Re: How to write System calls?

["Rajat Chadda" <rajat.chadda@wipro.com>]

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Yes -- system calls can be implemented
as modules.
prog8.c is a module.
prog9.c is a user-space program.

In <asm/unistd.h> -- add
#define __NR_my_func            250 


----- Original Message -----
From: Matthieu Delahaye <delahaym@esiee.fr>
Date: Tuesday, October 30, 2001 2:41 pm
Subject: Re: How to write System calls?

> Hi,
> 
> First, I propose you to have a look on "Linux Kernel 2.4 Internals" 
> from 
> Tigran Aivazian. You can read it from the LDP at exact URL: 
> http://www.linuxdoc.org/LDP/lki/index.html
> In this, you'll probably see that syscalls are hardcoded (file 
> arch/xxx/kernel/entry.S  on your kernel tree).
> This means, AFAIK, you cannot implements a syscall function in a 
> module ;-)
> One other way is using  your modules to implements char drivers and 
> use 
> ioctl() as if it was yours syscalls. This method is generally 
> recommended to users who wants to implement new sayscalls.
> 
> 
> Am I wrong?
> 
> Regards,
> Matthieu
> 
> Sureshkumar Kamalanathan wrote:
> 
> >Hi All,
> >  Good day!
> >  I have 2.4.4 kernel.  I have to write some system calls for
> >interaction with the kernel from the userland.  Can any of you 
> tell me
> >where I can get the information regarding this?  
> >  I have the Linux Kernel Internals by Beck and others.  But it gives
> >the procedure only for 2.2.x.
> >  Moreover I need to implement these system calls as Loadable 
> modules.>  Any pointers in this regard will be highly appreciated 
> and I'll very
> >grateful!!
> >
> >  Thanks in advance,
> >
> >Regards,
> >Suresh.
> >-
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> >
> 
> 
> 
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> 

[-- Attachment #2: rog8.c --]
[-- Type: application/octet-stream, Size: 574 bytes --]

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/sys.h>
#include <stdio.h>
 
extern void *sys_call_table[];
 
asmlinkage static void sys_my_func();
 
void *old_val;

 
int
init_module()
{
        printk("Adding my own system call \n");
	old_val = (void *) sys_call_table[250];
 
        sys_call_table[250] = (void *) sys_my_func;
 
        return 0;
}
 
asmlinkage static void sys_my_func()
{
        printk("I am a working system call.\n");
}
 
void
cleanup_module()
{
        sys_call_table[250] = old_val;
	printk("<1> Restoring sys_call_table\n");
}

[-- Attachment #3: rog9.c --]
[-- Type: application/octet-stream, Size: 181 bytes --]

#include <stdio.h>
#include <linux/unistd.h>
extern long __res;

_syscall0(void,my_func);

main()
{
		
	
	my_func();
	printf("going to print my call\n");

	printf("bye bye \n");

}

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