How to use write system call within a system call

byteinnovator · 07-27-2010, 11:12 PM

I'm writing a new system call named icsys, i want to write something to the console when this system call is invoked.
i tried calling write() from icsys() but during compilation of kernel i received multiple definition error for several functions in read_write.c.
how to print
to a console from a system call.
please someone help me with this

i'm using kernel 2.6.34.1

Mara · 07-28-2010, 02:42 PM

printk() ?

byteinnovator · 07-29-2010, 07:23 AM

Thanks for the reply.... could you please tell me how to use printk() to print on the console.

when i use printk(), it prints in log files but i need my output on terminal.

could you please tell how to do it

Aquarius_Girl · 07-29-2010, 07:26 AM

While using printk() does dmesg command say anything interesting ?

ashok449 · 07-30-2010, 02:05 AM

try below code in your system call implementation (kernel)

Code:

static void print_string(char *str)
{
 
struct tty_struct *my_tty;
        my_tty = current->signal->tty;

        if (my_tty != NULL) {


                ((my_tty->driver)->ops->write) (my_tty,str,strlen(str));

                ((my_tty->driver)->ops->write) (my_tty, "\015\012", 2);

        }
}

-- Ashok

Aquarius_Girl · 07-30-2010, 02:12 AM

Aquarius_Girl · 07-30-2010, 02:33 AM

Quote:

Originally Posted by anishakaul

To whom are you telling this ? Who is going to try this ?

I just asked you in post number 4 to execute dmesg command and see what printk () writes there !

Ashok

I am extremely sorry for the post number 6 of mine ! I thought you are the OP

ashok449 · 07-30-2010, 02:37 AM

He is trying to implement a system call and I thought he is well aware of printk().

I just tried to help him, Anyhow I already posted it, may be someone else will use this post later. we should not use this site for such comments. We are here to help.

--Ashok

Aquarius_Girl · 07-30-2010, 02:39 AM

Read my post number 7: I thought you were the creator of this thread

byteinnovator · 07-30-2010, 07:43 AM

Thanks guys, i will try what u said and let u know about the progress

Aquarius_Girl · 07-30-2010, 07:45 AM

Quote:

Originally Posted by byteinnovator

Thanks guys, i will try what u said and let u know about the progress

Try the dmesg command, it will not take much time, I think by that you will be able to see printk's output on the console !

byteinnovator · 07-31-2010, 08:11 AM

HI anishakaul,

This is wat i actually want to do, when i call a system call i need to some messages directly displayed on terminal, i dont want to use dmesg.

Its something like redirection to terminal. can u tell how to do it,

Aquarius_Girl · 07-31-2010, 09:25 AM

Does the following code solves your problem ? Try it !

Code:

printk(KERN_CRIT "I am a critical level printk");

Search the man page for printk on the net,

archieval · 08-03-2010, 09:31 PM

write() is user space, why use it in kernel space?

Aquarius_Girl · 08-04-2010, 12:37 AM

Quote:

Originally Posted by archieval

write() is user space, why use it in kernel space?

write() is used for low-level I/O. What else in your view point should be used for writing in Kernel modules ? Kindly enlighten.