Intercepting system calls

Intercepting system calls

Post by Clinton Pierc » Sat, 11 May 1996 04:00:00

I'm looking to buy a clue.  What I want to do is have a _single_ program
run and get "custom" results back from a system call.  For example, if I
wanted to intercept time(2) so that a program calling time(2) gets something

I understand that I can re-compile libc and probably get something like this
to happen...  The kernel seems to actually get the time from
the system harware clock in kernel/time.c near:

asmlinkage int sys_time(long * tloc)
        int i, error;

        i = CURRENT_TIME;
        if (tloc) {
                error = verify_area(VERIFY_WRITE, tloc, 4);
                if (error)
                        return error;
                put_fs_long(i,(unsigned long *)tloc);
        return i;


Obiously, this is the guts of time(2) in the kernel.  What I don't understand
is how you get from an application to here.  In the application, of course,
you just do this:
        long now;
This has to link (at link time) to something, but I can't seem to find a
"time" function in the libc sources...what did I miss?

    Clinton A. Pierce       |    "If you rush a Miracle Man     |  \ / \ /

 DCI, Inc. on loan to Ford. | --Miracle Max, The Princess Bride |  / \ / \
Geek Code: GCM/GCSd-(++)s+:+a-C++UA++++$UIS+++$UL+++P++++L++E---W++N++w---O
(Revised!)  t++(+++)5+X+R-tv-b+++DI++++G++e+>++h----r+++y+++(--)>y*


1. Intercepting system calls

Hello all...

What is the best way to intercept system calls for a single process?
For example, say a process calls gethostbyname(), and I want to intercept
this and return different information than gethostbyname() would normally
return, how do I do this?

Please reply via email rather than here, if possible.

Thanks in advance
Chris Seatory
System Administrator

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