Applied Code Reading: Debugging FreeBSD Regex
When the code we're trying to
read is inscrutable,
inserting print statements and running various test cases can be
two invaluable tools.
Earlier today I fixed
a tricky problem in the FreeBSD regular expression library.
originally written by Henry Spencer in the early 1990s,
is by far the most complex I've ever encountered.
It implements sophisticated algorithms with minimal commenting.
Also, to avoid code repetition and increase efficiency,
the 1200 line long main part of the regular expression execution engine is
included in the compiled C code
three times after modifying various macros to adjust the code's behavior:
the first time the code targets small expressions and operates
with bit masks on long integers,
the second time the code handles larger expressions
by storing its data in arrays,
and the third time the code is also adjusted to handle multibyte characters.
Here is how I used test data and print statements to locate and fix the problem.
Continue reading "Applied Code Reading: Debugging FreeBSD Regex"
Madplay on an Intel Mac
Numerous MP3 players around my house pull music from a central file server.
The hardware I'm using is extremely diverse and many devices
can nowadays be politely described as junk:
they include 100MHz Pentiums with 16MB RAM, and an ARM-based prototype
lacking support for floating point operations.
For the sake of simplicity I've standardized the setups around
a web server running on each machine to list static HTML pages
containing the available music files,
and simple shell-based CGI clients that invoke
play the music.
When I added an Intel-based Mac to the mix I found that madplay
refused to work, producing only a white noise hiss.
Continue reading "Madplay on an Intel Mac"
Fixing the Orientation of JPEG Photographs
I used to fix the orientation of my photographs through an application
that would transpose the compressed JPEG blocks.
This had the advantage of avoiding the image degradation of a
decompression and a subsequent compression.
Continue reading "Fixing the Orientation of JPEG Photographs"
Cross compiling software on a host platform to run on a different
target used to be an exotic stunt to be performed by
the brave and desperate.
One had first to configure and build the compiler, assembler, archiver,
and linker for the different architecture, then cross-build the other
architecture's libraries, and finally the software.
This week, while preparing a new release of the
CScout refactoring browser
I realized that what was once a feat is nowadays a routine operation.
Continue reading "Cross Compiling"
Everything Old is New Again
In 1984 the new kid on the block was Borland's Sidekick.
A terminate and stay resident (TSR) program for MS-DOS,
it would run in the background, and when it detected
the two shift keys being pressed it would overlay the
(then character) screen with a calculator, a notepad,
a calendar, a dialer or an ASCII table.
Continue reading "Everything Old is New Again"
XML Abstraction at the Wrong Level
Over the last month I've encountered two applications
that use XML at the wrong level of abstraction.
Instead of tailoring the schema to their needs, they
use a very abstract schema, and encode their elements
at a meta level within the XML data.
This approach hinders the verification and manipulation of the corresponding
Continue reading "XML Abstraction at the Wrong Level"
Apple's Presence in Greece Appears to be a Joke.
Earlier today I tried to buy an Airport Express base station by
visiting what appeared to be an Apple store in Stournari street:
the road in Athens with the largest physical concentration of computer shops.
Continue reading "Apple's Presence in Greece Appears to be a Joke."