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Nimterop is a Nim package that aims to make C/C++ interop seamless

Most of the wrapping functionality is contained within the toast binary that is built when nimterop is installed and can be used standalone similar to how c2nim can be used today. In addition, nimterop also offers an API to pull in the generated Nim content directly into an application and other functionality that helps in automating the wrapping process. There is also support to statically or dynamically link to system installed libraries or downloading and building them with autoconf or cmake from a Git repo or source archive.

The nimterop wrapping functionality is still limited to C but is constantly expanding. C++ support will be added once most popular C libraries can be wrapped seamlessly. Meanwhile, c2nim can also be used in place of toast with the c2nImport() API call.

The goal is to make interop seamless so nimterop will focus on wrapping headers and not the outright conversion of C/C++ implementation.


Nimterop can be installed via Nimble:

nimble install nimterop -y


git clone && cd nimterop
nimble develop -y
nimble build -d:danger

This will download and install nimterop in the standard Nimble package location, typically ~/.nimble. Once installed, it can be imported into any Nim program.


Nimterop can be used in three ways:

  • Creating a wrapper file - a .nim file that contains calls to the high-level API that can download and build the C library as well as generate the required Nim code to interface with the library. This wrapper file can then be imported into Nim code like any other module and it will be processed at compile time.
  • Same as the first option except using the nimFile param to cImport() to write the generated wrapper to a file during build time just once and then importing that generated wrapper into the application like any other Nim module.
  • Using the command line toast tool to generate the Nim code which can then be stored into a file and imported separately.

Any combination of the above is possible - only download, build or wrapping and nimterop avoids imposing any particular workflow.

Refer to for history and information around breaking changes.

Build API

Creating a wrapper has two parts, the first is to setup the C library. This includes downloading it or finding it if already installed, and building it if applicable. The getHeader() high-level API provides all of this functionality as a convenience. The following .nim wrapper file is an example of using the high-level getHeader() API to perform all building, wrapping and linking automatically:

import nimterop/[build, cimport]

  cDebug()                                                # Print wrapper to stdout

  baseDir = getProjectCacheDir("testwrapper")             # Download library within nimcache

  "header.h",                                             # The header file to wrap, full path is returned in `headerPath`
  giturl = "",            # Git repo URL
  dlurl = "$1.tar.gz",  # Download URL for archive or raw file
  conanuri = "repo/$1",                                   # URI
  jbburi = "repo/$1",                                     # URI
  outdir = baseDir,                                       # Where to download/build/search
  conFlags = "--disable-comp --enable-feature",           # Flags to pass configure script
  cmakeFlags = "-DENABLE_STATIC_LIB=ON"                   # Flags to pass to Cmake
  altNames = "hdr"                                        # Alterate names of the library binary, full path returned in `headerLPath`

# Wrap headerPath as returned from getHeader() and link statically
# or dynamically depending on user input
when not isDefined(headerStatic):
  cImport(headerPath, recurse = true, dynlib = "headerLPath")       # Pass dynlib if not static link
  cImport(headerPath, recurse = true)

Module documentation for the build API can be found here. Refer to the tests directory for additional examples on how the library can be used. Also, check out the wiki for a list of all known wrappers that have been created using nimterop. They will provide real world examples of how to wrap libraries. Please do add your project once you are done so that others can benefit from your work.

Download / Search

The above wrapper is generic and allows the end user to control how it works. Note that headerPath is derived from header.h so if you have SDL.h as the argument to getHeader(), it generates SDLPath and SDLLPath and is controlled by -d:SDLStatic, -d:SDLGit and so forth.

  • If the library is already installed in /usr/include then the -d:headerStd define to Nim can be used to instruct getHeader() to search for header.h in the standard system path.
  • If the library needs to be downloaded, the user can use -d:headerGit to clone the source from the specified git URL, -d:headerDL to get the source from download URL, -d:headerConan to download from or -d:headerJBB to download from
    • The -d:headerSetVer=X.Y.Z flag can be used to specify which version to download. It is used as the tag name for Git and for DL, Conan and JBB, it replaces $1 in the URL if specified.
  • If no flag is provided, getHeader() simply looks for the library in outdir. The user could use Git submodules or manually download or check-in the library to that directory and getHeader() will use it directly.

Pre build

getHeader() provides a headerPreBuild() hook that gets called after the library is downloaded but before it is built. This allows for any manipulations of the source files or build scripts before build. archive has such an example.

The build API also includes various compile time helper procs that aid in file manipulation, Cmake shortcuts, library linking, etc. Refer to build for more details.


Nimterop currently supports configure and cmake based building of libraries, with cmake taking precedence if a project supports both. Nimterop verifies that the tool selected is available and notifies the user if any issues are found. Bash is required on Windows for configure and the binary shipped with Git has been tested.

Flags can be specified to these tools via getHeader() or directly via the underlying configure() and cmake() calls. Once the build scripts are ready, getHeader() then calls make(). At every step, getHeader() checks for the presence of created artifacts and does not redo steps that have been successfully completed.


If -d:headerStatic is specified, getHeader() will return the static library path in headerLPath. The wrapper writer can check for this and call cImport() accordingly as in the example above. If -d:headerStatic is omitted, the dynamic library is returned in headerLPath.

All dependency libraries (supported by Conan and JBB) will be returned in headerLDeps. Static libraries and dependencies are automatically linked using cPassL(). Conan shared libs typically include dependencies compiled in whereas JBB shared libs expect the required dependencies to be in the same location or in LD_LIBRARY_PATH. conanFlags and jbbFlags can be used to skip required dependencies from being downloaded in case another source is preferred. This can be done with skip=pkg1,pkg2 to these flags.

getHeader() searches for libraries based on the header name by default:

  • or libheader.a on Linux
  • libheader.dylib on OSX
  • header.dll, header.a or header.lib on Windows

If a library has a different header and library binary name, altNames can be used to configure an alternate name of library binary.

  • For example, Bzip2 has bzlib.h but the library is so altNames = "bz2".
  • In the example above, altNames = "hdr" so getHeader() will look for, hdr.dll, etc.
  • See bzlib.nim for an example.

lzma.nim is an example of a library that allows both static and dynamic linking.

User control

The -d:xxxYYY Nim define flags have already been described above and can be specified on the command line or in a nim.cfg file. It is also possible to specify them within the wrapper itself using setDefines() if required. Further, all defines, regardless of how they are specified, can be generically checked using isDefined().

If more fine-tuned control is desired over the build process, it is possible to manually control all steps that getHeader() performs by directly using the API provided by build. Note also that there is no requirement to use these APIs to setup the library. Any other established mechanisms can be used to do so any limitations imposed by Nimterop are unintentional and feedback is most welcome.

Wrapper API

Once the C library is setup, the next step is to generate code that inform Nim of all the types and functions that are available. Following is a simple example covering the API:

import nimterop/cimport

  cDisableCaching()           # Regenerate Nim wrapper every time

cDefine("HAS_ABC")            # Set #defines for preprocessor and compiler
cDefine("HAS_ABC", "DEF")

cIncludeDir("clib/include")   # Setup any include directories
cExclude("clib/file.h")       # Exclude file from wrapped output

cImport("clib.h")             # Generate wrappers for header specified

cCompile("clib/src/*.c")      # Compile in any implementation source files

All {.compileTime.} procs must be used in a compile time context, like cDebug() and cDisableCaching() above.

Module documentation for the wrapper API can be found here.


In order to leverage the preprocessor, certain projects might need cDefine() calls to set #define values. Simpler library may have documentation that cover this but larger ones will rely on build tools that discover and set values in a config.h which is loaded with #include. Projects might also require some cIncludeDir() calls to specify paths to directories that contain other headers. This might be within the library or refer to another library.

The wrapper API always runs headers through the C preprocessor before wrapping. Details on why are discussed further down.

By default, the $CC environment variable is used for the compiler path. If not found, toast defaults to gcc.


The cImport() call invokes the toast binary with appropriate command line flags including any cDefine() and cInclude() parameters configured. The output of toast is then pulled into the module as Nim code and printed if cDebug() is specified. This allows for an end user to simply import the wrapper into their code and access the library API as Nim types and procs. Output is cached to save time on subsequent runs. It is also possible to just redirect the output to a file and import that instead if preferred.

The recurse flag can be set to enable the recursion capability which runs through all #include files in the header. If the library needs to be dyamically linked using Nim's dynlib pragma, the dynlib = "constName" attribute can be set to generate wrappers that load the DLL automatically. Without dynlib, static link is assumed so it is the user's responsibility to link the library.

There may be cases where the wrapper generated by toast for certain types or procs is not preferred, or may be skipped or altogether wrong due to limitations or bugs. In these instances, the cOverride() macro can be used to define consts, types or procs to use in place of the wrapper generated output. cImport() will forward this information to toast and the values will be inserted in context in the generated wrapper. This allows wrapper authors to work around tool limitations or to improve the wrapper output - say change ptr X to var X or to create more Nim friendly types or proc signatures.

Several C libraries also use leading and/or trailing _ in identifiers and since Nim does not allow this, the cPlugin() macro can be used to modify such symbols or cSkipSymbol() them altogether. Instead of a full cPlugin() section, it might also be preferred to set flags = "-E_ -F_" to the cImport() call to trim out such characters. These features can also be used to remove common prefixes like SDL_ to generate a cleaner wrapper. The --replace | -G flag can be used for replacements. cPlugin() is real Nim code though so anything Nim allows is fair game. Note that cPlugin() overrides any -E -F -G flags. Also, behind the scenes, cOverride() is communicated to toast via cPlugin().

If the same cPlugin() is needed in multiple wrapper files, the code can be moved into a standalone file and be used with the cPluginPath() call.

Lastly, c2nImport() provides access to calling c2nim from the wrapper instead of toast. Note that c2nImport() does not use any of the above described features like cPlugin() and needs to be controlled with c2nim specific flags via the flags param.

Header vs. Dynlib

Nim provides some flexibility when it comes to using C/C++ libraries. In order to understand this better, some Nim pragmas need to be introduced. The main one is {.importc.} which informs Nim to use a symbol defined in a C library. This applies to both types and procs but how Nim should find the symbol is slightly different for each.

For types, {.header: "header.h".} informs Nim that header.h has the symbol and to #include "header.h" in the generated code. However, types can be mostly recreated in pure Nim so it is also possible to omit both {.importc.} and {.header} and it will work just fine except with a different name in the generated C code. This allows the user to compile the wrapper without requiring header.h to be present.

For functions, {.header.} works the same as types and can be omitted if preferred. The {.importc.} pragma is still required, unlike types since functions need to be linked to the implementation in the library. The user will need to provide this information at link time with cPassL() and linking to a library with -lheader or path/to/libheader.a. It is also possible to just use cCompile() or {.compile.} to compile some C source files which contain the implementation.

While {.header.} can be omitted for convenience, it does prevent wrapping of static inline functions as well as type checking of the wrapper ABI with -d:checkAbi at compile time. Further, anonymous nested structs/unions within unions will be rendered incorrectly by Nim since it is unaware of the true memory structure of the type. The user will need to choose based on the library in question.

Going further, the {.dynlib: "path/to/".} pragma can be used to inform Nim to load the library at runtime and link the function instead of linking at compile time. This enables creation of a wrapper that does not need the library present at compile time.

Now that this is understood, a user might want any combination of the above in the wrapper rendered by Nimterop. This can be controlled with various flags to cImport() and toast.

  • By default, generated wrappers will include the {.header, importc.} pragmas for types and procs. This can be disabled with the --noHeader | -H flag to toast or flags = "-H" param to cImport() which will remove {.header} for both and {.importc.} for types only.
  • By default, generated wrappers will assume that the user will link the library implementation themselves. The --dynlib | -l flag to toast or dynlib = "headerLPath" param to cImport() will configure the wrapper to generate {.dynlib.} pragmas for procs.

This results in four supported cases:

  1. Default: {.header, importc.} for both types and procs
  2. With --noHeader, types will be pure Nim and procs will be just {.importc.}
  3. With --dynlib, types will still be {.header, importc.} but procs will be {.dynlib, importc.}
  4. With --dynlib and --noHeader, types will be pure Nim, procs will be {.dynlib, importc.}

Creation of a standalone wrapper (case 4) which does not require the header or library at compile time will require an explicit --noHeader and --dynlib.

More documentation on on these pragmas can be found in the Nim manual:

Compiling the source

The job of building and compiling the underlying C library is best left to the build mechanism selected by the library author so using getHeader() is recommended. For simpler projects with a few .c files though, cCompile() should be more than enough. It is not recommended for larger projects which heavily rely on functionality offered by build tools. Recreating reliable logic in Nim can be tedious and one can expect minimal support from that author if their tested build mechanism is not used.

Docs API

Nimterop also provides a docs API which can be used to generate documentation from the generated wrappers. This can be added as a task in the .nimble or .nims file for convenience. See nimarchive.nimble for an example.

Command line API

The toast binary can also be used directly on the CLI, similar to c2nim. These flags can be specified on the command line or via a file, one or more flags per line, and the path provided to toast instead, or a combination. The file contents will be expanded in place.

Note: unlike the wrapper API, the -p | --preprocess flag is not enabled by default but is highly recommended.

> toast -h
  main [optional-params] C/C++ source/header(s) and command line file(s)
  -h, --help                              print this cligen-erated help
  --help-syntax                           advanced: prepend,plurals,..
  -k, --check          bool      false    check generated wrapper with compiler
  --compile=           strings   {}       create {.compile.} entries in generated wrapper
  -C=, --convention=   string    "cdecl"  calling convention for wrapped procs
  -d, --debug          bool      false    enable debug output
  -D=, --defines=      strings   {}       definitions to pass to preprocessor
  -l=, --dynlib=       string    ""       {.dynlib.} pragma to import symbols - Nim const string or
                                          file path
  -X=, --exclude=      strings   {}       files or directories to exclude from the wrapped output
  -f=, --feature=      Features  {}       flags to enable experimental features
  -I=, --includeDirs=  strings   {}       include directory to pass to preprocessor
  -m=, --mode=         string    ""       language parser: c or cpp
  --nim=               string    "nim"    use a particular Nim executable
  -c, --noComments     bool      false    exclude top-level comments from output
  -H, --noHeader       bool      false    skip {.header.} pragma in wrapper
  -o=, --output=       string    ""       file to output content - default: stdout
  --passC=             strings   {}       create {.passC.} entries in generated wrapper
  --passL=             strings   {}       create {.passL.} entries in generated wrapper
  -a, --past           bool      false    print AST output
  --pluginSourcePath=  string    ""       nim file to build and load as a plugin
  -n, --pnim           bool      false    print Nim output
  -E=, --prefix=       strings   {}       strip prefix from identifiers
  -p, --preprocess     bool      false    run preprocessor on header
  -r, --recurse        bool      false    process #include files - implies --preprocess
  -G=, --replace=      strings   {}       replace X with Y in identifiers, X1=Y1,X2=Y2, @X for regex
  -s, --stub           bool      false    stub out undefined type references as objects
  -F=, --suffix=       strings   {}       strip suffix from identifiers
  -O=, --symOverride=  strings   {}       skip generating specified symbols
  -T=, --typeMap=      strings   {}       map instances of type X to Y - e.g. ABC=cint

Why nimterop

Nim has one of the best FFI you can find - importing C/C++ is supported out of the box. All you need to provide is type and proc definitions for Nim to interop with C/C++ binaries. Generation of these wrappers is easy for simple libraries but can quickly get out of hand. c2nim greatly helps here by parsing and converting C/C++ into Nim but is limited due to the complex and constantly evolving C/C++ grammar. nimgen mainly focused on automating the wrapping process with c2nim and filled some holes but is again limited to c2nim capabilities.

The goal of nimterop is to leverage the tree-sitter engine to parse C/C++ code and then convert relevant portions of the AST into Nim definitions. tree-sitter is a Github sponsored project that can parse a variety of languages into an AST which is then leveraged by the Atom editor for syntax highlighting and code folding. The advantages of this approach are multifold:

  • Benefit from the tree-sitter community's ongoing investment into language parsing
  • Wrap what is recognized in the AST rather than completely failing due to parsing errors

The tree-sitter library is limited though - it may fail on some advanced language constructs but is designed to handle them gracefully since it is expected to have bad code while actively typing in an editor. When an error is detected, tree-sitter includes an ERROR node at that location in the AST. At this time, cImport() will complain and continue if it encounters any errors. Depending on how severe the errors are, compilation may succeed or fail. Glaring issues will be communicated to the tree-sitter team but their goals may not always align with those of this project.

It is debatable whether a syntax highlighting engine like tree-sitter is the most reliable method to convert C code into AST. However, it is lightweight, cross-platform with no dependencies and handles error conditions gracefully. It has produced usable wrappers for C libraries though things could get murky when considering C++ but that will be a topic for another day. Nimterop relies heavily on the preprocessor, as discussed next, so having an engine which can run anywhere has been worth the compromise. Only time will tell though.


The wrapper API always runs headers through the C preprocessor before wrapping, unlike the command line interface where the -p | --preprocess flag is not set by default but highly recommended. This is because almost all platform, compiler and package discovery is handled by build tools like configure and cmake which then use preprocessor #define values to tweak what C code is applicable for that platform. While parsing preprocessor macros is possible in tools like toast, given how dependent the #ifdef branches are on values provided by these and many other build tools, preprocessing seems is best left to them than attempting to self-discover or intercept that information.

Nimterop is still able to wrap most relevant #define like numbers and strings thanks to gcc -E providing the sufficient detail in its output. Many C libraries also use #define templates for some of their user facing API and providing that functionality in Nim is on the Nimterop roadmap.

The con of this approach of delegating to the preprocessor is that the Nim wrapper generated by Nimterop is no longer portable despite being Nim code. A wrapper rendered on Linux might not work on Windows since some APIs may not be available or inappropriate, integer sizes might be wrong, types could be missing and many other possible issues. But none of this is easily or accurately known at the Nim level since it would require input from the build tools which already work well with the preprocessor or have to be completely reimplemented within Nim. Neither approach that bypasses such build tools would be supported by the library author.

This is part of the reason why Nimterop provides a wrapper API so that the generation of wrappers is Nim code that can be rendered as part of the build process on the target platform. It helps to think of Nimterop as a build time tool like cmake that renders artifacts on the target rather than a tool whose generated artifacts should be checked into source control. Regardless, both the wrapper API and the toast command line still allow saving the wrapper output to a file to be stored in source control since it might work well enough for many projects.


Nimterop depends on tree-sitter and all licensing terms of tree-sitter apply to the usage of this package. The tree-sitter functionality is pulled and wrapped using nimterop itself.

Thank you to all the contributors, issue submitters, various people in #nim and users for helping improve Nimterop over the years.


Nimterop is a work in progress and any feedback or suggestions are welcome. It is hosted on GitHub with an MIT license so issues, forks and PRs are most appreciated.

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