New Linux Debugger Runs Programs in Reverse
May 24th, 16:51 UTC
Undo Software today unveiled UndoDB -- a debugger that can step unmodified Linux programs backwards as well as forwards.
Cambridge, UK - May 23, 2006.
Undo Software (http://undo-software.com/) today unveiled UndoDB -- the first bidirectional debugger for compiled programs.
A bidirectional debugger allows programmers to run a program
backwards in time as well as forwards. The program can be
stepped back line-by-line, or rewound to any point in its history.
Furthermore, programmers can play the program forwards and backwards in
a totally repeatable fashion, allowing them to "home in" on the cause of
a bug.
Bidirectional debuggers are much more powerful than their traditional
counterparts, which only allow programmers to step their programs
forwards in time. This is particularly true for bugs whose root cause
occurs long before the ill effects manifest themselves, and for bugs
that occur only intermittently.
Bidirectional debugging means bugs that would normally take days to
uncover can be found in minutes [1].
Although bidirectional debugging has proved highly effective for
Java programs, until now has it not been available to programmers using compiled
languages such as C and C++. This is somewhat ironic, since lower-level
languages like C and C++ can be much more bug-prone than their higher-level
counterparts. However implementing a bidirectional debugger for arbitrary
binaries presents a much more difficult problem than implementing one for
interpreted or 'byte-code' languages, where the interpreter can be extended to
record all state changes as the program executes.
UndoDB uniquely enables bidirectional debugging of
arbitrary Linux binary programs, including those written in C or C++.
UndoDB requires no recompilation or other modifications to the
program being debugged, nor does it require any specialized hardware,
kernel patches, or kernel modules.
"UndoDB makes even the nastiest bugs trivial to uncover," says
Julian Smith, co-founder of Undo Software. "This is hugely significant,
because the unfortunate reality is that most programmers spend most
of their time hunting a few nasty bugs. A tool that could reduce the
debugging burden by even a few percent would save the software industry
a staggering amount of time and money; and UndoDB can do much,
much better than that."
Bidirectional debugging is so useful because it gives the user
control over time. To debug a program is to reason backwards
from the point of failure to determine the cause of that failure. On
the first page of their book, The Practice of Programming, Brian
Kernighan and Rob Pike give the following advice to programmers when
debugging:
Reason back from the state of the crashed program to
determine what could have caused this. Debugging involves backwards
reasoning, like solving murder mysteries. Something impossible occurred,
and the only solid information is that it really did occur. So we
must think backwards from the result to discover the reasons.
With this analogy, a programmer using a bidirectional debugger is
like the detective finding detailed CCTV footage not just of the murder
itself, but also all pertinent events that led to the murder.
UndoDB uses the ubiquitous gdb as its front-end, so
Linux developers will feel right at home, and be productive the moment
they get started. gdb has been complemented with a few new
commands which work just like gdb's existing commands, only
they step the program backwards in time, rather than forwards. For
example, whereas gdb's step command steps the program
forwards one source line, UndoDB's bstep command steps
the program back one source line. UndoDB also compliments
gdb's next, until, finish,
stepi and nexti commands with their backwards
counterparts: bnext, buntil, bfinish,
bstepi and bnexti, respectively. UndoDB
also adds some new commands which have no parallel in gdb,
such as the bgoto and bgoton commands, which jump
to an arbitrary point in the program's history. When the program is
paused at any point in its history, the programmer can inspect the full
state of their program using the usual gdb commands (e.g.
print, backtrace and display).
The company is unusual in that it has chosen Linux as the first
operating system on which its proprietary product runs. Julian Smith
explains "We did this for several reasons. Today's Linux market is
pretty exciting, not least due to its rapid expansion into Consumer
Electronics and other embedded systems. We believe that the somewhat
fragmented nature of the Linux market makes it more accessible for small
but innovative companies such as ourselves. There were also technical
considerations -- for example, UndoDB's technology requires a precise
and complete specification of the application/kernel interface, and you
can't get a more precise and complete specification than the kernel's
source code!".
Although UndoDB is not open source, developers who don't get
paid for their work can use UndoDB for free. For professional
use, UndoDB retails for $495 per seat, with an introductory
special offer of $295, plus a fee upgrade to version 2 when that is
released (currently slated for Q4 this year). There is also a free
30-day evaluation version. UndoDB is available now, from http://undo-software.com/.
References
[1] Dr. Dobb's Journal, May 09, 2005 --
Omniscient Debugging (RetroVue at Work)
http://www.ddj.com/dept/debug/184406101?pgno=5.
Notes to Editors
Undo Ltd is a privately held software company based in Cambridge, UK.
It was founded by Greg Law and Julian Smith in 2005. Both hold a Ph.D.
and have worked in the computer industry for many years. The company
was formed out of the founders' frustration with existing debugging
tools. UndoDB is Undo Software's first product.
(Submitted by Greg Law of support@undo-software.com)
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