile it, execute it and done!

This file do a _syscall to FS saying to execute method 64 and send by message the number 2. So the do_TESTCALL receive the parameter and show it on screen. Yes it is simple, but it is just an example about how to work with VFS server.

Example of code changed/add to VFS server

Creating a system library in Minix.

Best Regards,
Matheus

PS: It is a silly example, but it just show how to add some method to VFS Server. Probably in the next days I will show a real example.

Published inCLinux

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