Foreign function interface

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A foreign function interface (or FFI) is a mechanism by which a program written in one programming language can call routines or make use of services written in another. The term comes from the specification for Common Lisp, which explicitly refers to the language features for inter-language calls as such; the term is also used officially by the Haskell programming language. Other languages use other terminology (the Ada programming language talks about "language bindings", while Java refers to its FFI as the Java Native Interface, or JNI). Foreign function interface has become generic terminology for mechanisms which provide such services.

It should be noted that despite the name, FFIs are not necessarily restricted to function calls; many FFIs permit method calls on objects; and some even permit migration of non-trivial datatypes and/or objects across the language boundary.

The term foreign function interface is generally not used to describe multi-lingual runtimes such as the Microsoft Common Language Runtime, where a common "substrate" is provided which enables any CLR-compliant language to use services defined in any other. In addition, many distributed computing architectures such as the Java remote method invocation (RMI), RPC, CORBA, and SOAP permit different services to be written in different languages; such architectures are generally not considered FFIs.

In most cases, a FFI is defined by a "higher-level" language, so that it may employ services defined and implemented in a lower level language, typically a systems language like C or C++. This is typically done to either access OS services in the language in which the OS' API is defined, or for performance considerations.

Many FFIs also provide the means for the called language to invoke services in the host language as well.

[edit] Operation of a FFI

The primary function of a FFI is to mate the semantics and calling conventions of one programming language (the host language, or the language which defines the FFI), with the semantics and conventions of another (the guest language). This process must also take into consideration the runtime environments and/or application binary interfaces of both. This can be done in several ways:

Requiring that guest-language functions which are to be host-language callable be specified or implemented in a particular way; often using a compatibility library of some sort.
Use of a tool to automatically "wrap" guest-language functions with appropriate glue code, which performs any necessary translation.
Restricting the set of host language capabilities which can be used cross-language. For example, C functions called from C++ may not (in general) include reference parameters or throw exceptions.

FFIs may be complicated by the following considerations:

If one language supports garbage collection and the other does not; care must be taken that the non-GC language code doesn't do something to cause the GC to fail. In JNI, for example, C code which "holds on to" object references passed from Java must "register" this fact with the Java runtime; otherwise the referred-to objects may be garbage-collected if no more valid references to the object(s) exist within the Java environment. (The C code must likewise release such references manually when the corresponding object is no longer needed).
Complicated or non-trivial objects or datatype may be difficult to map from one environment to another.
It may not be possible for both languages to maintain references to the same instance of a mutable object, due to the mapping issue above.
One or both languages may be running on a virtual machine (including different VMs).
Cross language inheritance or other forms of type or object composition may be especially difficult.

[edit] Examples

Examples of FFIs include:

Ada language bindings, allowing not only to call foreign functions but also to export its functions and methods to be called from non-Ada code.[1]
C++ has a trivial FFI with C, as the languages share a significant common subset. The primary effect of the extern "C" declaration in C++ is to disable name mangling.
JNI, which provides an interface between Java (programming language) and C/C++, the preferred systems language on most systems where Java is deployed
The FFIs of Common Lisp and Haskell
The major "scripting" languages, such as Python, Perl, Tcl, and Ruby, all provide easy access to native code written in C/C++ (or any other language obeying C/C++ calling conventions).