Roads Less Taken

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Nim Error Handling

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Shit happens as they say, and a good approach to handling errors is important in a modern programmling language. Nim has taken a seemingly traditional approach to handling errors. It uses a model with Exceptions that are objects and a try:except: construction.

I am not listing range, string and cstring types either. So… in Nim, if we look at Collections, we have seq as the single general purpose dynamically sized collection. Now… this is in the Nim language. If we extend our view to also include the standard library, we find lots more.

In order to first learn all you can do with a seq, one way is basically to search for “seq” in the documentation of the system module.

Squeak (and Pharo) has two basic mechanisms of interfacing with the C world:

The VM plugins are basically a controlled way to introduce new “named” primitives in Smalltalk that can be invoked from Smalltalk. A plugin can be built either linked into the VM binary (statically) or as a dynamically loaded library (dll, so, dylib etc). When “all else fails” a plugin is the way to go, but they are a bit awkward to work with.

Then we have the FFI which is a generic way to dynamically call dynamically loaded libraries. In other words, no compilation step needed - just type the correct Smalltalk line and the library will load and the calls work. Now… sure, the FFI mechanism is a bit slower, since it needs to look at arguments and make the proper type conversions for the call. But the FFI is heavily used in the commercial Terf system, in fact, all the OpenGL calls are done through it. So its quite proven, and not that slow.

NOTE: There are in fact several FFIs today, the old one, the one called Alien and Pharo is brewing a new one called UFFI.

Let’s see if we can use the good old FFI with Nim.

Making a Nim library

It turns out that making a library isn’t hard, but there are a few things of note. An oh, all files here if you want to try it out. Ok, so here is our testlib.nim library:

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# We exercise some trivial types and how they map.
#
#   SqueakFFI      Nim
#   =========      ======
#   long       =>  int
#   char*      =>  cstring        

import math

# A single proc, returns an int. Since we are on 32 bits
# an int is 4 bytes (same size as pointer) and in Squeak FFI
# this is a long. The exportc pragma ensures that the exported
# name for this proc is exactly "hello" and not mangled.
proc hello*(): int {.exportc.} =
  42

# Trivial, a Nim string can be sent as a cstring because they are
# automatically 0-terminated.
proc foo*(): cstring {.exportc.} =
  "hey"

# Not a problem taking int arguments, they are "long" in Squeak FFI.
proc add*(x, y:int): int {.exportc.} =
  x + y

# Just return the length of a cstring
proc length*(x: cstring): int {.exportc.} =
  len(x)

# Here we convert cstrings to Nim strings, concatenate and return.
proc concat*(x, y: cstring): cstring {.exportc.} =
  $x & $y

The only thing above that wasn’t obvious - ehrm, unless you read the manual of course - is that you need to use the exportc pragma to make the symbols stay unmangled in the library.

To compile the above to a 32 bit .so library on Linux, we create a corresponding config file called testlib.nim.cfg:

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--app:lib
--cpu:i386
--passC:"-m32"
--passL:"-m32"

The above are options to the nim compiler, so we could have just used them on the command line. Now we compile it with nim c testlib and it should produce a file called libtestlib.so.

The first options means “compile a lib instead of a program”, and the second means “make it 32 bit”. The last two were needed to make it boil down to the C compiler, might be a bug somewhere, but it works.

Now… the above library worked just fine to call from Squeak… at least all procs unless the concat proc. Boom. And it almost DROVE ME NUTS trying to figure out why I was having issues sending char* arguments from Squeak. I thought the problem was on the Squeak side, and I must say that googling for info on stuff like this is a bit of a haystack operation.

Sidenote: Squeak does suffer quite hard from a lack of proper singular documentation. I mean, lots of docs, wiki etc, all over - but what is current? What works? Its a bit of a problem in the Squeak community I would say. And… well, is Pharo different? Not entirely sure. I really like Smalltalk, Squeak and Pharo, and I am used to this, but I must say its a mess.

It turns out that the concat proc is the only one that creates a Nim string, that code needs the GC, and the GC… is in the nimrtl.nim module. This is also documented, but its in the Compiler manual. So it wasn’t hard, we just need to make sure to build it too as 32 bits, and copy it to some reasonable place:

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nim c --cpu:i386 --passC:"-m32" --passL:"-m32" --app:lib --define:createNimRtl lib/nimrtl.nim
sudo cp lib/libnimrtl.so /lib/i386-linux-gnu/

Then… we also need to add an option to compiling our lib, -d:useNimRtl, so testlib.nim.cfg looks like this:

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--app:lib
--cpu:i386
--passC:"-m32"
--passL:"-m32"
-d:useNimRtl

The end result is that we now have a dynamically loadable runtime library libnimrtl.so that has the GC etc. The above option makes sure our libtestlib.so uses that runtime library which in turn makes Nim strings work fine and tada, concat() works.

Smalltalk side

Okidoki, so… thanks Andreas for helping me to get it working. Now, time to play. I put the libtestlib.so in the directory where I run Squeak. Now… I am working with an old Squeak, but it also works just fine with Pharo 3.0. So easiest way to see it in action is to download Pharo 3.0 and try it there:

  1. Download Pharo 3.0, unzip somewhere.
  2. Unzip this file into the pharo3.0 directory. It has the libtestlib.so and also the Smalltalk source.
  3. Install FFI into Pharo: I did it by opening the Configuration browser, finding FFI configuration, then “Install stable version”. The FFI plugin itself is included in the VM.
  4. File in the Smalltalk source: Open a File browser, select the file Nim-Test.st and do “Install into new changeset”.
  5. Open a Test Runner, search for “Nim” using upper left field, then run the test FFINimTest. If its green - yiha!

You find the calls on the class side of FFINimTestLibrary. For example the one for concat():

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ffiConcat: a with: b
  "self ffiConcat: 'a' with: 'b' "
  
  <cdecl: char* 'concat' (char* char*) module: 'testlib'>
  ^self externalCallFailed

You can select the code between “” and run it with “inspect it”. Hopefully you get the Smalltalk String 'ab'.

Remark

First versions of this article mentioned segfaults, but after looking closer it seems those segfaults appeared due to me using Smalltalk unloadModule: 'testlib' and then trying again, thus causing a reload. Apparently there is a problem doing this in the VM. But the code seems solid, I have run it hours on end with no problems.

I also studied the Squeak FFI code a bit up close and yes, for char* arguments it will copy them out into temporary allocated memory, and then make the call. Afterwards it will deallocate them before returning back into Squeak. For a char* result it will instantiate a ByteString object from inside the plugin and copy the bytes over into it, so the actual pointer and memory returned from Nim, is not used. If we use byte* we can do the same operation from inside Squeak, as the Terf codebase does extensively. But both approaches seem to be solid.

As noted the pointer returned from Nim in concat() is the pointer to the Nim string itself. This works because Nim strings “double” as cstring, because they are null terminated. That string is however a GC tracked ref in Nim, so Nim will eventually garbage collect it - but the GC doesn’t run in its own thread, so the data is safe for Squeak to copy - at least until we return back to Nim in the next call. But generally having Nim return pointers to GC tracked data, is of course not safe unless Squeak immediately copies it. Now… thankfully for char* Squeak FFI does it automatically, for other types we can do it manually in Smalltalk code.

Conclusion

The old Squeak FFI is quite proven and works. Nim can produce libraries quite easily. And using Nim instead of C/C++ is a nobrainer to me. :)

Happy hacking!

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