How to make timer?

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JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 24 Mar 2011, 14:58

Classic question to many Linux forums, that usually is answered with "There is a Google!" or "We don't want to make your homework for you." Very Happy

So - don't answer in this manner!

After some search, I figured out that there are several "kernel functions" - such as "init_timer", "add_timer" and so on, that allows creating of timer and running callback after some predefined time.
Unfortunately, I didn't figured out how I can use these functions from assembly program.
So, please help if someone knows how I can use timers in assembly program.

P.S. Linking to static libraries is not an option. Importing from shared libraries is OK. Using only syscals is the best, but I am afraid it is not possible in this case.

24 Mar 2011, 14:58

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20689
Location: In your JS exploiting you and your system

revolution 24 Mar 2011, 15:06

Perhaps you should also state what resolution in timing you need. Microsecond delays are very different, and use different techniques, compared to hour delays.

24 Mar 2011, 15:06

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 24 Mar 2011, 15:48

From 50ms to several seconds.

24 Mar 2011, 15:48

pelaillo
Missing in inaction

Joined: 19 Jun 2003
Posts: 878
Location: Colombia

pelaillo 24 Mar 2011, 17:53

What about using the following syscalls:

Code:

sys_setitimer                 = 068h
sys_getitimer                 = 069h

I'll try to produce a working snippet later at home

24 Mar 2011, 17:53

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 24 Mar 2011, 18:46

pelaillo wrote:

What about using the following syscalls:
Code:
sys_setitimer                 = 068h
sys_getitimer                 = 069h
    
I'll try to produce a working snippet later at home

The thing that stopped me to use setitimer functions is that there is only one timer available, so I have to create special library in order to allow several timers to be used in the program.
But when I think now - it looks not so bad idea...

24 Mar 2011, 18:46

Tyler

Joined: 19 Nov 2009
Posts: 1215
Location: NC, USA

Tyler 24 Mar 2011, 19:26

Has that library not already been created? It seems like a necessity, surly someone has done it already.

24 Mar 2011, 19:26

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 24 Mar 2011, 20:58

Tyler wrote:

Has that library not already been created? It seems like a necessity, surly someone has done it already.

There is a C implementation. I don't know about any FASM implementation.
So, probably it is good idea to make one for FreshLib. It will be portable of course.

24 Mar 2011, 20:58

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7103
Location: Slovakia

vid 24 Mar 2011, 22:12

Same issue as with heap: Linux kernel only offers smallest set needed to implement the library, not the library itself. It is not expected someone would use syscalls instead of "proper" using of glibc.

BTW, why don't you use glibc? It might save you few headaches.

24 Mar 2011, 22:12

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 24 Mar 2011, 22:45

vid wrote:

BTW, why don't you use glibc? It might save you few headaches.

I would (maybe) but there are thousands of C libraries and what is the proper one? Evil or Very Mad

Are there some timer functions in glibc? And what is the name of the shared library that you call "glibc"? I am using several functions from libc (actually named libc.so.6) but there is no "glibc.so.* in my installation. But there is libgcc_s.so.1
Linux is a mess of files, links to files, links to links to files and some objects that look and behave like files, but actually are not.... and all these files are text files! Laughing

24 Mar 2011, 22:45

Tyler

Joined: 19 Nov 2009
Posts: 1215
Location: NC, USA

Tyler 24 Mar 2011, 22:52

libgcc is internal gcc stuff, that it links into your C(or other) code.

But, as for finding which thingy to link to: USE MAN!!! Smile

Code:

$man libc
LIBC(7)                    Linux Programmer's Manual                   LIBC(7)

NAME
       libc - Overview of standard C libraries on Linux

DESCRIPTION
       The  term  "libc"  is  commonly used as a shorthand for the "standard C
       library", a library of standard functions that can be  used  by  all  C
       programs  (and  sometimes  by programs in other languages).  Because of
       some history (see below), use of the term "libc" to refer to the  stan
       dard C library is somewhat ambiguous on Linux.

   glibc
       By  far  the  most  widely used C library on Linux is the GNU C Library
       (http://www.gnu.org/software/libc/), often referred to as glibc.   This
       is  the  C  library  that is nowadays used in all major Linux distribu
       tions.  It is also the C library whose details are  documented  in  the
       relevant  pages of the man-pages project (primarily in Section 3 of the
       manual).  Documentation of glibc is also available in the glibc manual,
       available  via the command info libc.  Release 1.0 of glibc was made in
       September 1992.  (There were earlier 0.x  releases.)   The  next  major
       release of glibc was 2.0, at the beginning of 1997.

       ---=== The  pathname  /lib/libc.so.6 (or something similar) is normally a sym
       bolic link that points to the location of the glibc library ===--- ,  and  exe
       cuting  this  pathname  will cause glibc to display various information
       about the version installed on your system.

24 Mar 2011, 22:52

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 24 Mar 2011, 23:11

libc (or whatever it is called today) does not contains decent timers support.

24 Mar 2011, 23:11

Tyler

Joined: 19 Nov 2009
Posts: 1215
Location: NC, USA

Tyler 24 Mar 2011, 23:15

Moreover, it contains no timer support. You were the one asking about libc, not me. Smile

24 Mar 2011, 23:15

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7103
Location: Slovakia

vid 24 Mar 2011, 23:24

It should at minimal offer some kind of wrapper over kernel function, doesn't it?

24 Mar 2011, 23:24

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 24 Mar 2011, 23:33

There are both getitimer and setitimer functions, they are defined as "weak" symbols. (What that means?)
But why I have to use wrappers if I can use the originals?

24 Mar 2011, 23:33

Tyler

Joined: 19 Nov 2009
Posts: 1215
Location: NC, USA

Tyler 25 Mar 2011, 00:58

vid: POSIX does. libc does not.

John: Try this: http://www.gnu.org/s/libc/manual/html_node/Setting-an-Alarm.html. It's POSIX, which is almost as portable as libc. As for an explanation of weak symbols, try http://en.wikipedia.org/wiki/Weak_symbol.

25 Mar 2011, 00:58

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7103
Location: Slovakia

vid 25 Mar 2011, 08:45

Quote:

vid: POSIX does. libc does not.

Right, thanks. But then I am wondering, in what library is code of these wrappers physically present?

25 Mar 2011, 08:45

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 26 Mar 2011, 13:10

You guys, misguided me! I discovered, that the kernel actually supports multiple timers. The functions are:

Code:

sys_timer_create         =  $103
sys_timer_settime        =  $104
sys_timer_gettime        =  $105
sys_timer_getoverrun     =  $106
sys_timer_delete         =  $107

Too bad - I just finished library, using setitimer functions. Now I have to rewrite it from scratch.

Regards.

26 Mar 2011, 13:10

Tyler

Joined: 19 Nov 2009
Posts: 1215
Location: NC, USA

Tyler 27 Mar 2011, 17:10

vid wrote:

Quote:
vid: POSIX does. libc does not.

Right, thanks. But then I am wondering, in what library is code of these wrappers physically present?

Probably glibc... Razz

John: It was a conspiracy all along. We were just out to waste your time. Smile

27 Mar 2011, 17:10

JohnFound

Joined: 16 Jun 2003
Posts: 3499
Location: Bulgaria

JohnFound 28 Mar 2011, 06:53

Later news from the big timer battle. Razz

POSIX timers appears to support only signal events in the kernel (and callback call events in the glibc wrapper - I really hate how libc and C are enforced in Linux programming)

Anyway - signal handlers in Linux are very, very restrictive - they can only change memory and call limited number of functions.
Because this, I tried to make the timers to use threads - when the timer expires - the user callback function is started in new thread and after processing ends.
This approach appears to work - you can see the sources in the file linux/timer.asm
The good thing of the whole story is that now we have almost working Linux threads in FreshLib.

28 Mar 2011, 06:53

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20689
Location: In your JS exploiting you and your system

revolution 28 Mar 2011, 07:11

Ouch. Using threads for timer expiry sounds very expensive in both memory usage and OS overhead. Isn't there another way?

28 Mar 2011, 07:11

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