Compilers for high level languages generate code that follow certain conventions. These conventions are necessary, in part, for separate compilation to work. One such convention is the calling convention. The calling convention is a set of assumptions made by the compiler about where function arguments will be found on entry to a function. A calling convention also specifies where the return value for a function is found. The calling convention is also sometimes called the ABI or Application Binary Interface.
Some programs may not know at the time of compilation what arguments
are to be passed to a function. For instance, an interpreter may be
told at run-time about the number and types of arguments used to call
a given function. libffi can be used in such programs to
provide a bridge from the interpreter program to compiled code.
The libffi library provides a portable, high level programming
interface to various calling conventions. This allows a programmer to
call any function specified by a call interface description at run
time.
FFI stands for Foreign Function Interface. A foreign
function interface is the popular name for the interface that allows
code written in one language to call code written in another language.
The libffi library really only provides the lowest, machine
dependent layer of a fully featured foreign function interface. A
layer must exist above libffi that handles type conversions for
values passed between the two languages.