Preface
C is an important language because, despite what many will tell you, it is still
the closest we have to a machine-independent assembler. This makes it ideal
for working closely with the hardware and hence it is ideal for most low-level
programming. Today's small embedded processors are no longer as small as
they once were and it is not uncommon to find them running a complete
Linux operating system. This would have been regarded as crazy, if not
impossible, only a few years ago. Of course, there are very low-cost processors
that can't manage to run a full operating system, and these are likely to be
with us for some time, but the point is that as hardware evolves and prices
fall, Linux in even the simplest devices will become the norm.
What this means is that writing low-level code has to evolve to work with
Linux rather than trying to ignore it, or worse struggle against it. Linux, and
POSIX systems in general, have many features to make access to hardware
and nearly realtime operation easy - and all from user space. The only
problem is that a full multi-tasking operating system, complete with security,
is a complex beast and finding out how to do things can be difficult. The
documentation and many of the "how-tos" on the web aren't aimed at low-
level programming, but at end users and applications programmers. What I
have tried to do in this book is to collect together all of the different
techniques needed to work close to the hardware without having to abandon
the OS.
Of course, there are compromises to be made. The OS has to provide security
and this often stops you doing something that from your point of view is
perfectly safe. The needs of a multitasking OS can also interfere with the
timely execution of your code, but given the right scheduling method and
core affinity this can be minimized. For these inconveniences you get lots of
advantages: a full multi-threaded environment complete with the necessary
mechanisms such as locks, high-level access to communications stacks such
as TCP/HTTP, interprocess communications such as pipes and shared
memory. Finally you get access to powerful graphics, almost for free. You
might not have imagined wanting graphics, but now it’s at your disposal, it
could be worth using, rather than building some custom I/O hardware.
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