[racket] FFI: problems using (_list i _string)

From: Thomas Chust (chust at web.de)
Date: Sun Jun 5 18:56:27 EDT 2011

2011/6/5 keydana at gmx.de <keydana at gmx.de>:
> [...]
> In some example c code available, the argument actually is a
> two-dimensional array of chars, which then of course is passed as a
> pointer to char, and I guess the separation of strings is achieved
> by the null-termination of c strings then...

Hello Sigrid,

in C, strings are almost always zero terminated, yes. However, a two
dimensional array that doesn't consist of string pointers but the
string data itself will necessarily contain fixed length strings in C,
possibly padded with zero bytes in case the actual payload for some
items is smaller than the available space.

> [...]
> I wonder how I am going to do this in racket, should I append null
> to every string, string-append the strings and then pass a single
> string to the function?
> [...]

Yes, I would say this is a perfectly fine solution. To convert a list
of strings to something that can be used as an array of fixed width
items try something like this module:

  #lang racket/base

  (define (string-list->fixed-width-array width items)
    (string-join (map (cut string-pad-right <> width #\null) items)))


> [...]
> Regarding racket garbage collection, on the one hand, and c pointer
> freeing, that is generally a topic I'm very unsure about and could
> perhaps use some "basic" advice (more basic  than the FFI reference,
> I mean).  For example, do I have to explicitly "destroy"  c pointers
> somehow, and if so, when? In the case of my c library given, I
> assume I have to implement every function like '<object>Free' and
> call it when I'm done, but apart from that?
> [...]

Ok, I'll try to sum up a few basics: First it's important to note that
values allocated by Racket and blocks allocated by C code live in
different memory areas and are handled differently. While Racket
values are reclaimed automatically by the garbage collector some time
after they are no longer referenced, blocks allocated from C live
forever unless explicitly destroyed. These two worlds know nothing
about each other by default.

The pointers your code handles never have to be destroyed explicitly,
but the objects they point to may have to be destroyed. There is no
simple general rule when and how this has to happen. It all depends on
the way the C code is designed and what mechanisms it uses for memory

In the most simple situation, you do not use any C functions that
allocate and store or return (pointers to) blocks of memory, so you
can just let all the data live in the memory area managed by Racket
and let the garbage collector do its job without caring about manually
freeing any blocks of memory. C structures can be allocated through
Racket's garbage collector, too, and they will be reclaimed just like
Racket values some time after all pointers referring to them go out of
scope. Look at the documentation of malloc in the ffi/unsafe module
for details about memory allocation from the Racket side.

Another case encountered frequently is that there are C functions
creating, operating on and destroying pointers to some opaque data
structure. These structures do not live in Racket's managed memory, so
calls to the creation functions have to be matched by calls to the
destruction functions or the objects will stay in memory until the
process terminates.

In this situation you have two basic choices: You can either provide
bindings to the functions that create and destroy objects and require
any client code to handle memory management of those objects
explicitly, manually destroying them when they are no longer
needed. Or you can set up an automatism in Racket that ensures all
those objects created by calls from Racket are freed some time after
Racket code no longer holds any references to them. An easy way to
achieve this is by decorating the bindings for the creation and
destruction procedures using allocator and deallocator from the
ffi/unsafe/alloc module. However, care has to be taken that no code
outside Racket is still using the objects when they are reclaimed by
the Racket garbage collector. Some C libraries use reference counting
to better handle the situation that different pieces of code may hold
references to an object for random periods of time; the
ffi/unsafe/alloc module also has support for that situation.

Things get progressively more complicated if the C structures put
inside Racket's memory areas contain pointers themselves, if
structures outside Racket's memory areas contain pointers to Racket
values, or if callbacks from C into Racket are used.

> [...]
> I hope I didn't ask too many too basic questions now, but never
> having done any c programming and just knowing the concepts
> "theoretically", there a quite some things about using FFI I don't
> automatically understand from the Reference...
> [...]

Among all the FFIs I have used, Racket's is one of the most
comfortable and full featured ones. However, its features imply
complexity, too, and I can well imagine that without any prior
experience in low level programming and manual memory management, one
can easily get lost. So questions have to be expected :-)


When C++ is your hammer, every problem looks like your thumb.

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