FFI Documentation

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FFI Documentation

Sean P. DeNigris
Communication with code outside the image
I want to get this info into the help system, as similar questions pop up regularly on the lists.

I compiled every piece of info I could find.  The focus in on FFI, with stubs for other strategies.  Please read this *very* rough outline for missing info and inaccuracies:


What is FFI and what is it used for?  Calling functions in libraries outside of the image...

FFI, the Squeak Foreign Function Interface, is used to call functions located in shared libraries that are not part of the Squeak VM nor its plugins. It also provides means to read and write memory structures that are associated with the use of those shared libraries. A typical use is to directly invoke operating system APIs. As such, applications that use FFI can only be used on the platform(s) that support the particular API being used. C conventions are used throughout, though the external function could have been written by any language capable of generating object code that follows C conventions.  FFI is probably the easiest way to do the things it does. FFI is pretty fast too. Croquet uses FFI calls to OpenGL for all it's drawing routines.[1]

How does FFI work?
Technically what happens is:
* you define what the interface is - the parameters, types etc.
* when you make the call, the FFI logic assembles the data from the Squeak Objects into the proper structures according to the routine calling conventions for your architecture, and of course manages the return values. So no magic but perhaps just a little assembler in the plugin to properly deal with all the registers and condition flags.

How do I use it?

1. make a method (whose structure is similar to a named primitive method)

system: aString "name (by convention is apiXxx: e.g. apiSystem:)"

        <apicall: long 'system' (char*) module: 'libSystem.dylib'> "first line should be the external function specification"
        ^self externalCallFailed.

Let's take it piece by piece:

        system: aString
                Method name - by convention named 'apiXxx'
        <apicall: long 'system' (char*) module: 'libSystem.dylib'>
                Function specification
                        - should be the first line
                        - enclosed in angle brackets: < > containing:
                                1. Calling Convention, either apicall: (Pascal convention) or cdecl: (C convention)
                                        - Mac - use either one
                                        - Unix - use cdecl
                                        - Windows - use apical
                                2. Return Type (see types)
                                3. External Function Name (literal string)
                                4. Argument Types (a literal array)
                                5. Module - "module: " + [filename of the external library (literal string)] (see below).

        self externalCallFailed.
                Failure handler
                        - normal smalltalk code
                        - executed if the linking to or calling the external function fails
                        - API calls don't know how to communicate failure like Squeak primitives do, so:
                                - it does not tell you whether the external function succeeded
                                - the most common code is simply '^self externalCallFailed.'

Argument Types
        - must be names of ExternalTypes, either:
                - atomic types (see ExternalType class>>initializeFFIConstants and ExternalType class>>initializeAtomicTypes):
                        byte (unsigned)
                        sbyte (signed)
                        ushort (16-bit unsigned)
                        short (16-bit signed)
                        ulong (32-bit unsigned)
                        long (32-bit signed)
                        ulonglong (64-bit unsigned)
                        longlong (64-bit signed)
                        char (unsigned)
                        schar (signed)
                        float (single-precision float)
                        double (double-precision float)

Structure Types [4]
        - subclass of ExternalStructure
                - class>>fields that returns an array of field descriptions (see below)
                        - Example:
                                        ^#((red   'byte')(green 'byte')(blue  'byte'))          
                - class>>initialize which includes "self defineFields" (which must be called before using the class)
        - refer to as MyExternalStructure* (to indicate that the argument or return is a pointer to that structure)

Field description [4]
        - 2-element array (or three but that does something else, I'm not sure what):
                - first element is the field name
                - second is the type

Mac Memory Allocation Issues [4] (not sure about this)

If you allocate external structures, those with memory outside the Squeak process space, you may need to increase the amount of memory that is reserved outside the object heap for such use. The Mac OS (9 and previous) needs this, other platforms may be able to dynamically get more memory from the OS. To see how much memory is currently reserved printIt 'Smalltalk extraVMMemory'. To change it, execute 'Smalltalk extraVMMemory: someNumberofBytes' Then save your image and quit. When you next start up, the amount of memory you requested will be reserved. (JMM) Note the OSX versions of the VM ignore extraVMMemory because the memory model for OS-X/unix applications is quite different.

Module Name
- depends on the platform
        - Mac
                - pre Snow Leopard: flexible, can eliminate leading lib or extension e.g. 'libc.dylib' becomes 'libc', 'c.dylib', or 'c'
                - Snow Leopard
                        - file name must be exact including extension (unless Info.plist is altered as in 'Library Location' below)
                - With pre-mach-o VMs
                        - For Classic applications, use 'InterfaceLib'
                        - For Carbon libs, use 'CarbonLib'

Module Location - where the external library file lives
        - depends on the platform
                - Mac
                        - pre Snow Leopard
                                - checks VM path and common library paths
                        - Snow Leopard
                                - only looks in VM bundle's Resources file, you must either [5]:
                                        - store all external libraries there
                                        - ln -s path/to/library path/to/VM/Resources/library_name
                                        - Change the VM's Info.plist "SqueakPluginsBuiltInOrLocalOnly" key from "true" to "false."
        - security
                - malicious users could call arbitrary functions in the OS e.g.  "format c:" from "system.dll" [7]
                - VMs do not protect against buffer overflow from bad parameters [8]:
                        "this would require an attacker to execute arbitrary Smalltalk
                        code on your server. Of course if they can do that they own you
                        anyway, especially if you allow FFi or use the OSProcess plugin" - John McIntosh

* difficulty
        - if you make a mistake you'll not drop into the debugger but Squeak will just crash [2]
        - If you crash Squeak when it is running the garbage collector, then you know your FFI code is leaking bits into object memory [2]

What do I need to use FFI with Squeak?

You need the FFI plugin, which is included with most VM's as of Squeak 3.6 or so.

You can also build the plugin yourself. See VMMaker.

[1] http://wiki.squeak.org/squeak/1414
[2] http://wiki.squeak.org/squeak/2424
[3] http://wiki.squeak.org/squeak/5716
[4] http://wiki.squeak.org/squeak/2426
[5] http://forum.world.st/squeak-dev-Alien-Squeak-FFI-issues-on-Snow-Leopard-td85608.html
[6] http://wiki.squeak.org/squeak/5846
[7] http://forum.world.st/FFI-Callbacks-td54056.html#a54073
[8] http://forum.world.st/Security-td99624.html#a99635:

Other choices:
In the fall of 2008, Alien the FFI interface (written by Cadence Design Systems, Inc.) was put into squeaksource: http://www.squeaksource.com/Alien.html. This API allows the primitive to call back to Smalltalk Code, and return error code information, and apparently is much faster due to a less complex call sequence.
        * if you need callbacks
        * mac-only?

Plugins - write external code and dynamically link it to the VM
                * safest - users are limited to using the functionality you provide and can not call other external functions e.g. system rm /
                * fast - faster than FFI
                * you can create new functionality that doesn't exist in any library
                * harder to write
                * plugin must be distributed with the Smalltalk code - there can be complications with platforms

Primitive method - invokes behavior in the VM or a plugin [3]

* why would you want to build the FFI plugin yourself?
* api prefix or no for method names?
* not sure about pre Snow Leopard library search
* OSProcess - how does this fit into the bigger picture?
* "^self externalCallFailed." or "self externalCallFailed." i.e. return self or return the result, or doesn't matter?
* why would a field description have three elements?
* Mac Memory Allocation Issues?