Chapter 1

Introduction

MzScheme includes functionality that makes it possible to interact with foreign data and foreign function, available through the foreign.ss module in MzLib. Traditional Scheme code is usually safe: if the process crashes the blame is always on C code. Using C extensions makes it possible to extend Scheme -- the added code can lead to crashes, but the blame for crashes is still at the C side.

The foreign capabilities of MzScheme makes it possible to use Scheme in situation that would otherwise require writing a C extension, so Scheme code can be used to substitute C. Doing this is, obviously, as unsafe as writing C -- so a crash can be now blamed on either C or Scheme code which interacts with the foreign world. To make things a bit safer, the foreign.ss module will not provide you with any of the unsafe operations by default. Instead, you get an `unsafe!' macro, which you can use to make the unsafe operations available¹. Using this macro should be considered as a declaration that your code is itself unsafe, therefore can lead to serious problems in case of bugs: it is your responsibility to provide a safe interface. By this we constrain the blame for crashes to either C code or such unsafe Scheme code.

In rare cases you might want to provide an unsafe interface, for example, if you write a module that provides more foreign-interaction functionality. In these cases you should use the `provide*' macro with `unsafe' bindings, and the `define-unsafer' to provide an unsafe!-like macro that will make these bindings available. See the foreign.ss source for details. Providing users with unsafe operations without this facility is considered a bug in your code.

In addition to this manual, Inside PLT MzScheme can be used to learn about the internals of MzScheme. Note that using ffi-lib with #f instead of a file name will use all loaded libraries including bindings from the process itself -- exposing making all of MzScheme's C-level functions (including the standard library) available.