Implementation of read( )

Implementation of read( )

[Vineet Joglekar]

Hi all,

I dont know if this question is relevant on this mailing list, but please do help me out if you can.

For my project, I need to trace the read() function call till the VFS level. I have read that read() makes the kernel invoke sys_read() and I want to do the same thing with some additional functionalities. Where can i get the implementation of the read() function so that on the parallel lines I will be able to write my own my_read() function which in turn will call the sys_read() too?

Thanks and regards,

Vineet


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Re: Implementation of read( )

[Steven Smith]

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> I have read that read() makes the kernel invoke sys_read()
Yup.

> and I want to do the same thing with some additional
> functionalities. Where can i get the implementation of the read()
> function so that on the parallel lines I will be able to write my
> own my_read() function which in turn will call the sys_read() too?
Nowadays, most programs will use glibc's implementation of read, which
is in the glibc source (sysdeps/unix/sysv/linux/i386/syscall.S,
mostly).  There is, however, a macro in asm/unistd.h in the kernel
source pool which will do the basics for you.  The code goes like
this:

#include <asm/unistd.h>
#define __NR_private_read __NR_read
_syscall3(int, private_read, int, fd, char *, buf, size_t, size);

Calling private_read will then make the read system call directly
without going through libc.  Strictly speaking, it is valid to just
redefine read in the obvious way, but that gets horribly confusing.

The alternative approach is to call your function read, taking
advantage of the fact that a normal symbol will override one of the
weak symbols in libc, and then just call __libc_read when you need to
use the libc implementation.

Steven Smith,
sos22@cam.ac.uk

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Re: Implementation of read( )

[Glynn Clements]

Vineet Joglekar wrote:

> I dont know if this question is relevant on this mailing list, but
> please do help me out if you can.
> 
> For my project, I need to trace the read() function call till the VFS
> level. I have read that read() makes the kernel invoke sys_read() and
> I want to do the same thing with some additional functionalities. 
> Where can i get the implementation of the read() function so that on
> the parallel lines I will be able to write my own my_read() function
> which in turn will call the sys_read() too?

read() is part of libc. However, it's just a trivial wrapper around
the corresponding system call; the "source code" in
sysdeps/unix/syscalls.list is just:

read		-	read		i:ibn	__libc_read	__read read

The glibc build system generates the corresponding read.c file from
the above line using the sysdeps/unix/make-syscalls.sh script.

If you want to see the code, the simplest solution is to build glibc.

-- 
Glynn Clements <glynn.clements@virgin.net>