Nim Backend Integration

Author: Puppet Master
Version: 0.14.0

"Heresy grows from idleness." -- Unknown.


The Nim Compiler User Guide documents the typical compiler invocation, using the compile or c command to transform a .nim file into one or more .c files which are then compiled with the platform's C compiler into a static binary. However there are other commands to compile to C++, Objective-C or JavaScript. This document tries to concentrate in a single place all the backend and interfacing options.

The Nim compiler supports mainly two backend families: the C, C++ and Objective-C targets and the JavaScript target. The C like targets creates source files which can be compiled into a library or a final executable. The JavaScript target can generate a .js file which you reference from an HTML file or create a standalone nodejs program.

On top of generating libraries or standalone applications, Nim offers bidirectional interfacing with the backend targets through generic and specific pragmas.


The C like targets

The commands to compile to either C, C++ or Objective-C are:

compileToC, cccompile project with C code generator
compileToCpp, cppcompile project to C++ code
compileToOC, objccompile project to Objective C code

The most significant difference between these commands is that if you look into the nimcache directory you will find .c, .cpp or .m files, other than that all of them will produce a native binary for your project. This allows you to take the generated code and place it directly into a project using any of these languages. Here are some typical command line invocations:

$ nim c hallo.nim
$ nim cpp hallo.nim
$ nim objc hallo.nim

The compiler commands select the target backend, but if needed you can specify additional switches for cross compilation to select the target CPU, operative system or compiler/linker commands.

The JavaScript target

Nim can also generate JavaScript code through the js command. However, the JavaScript code generator is experimental!

Nim targets JavaScript 1.5 which is supported by any widely used browser. Since JavaScript does not have a portable means to include another module, Nim just generates a long .js file.

Features or modules that the JavaScript platform does not support are not available. This includes:

  • manual memory management (alloc, etc.)
  • casting and other unsafe operations (cast operator, zeroMem, etc.)
  • file management
  • most modules of the Standard library
  • proper 64 bit integer arithmetic
  • unsigned integer arithmetic

However, the modules strutils, math, and times are available! To access the DOM, use the dom module that is only available for the JavaScript platform.

To compile a Nim module into a .js file use the js command; the default is a .js file that is supposed to be referenced in an .html file. However, you can also run the code with nodejs, a software platform for easily building fast, scalable network applications:

nim js -d:nodejs -r examples/hallo.nim


Nim offers bidirectional interfacing with the target backend. This means that you can call backend code from Nim and Nim code can be called by the backend code. Usually the direction of which calls which depends on your software architecture (is Nim your main program or is Nim providing a component?).

Nim code calling the backend

Nim code can interface with the backend through the Foreign function interface mainly through the importc pragma. The importc pragma is the generic way of making backend symbols available in Nim and is available in all the target backends (JavaScript too). The C++ or Objective-C backends have their respective ImportCpp and ImportObjC pragmas to call methods from classes.

Whenever you use any of these pragmas you need to integrate native code into your final binary. In the case of JavaScript this is no problem at all, the same html file which hosts the generated JavaScript will likely provide other JavaScript functions which you are importing with importc.

However, for the C like targets you need to link external code either statically or dynamically. The preferred way of integrating native code is to use dynamic linking because it allows you to compile Nim programs without the need for having the related development libraries installed. This is done through the dynlib pragma for import, though more specific control can be gained using the dynlib module.

The dynlibOverride command line switch allows to avoid dynamic linking if you need to statically link something instead. Nim wrappers designed to statically link source files can use the compile pragma if there are few sources or providing them along the Nim code is easier than using a system library. Libraries installed on the host system can be linked in with the PassL pragma.

To wrap native code, take a look at the c2nim tool which helps with the process of scanning and transforming header files into a Nim interface.

C invocation example

Create a logic.c file with the following content:

int addTwoIntegers(int a, int b)
  return a + b;

Create a calculator.nim file with the following content:

{.compile: "logic.c".}
proc addTwoIntegers(a, b: cint): cint {.importc.}

when isMainModule:
  echo addTwoIntegers(3, 7)

With these two files in place, you can run nim c -r calculator.nim and the Nim compiler will compile the logic.c file in addition to calculator.nim and link both into an executable, which outputs 10 when run. Another way to link the C file statically and get the same effect would be remove the line with the compile pragma and run the following typical Unix commands:

$ gcc -c logic.c
$ ar rvs mylib.a logic.o
$ nim c --passL:mylib.a -r calculator.nim

Just like in this example we pass the path to the mylib.a library (and we could as well pass logic.o) we could be passing switches to link any other static C library.

JavaScript invocation example

Create a host.html file with the following content:

<script type="text/javascript">
function addTwoIntegers(a, b)
  return a + b;
<script type="text/javascript" src="calculator.js"></script>

Create a calculator.nim file with the following content (or reuse the one from the previous section):

proc addTwoIntegers(a, b: int): int {.importc.}

when isMainModule:
  echo addTwoIntegers(3, 7)

Compile the Nim code to JavaScript with nim js -o:calculator.js calculator.nim and open host.html in a browser. If the browser supports javascript, you should see the value 10. In JavaScript the echo proc will modify the HTML DOM and append the string. Use the dom module for specific DOM querying and modification procs.

Backend code calling Nim

Backend code can interface with Nim code exposed through the exportc pragma. The exportc pragma is the generic way of making Nim symbols available to the backends. By default the Nim compiler will mangle all the Nim symbols to avoid any name collision, so the most significant thing the exportc pragma does is maintain the Nim symbol name, or if specified, use an alternative symbol for the backend in case the symbol rules don't match.

The JavaScript target doesn't have any further interfacing considerations since it also has garbage collection, but the C targets require you to initialize Nim's internals, which is done calling a NimMain function. Also, C code requires you to specify a forward declaration for functions or the compiler will assume certain types for the return value and parameters which will likely make your program crash at runtime.

The Nim compiler can generate a C interface header through the --header command line switch. The generated header will contain all the exported symbols and the NimMain proc which you need to call before any other Nim code.

Nim invocation example from C

Create a fib.nim file with the following content:

proc fib(a: cint): cint {.exportc.} =
  if a <= 2:
    result = 1
    result = fib(a - 1) + fib(a - 2)

Create a maths.c file with the following content:

#include "fib.h"
#include <stdio.h>

int main(void)
  for (int f = 0; f < 10; f++)
    printf("Fib of %d is %d\n", f, fib(f));
  return 0;

Now you can run the following Unix like commands to first generate C sources form the Nim code, then link them into a static binary along your main C program:

$ nim c --noMain --noLinking --header:fib.h fib.nim
$ gcc -o m -Inimcache -Ipath/to/nim/lib nimcache/*.c maths.c

The first command runs the Nim compiler with three special options to avoid generating a main() function in the generated files, avoid linking the object files into a final binary, and explicitly generate a header file for C integration. All the generated files are placed into the nimcache directory. That's why the next command compiles the maths.c source plus all the .c files form nimcache. In addition to this path, you also have to tell the C compiler where to find Nim's nimbase.h header file.

Instead of depending on the generation of the individual .c files you can also ask the Nim compiler to generate a statically linked library:

$ nim c --app:staticLib --noMain --header fib.nim
$ gcc -o m -Inimcache -Ipath/to/nim/lib libfib.nim.a maths.c

The Nim compiler will handle linking the source files generated in the nimcache directory into the libfib.nim.a static library, which you can then link into your C program. Note that these commands are generic and will vary for each system. For instance, on Linux systems you will likely need to use -ldl too to link in required dlopen functionality.

Nim invocation example from JavaScript

Create a mhost.html file with the following content:

<script type="text/javascript" src="fib.js"></script>
<script type="text/javascript">
alert("Fib for 9 is " + fib(9));

Create a fib.nim file with the following content (or reuse the one from the previous section):

proc fib(a: cint): cint {.exportc.} =
  if a <= 2:
    result = 1
    result = fib(a - 1) + fib(a - 2)

Compile the Nim code to JavaScript with nim js -o:fib.js fib.nim and open mhost.html in a browser. If the browser supports javascript, you should see an alert box displaying the text Fib for 9 is 34. As mentioned earlier, JavaScript doesn't require an initialisation call to NimMain or similar function and you can call the exported Nim proc directly.

Nimcache naming logic

The nimcache directory is generated during compilation and will hold either temporary or final files depending on your backend target. The default name for the directory is nimcache but you can use the --nimcache compiler switch to change it.

Nimcache and C like targets

The C like backends will place their temporary .c, .cpp or .m files in the nimcache directory. The naming of these files follows the pattern nimblePackageName_ + nimSource:

  • Filenames for modules imported from nimble packages will end up with nimblePackageName_module.c. For example, if you import the argument_parser module from the same name nimble package you will end up with a argument_parser_argument_parser.c file under nimcache. The name of the nimble package comes from the proj.nimble file, the actual contents are not read by the compiler.
  • Filenames for non nimble packages (like your project) will be renamed from .nim to have the extension of your target backend (from now on .c for these examples), but otherwise nothing else will change. This will quickly break if your project consists of a main proj.nim file which includes a utils/proj.nim file: both proj.nim files will generate the same name proj.c output in the nimcache directory overwriting themselves!
  • Filenames for modules found in the standard library will be named stdlib_module.c. Unless you are doing something special, you will end up with at least stdlib_system.c, since the system module is always imported automatically. Same for the hashes module which will be named stdlib_hashes.c. The stdlib_ prefix comes from the fake lib/stdlib.nimble file.

To find the name of a nimble package the compiler searches for a *.nimble file in the parent directory hierarchy of whatever module you are compiling. Even if you are in a subdirectory of your project, a parent *.nimble file will influence the naming of the nimcache name. This means that on Unix systems creating the file ~/foo.nimble will automatically prefix all nimcache files not part of another package with the string foo_.

Nimcache and the Javascript target

Unless you explicitly use the -o:filename.js switch as mentioned in the previous examples, the compiler will create a filename.js file in the nimcache directory using the name of your input nim file. There are no other temporary files generated, the output is always a single self contained .js file.

Memory management

In the previous sections the NimMain() function reared its head. Since JavaScript already provides automatic memory management, you can freely pass objects between the two language without problems. In C and derivate languages you need to be careful about what you do and how you share memory. The previous examples only dealt with simple scalar values, but passing a Nim string to C, or reading back a C string in Nim already requires you to be aware of who controls what to avoid crashing.

Strings and C strings

The manual mentions that Nim strings are implicitly convertible to cstrings which makes interaction usually painless. Most C functions accepting a Nim string converted to a cstring will likely not need to keep this string around and by the time they return the string won't be needed any more. However, for the rare cases where a Nim string has to be preserved and made available to the C backend as a cstring, you will need to manually prevent the string data from being freed with GC_ref and GC_unref.

A similar thing happens with C code invoking Nim code which returns a cstring. Consider the following proc:

proc gimme(): cstring {.exportc.} =
  result = "Hey there C code! " & $random(100)

Since Nim's garbage collector is not aware of the C code, once the gimme proc has finished it can reclaim the memory of the cstring. However, from a practical standpoint, the C code invoking the gimme function directly will be able to use it since Nim's garbage collector has not had a chance to run yet. This gives you enough time to make a copy for the C side of the program, as calling any further Nim procs might trigger garbage collection making the previously returned string garbage. Or maybe you are yourself triggering the collection.

Custom data types

Just like strings, custom data types that are to be shared between Nim and the backend will need careful consideration of who controls who. If you want to hand a Nim reference to C code, you will need to use GC_ref to mark the reference as used, so it does not get freed. And for the C backend you will need to expose the GC_unref proc to clean up this memory when it is not required any more.

Again, if you are wrapping a library which mallocs and frees data structures, you need to expose the appropriate free function to Nim so you can clean it up. And of course, once cleaned you should avoid accessing it from Nim (or C for that matter). Typically C data structures have their own malloc_structure and free_structure specific functions, so wrapping these for the Nim side should be enough.

Thread coordination

When the NimMain() function is called Nim initializes the garbage collector to the current thread, which is usually the main thread of your application. If your C code later spawns a different thread and calls Nim code, the garbage collector will fail to work properly and you will crash.

As long as you don't use the threadvar emulation Nim uses native thread variables, of which you get a fresh version whenever you create a thread. You can then attach a GC to this thread via


It is not safe to disable the garbage collector and enable it after the call from your background thread even if the code you are calling is short lived.