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Getting Started
========================================
All declarations in the library are contained within the namespace
``Botan``, so you need to either prefix types with ``Botan::`` or add
a ``using`` declaration in your code. All examples will assume a
``using`` declaration.
All library headers are included like so::
#include <botan/botan.h>
Initializing the Library
----------------------------------------
There is a set of core services that the library needs access to while
it is performing requests. To ensure these are set up, you must create
a ``LibraryInitializer`` object (usually called 'init' in Botan
example code; 'botan\_library' or 'botan\_init' may make more sense in
real applications) prior to making any calls to Botan. This object's
lifetime must exceed that of all other Botan objects your application
creates; for this reason the best place to create the
``LibraryInitializer`` is at the start of your ``main``
function, since this guarantees that it will be created first and
destroyed last (via standard C++ RAII rules). The initializer does
things like setting up the memory allocation system and algorithm
lookup tables, finding out if there is a high resolution timer
available to use, and similar such matters. With no arguments, the
library is initialized with various default settings. So (unless you
are writing threaded code; see below), all you need is::
Botan::LibraryInitializer init;
at the start of your ``main``.
The constructor takes an optional string that specifies arguments.
Currently the only possible argument is "thread_safe", which must have
an boolean argument (for instance "thread_safe=false" or
"thread_safe=true"). If "thread_safe" is specified as true the library
will attempt to register a mutex type to properly guard access to
shared resources. However these locks do not protect individual Botan
objects: explicit locking must be used if you wish to share a single
object between threads.
If you do not create a ``LibraryInitializer`` object, all library
operations will fail, because it will be unable to do basic things
like allocate memory or get random bits. You should never create more
than one ``LibraryInitializer``.
Pitfalls
----------------------------------------
There are a few things to watch out for to prevent problems when using
the library.
Never allocate any kind of Botan object globally. The problem with
doing this is that the constructor for such an object will be called
before the library is initialized. Many Botan objects will, in their
constructor, make one or more calls into the library global state
object. Access to this object is checked, so an exception should be
thrown (rather than a memory access violation or undetected
uninitialized object access). A rough equivalent that will work is to
keep a global pointer to the object, initializing it after creating
your ``LibraryInitializer``. Merely making the
``LibraryInitializer`` also global will probably not help, because
C++ does not make very strong guarantees about the order that such
objects will be created.
The same rule applies for making sure the destructors of all your
Botan objects are called before the ``LibraryInitializer`` is
destroyed. This implies you can't have static variables that are Botan
objects inside functions or classes; in many C++ runtimes, these
objects will be destroyed after main has returned.
The memory object classes (``MemoryRegion``, ``MemoryVector``,
``SecureVector``) are extremely primitive, and meant only for
secure storage of potentially sensitive data like keys. They do not
meet the requirements for an STL container object and you should not
try to use them with STL algorithms. For a general-purpose container,
use ``std::vector``.
Use a ``try``/``catch`` block inside your ``main`` function, and catch
any ``std::exception`` throws (remember to catch by reference, as
``std::exception::what`` is polymorphic)::
int main()
{
try
{
LibraryInitializer init;
// ...
}
catch(std::exception& e)
{
std::cerr << e.what() << "\n";
}
}
This is not strictly required, but if you don't, and Botan throws an
exception, the runtime will call ``std::terminate``, which usually
calls ``abort`` or something like it, leaving you (or worse, a user of
your application) wondering what went wrong.
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