Getting Started — Botan

# Botan

# 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 an object of type

class ```LibraryInitializer`

prior to making any other library calls. Typically this will be named something like `init` or `botan_init`. The object 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.