How to write System calls?

How to write System calls?

[Sureshkumar Kamalanathan]

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.

Re: How to write System calls?

[Matthieu Delahaye]

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.
>-
>To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>the body of a message to majordomo@vger.kernel.org
>More majordomo info at  http://vger.kernel.org/majordomo-info.html
>Please read the FAQ at  http://www.tux.org/lkml/
>

Re: How to write System calls?

[Rajat Chadda]

[-- Attachment #1: Type: text/plain, Size: 2087 bytes --]

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.
> >-
> >To unsubscribe from this list: send the line "unsubscribe linux-
> kernel" in
> >the body of a message to majordomo@vger.kernel.org
> >More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >Please read the FAQ at  http://www.tux.org/lkml/
> >
> 
> 
> 
> -
> To unsubscribe from this list: send the line "unsubscribe linux-
> kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
> 

[-- 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");

}

[-- Attachment #4: Wipro_Disclaimer.txt --]
[-- Type: text/plain, Size: 855 bytes --]

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Re: How to write System calls?

[Keith Owens]

On Mon, 29 Oct 2001 16:46:49 +0530, 
Sureshkumar Kamalanathan <skk@sasken.com> wrote:
>  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.

Syscall entries are hard coded into the syscall table, usually in
arch/$(ARCH)/entry.S.

There is no generic way to add a syscall that is serviced by a module.
I know that the modutils HOWTO gives examples of syscalls in modules
but that doc is out of date, on some architectures you have to adjust
additional registers to make a remote function call.  On those systems
the syscall table works because the kernel is compiled with special
flags, that will not work for modules.  Also Linus has said that he
does not want modules to be able to override syscalls.