Nim Compiler User Guide

Author

Andreas Rumpf

Version

Introduction

This document describes the usage of the Nim compiler on the different supported platforms. It is not a definition of the Nim programming language (which is covered in the manual).

Nim is free software; it is licensed under the MIT License.

Compiler Usage

Command-line switches

Basic command-line switches are:

Usage:

nim command [options] [projectfile] [arguments]

Command:

//compile, c compile project with default code generator (C) //r compile to $nimcache/projname, run with [arguments] using backend specified by --backend (default: c) //doc generate the documentation for inputfile for backend specified by --backend (default: c)

Arguments:

arguments are passed to the program being run (if --run option is selected)

Options:

-p, --path:PATH add path to search paths -d, --define:SYMBOL(:VAL) define a conditional symbol (Optionally: Define the value for that symbol, see: "compile time define pragmas") -u, --undef:SYMBOL undefine a conditional symbol -f, --forceBuild:onoff turn stack tracing onoff turn line tracing onoff turn support for multi-threading onoff turn all runtime checks onoff turn assertions onspeedsize Note: use -d:release for a release build! --debugger:native Use native debugger (gdb) --app:consolelibGUI appstatic library -r, --run run the compiled program with given arguments --fullhelp show all command line switches -h, --help show this help -v, --version show detailed version information

Note, single letter options that take an argument require a colon. E.g. -p:PATH.

Advanced command-line switches are:

Advanced commands:

//compileToC, cc compile project with C code generator //compileToCpp, cpp compile project to C++ code //compileToOC, objc compile project to Objective C code //js compile project to Javascript //e run a Nimscript file //rst2html convert a reStructuredText file to HTML use --docCmd:skip to skip compiling snippets //rst2tex convert a reStructuredText file to TeX //jsondoc extract the documentation to a json file //ctags create a tags file //buildIndex build an index for the whole documentation //genDepend generate a DOT file containing the module dependency graph //dump dump all defined conditionals and search paths see also: --dump.format:json (useful with: | jq) //check checks the project for syntax and semantic

Runtime checks (see -x):

--objChecks:onoff --fieldChecks:onoff --rangeChecks:onoff --boundChecks:onoff --overflowChecks:onoff --floatChecks:onoff --nanChecks:onoff --infChecks:onoff --refChecks:onoff (only for --newruntime)

Advanced options:

-o:FILE, --out:FILE set the output filename --outdir:DIR set the path where the output file will be written --usenimcache will use outdir=$$nimcache, whichever it resolves to after all options have been processed --stdout:onoff turn compiler messages coloring onoff list full paths in messages -w:onlist, --warnings:onlist turn all warnings onoff turn specific warning X onoffoff or list all available --hint[X]:onoff --warningAsError[X]:onoff --styleCheck:offerror produce hints or errors for Nim identifiers that do not adhere to Nim's official style guide https://nim-lang.org/docs/nep1.html --showAllMismatches:onoff compile Nim files only; do not assemble or link --noLinking:onoff do not generate a main procedure --genScript:onoff generate a '.deps' file containing the dependencies --os:SYMBOL set the target operating system (cross-compilation) --cpu:SYMBOL set the target processor (cross-compilation) --debuginfo:oncppobjc sets backend to use with commands like nim doc or nim r --docCmd:cmd if cmd == skip, skips runnableExamples else, runs runnableExamples with given options, e.g.: --docCmd:"-d:foo --threads:on" --docSeeSrcUrl:url activate 'see source' for doc command (see doc.item.seesrc in config/nimdoc.cfg) --docInternal also generate documentation for non-exported symbols --lineDir:onoff --embedsrc:onoff turn thread analysis onoff turn thread local storage emulation onoff turn taint mode onoff turn implicit compile time evaluation onoff turn term rewriting macros onoff turn multi-methods onoff turn memory tracker onoff turn support for hot code reloading onoff stack traces use full file paths --stackTraceMsgs:onoff allow 'nil' for strings/seqs for backwards compatibility --seqsv2:onoff do not read the nim installation's configuration file --skipUserCfg:onoff do not read the parent dirs' configuration files --skipProjCfg:onarcmarkAndSweepgoregions select the GC to use; default is 'refc' --exceptions:setjmpgoto select the exception handling implementation --index:onoff --putenv:key=value set an environment variable --NimblePath:PATH add a path for Nimble support --noNimblePath deactivate the Nimble path --clearNimblePath empty the list of Nimble package search paths --cppCompileToNamespace:namespace use the provided namespace for the generated C++ code, if no namespace is provided "Nim" will be used --expandMacro:MACRO dump every generated AST from MACRO --expandArc:PROCNAME show how PROCNAME looks like after diverse optimizations before the final backend phase (mostly ARC/ORC specific) --excludePath:PATH exclude a path from the list of search paths --dynlibOverride:SYMBOL marks SYMBOL so that dynlib:SYMBOL has no effect and can be statically linked instead; symbol matching is fuzzy so that --dynlibOverride:lua matches dynlib: "liblua.so.3" --dynlibOverrideAll disables the effects of the dynlib pragma --listCmd list the compilation commands; can be combined with --hint:exec:on and --hint:link:on --asm produce assembler code --parallelBuild:0... perform a parallel build value = number of processors (0 for auto-detect) --incremental:on13 set Nim's verbosity level (1 is default) --errorMax:N stop compilation after N errors; 0 means unlimited --maxLoopIterationsVM:N set max iterations for all VM loops --experimental:$1 enable experimental language feature --legacy:$2 enable obsolete/legacy language feature --useVersion:1.0 emulate Nim version X of the Nim compiler --profiler:onoff enable benchmarking of VM code with cpuTime() --profileVM:onoff en-/disable sink parameter inference (default: on) --panics:onoff enable 'system.deepCopy' for --gc:arc|orc

List of warnings

Each warning can be activated individually with --warning[NAME]:on|off or in a push pragma.

List of hints

Each hint can be activated individually with --hint[NAME]:on|off or in a push pragma.

Verbosity levels

Compile-time symbols

Through the -d:x or --define:x switch you can define compile-time symbols for conditional compilation. The defined switches can be checked in source code with the when statement and defined proc. The typical use of this switch is to enable builds in release mode (-d:release) where optimizations are enabled for better performance. Another common use is the -d:ssl switch to activate SSL sockets.

Additionally, you may pass a value along with the symbol: -d:x=y which may be used in conjunction with the compile-time define pragmas to override symbols during build time.

Compile-time symbols are completely case insensitive and underscores are ignored too. --define:FOO and --define:foo are identical.

Compile-time symbols starting with the nim prefix are reserved for the implementation and should not be used elsewhere.

Configuration files

Note: The project file name is the name of the .nim file that is passed as a command-line argument to the compiler.

The nim executable processes configuration files in the following directories (in this order; later files overwrite previous settings):

1. $nim/config/nim.cfg, /etc/nim/nim.cfg (UNIX) or <Nim's installation directory>\config\nim.cfg (Windows). This file can be skipped with the --skipCfg command line option.
2. If environment variable XDG_CONFIG_HOME is defined, $XDG_CONFIG_HOME/nim/nim.cfg or ~/.config/nim/nim.cfg (POSIX) or %APPDATA%/nim/nim.cfg (Windows). This file can be skipped with the --skipUserCfg command line option.
3. $parentDir/nim.cfg where $parentDir stands for any parent directory of the project file's path. These files can be skipped with the --skipParentCfg command-line option.
4. $projectDir/nim.cfg where $projectDir stands for the project file's path. This file can be skipped with the --skipProjCfg command-line option.
5. A project can also have a project-specific configuration file named $project.nim.cfg that resides in the same directory as $project.nim. This file can be skipped with the --skipProjCfg command-line option.

Command-line settings have priority over configuration file settings.

The default build of a project is a debug build. To compile a release build define the release symbol:

nim c -d:release myproject.nim

To compile a `dangerous release build`:idx: define the ``danger`` symbol::

nim c -d:danger myproject.nim

Search path handling

Nim has the concept of a global search path (PATH) that is queried to determine where to find imported modules or include files. If multiple files are found an ambiguity error is produced.

nim dump shows the contents of the PATH.

However before the PATH is used the current directory is checked for the file's existence. So if PATH contains $lib and $lib/bar and the directory structure looks like this:

$lib/x.nim
$lib/bar/x.nim
foo/x.nim
foo/main.nim
other.nim

And main imports x, foo/x is imported. If other imports x then both $lib/x.nim and $lib/bar/x.nim match but $lib/x.nim is used as it is the first match.

Generated C code directory

The generated files that Nim produces all go into a subdirectory called nimcache. Its full path is

• $XDG_CACHE_HOME/nim/$projectname(_r|_d) or ~/.cache/nim/$projectname(_r|_d) on Posix
• $HOME/nimcache/$projectname(_r|_d) on Windows.

The _r suffix is used for release builds, _d is for debug builds.

This makes it easy to delete all generated files.

The --nimcache compiler switch can be used to to change the nimcache directory.

However, the generated C code is not platform-independent. C code generated for Linux does not compile on Windows, for instance. The comment on top of the C file lists the OS, CPU, and CC the file has been compiled for.

Compiler Selection

To change the compiler from the default compiler (at the command line):

nim c --cc:llvm_gcc --compile_only myfile.nim

This uses the configuration defined in config\nim.cfg for lvm_gcc.

If nimcache already contains compiled code from a different compiler for the same project, add the -f flag to force all files to be recompiled.

The default compiler is defined at the top of config\nim.cfg. Changing this setting affects the compiler used by koch to (re)build Nim.

To use the CC environment variable, use nim c --cc:env myfile.nim. To use the CXX environment variable, use nim cpp --cc:env myfile.nim. --cc:env is available since Nim version 1.4.

Cross-compilation

To cross compile, use for example:

nim c --cpu:i386 --os:linux --compileOnly --genScript myproject.nim

Then move the C code and the compile script compile_myproject.sh to your Linux i386 machine and run the script.

Another way is to make Nim invoke a cross compiler toolchain:

nim c --cpu:arm --os:linux myproject.nim

For cross compilation, the compiler invokes a C compiler named like $cpu.$os.$cc (for example arm.linux.gcc) and the configuration system is used to provide meaningful defaults. For example for ARM your configuration file should contain something like:

arm.linux.gcc.path = "/usr/bin"
arm.linux.gcc.exe = "arm-linux-gcc"
arm.linux.gcc.linkerexe = "arm-linux-gcc"

Cross-compilation for Windows

To cross-compile for Windows from Linux or macOS using the MinGW-w64 toolchain:

nim c -d:mingw myproject.nim

Use --cpu:i386 or --cpu:amd64 to switch the CPU architecture.

The MinGW-w64 toolchain can be installed as follows:

Ubuntu: apt install mingw-w64
CentOS: yum install mingw32-gcc | mingw64-gcc - requires EPEL
OSX: brew install mingw-w64

Cross-compilation for Android

There are two ways to compile for Android: terminal programs (Termux) and with the NDK (Android Native Development Kit).

The first one is to treat Android as a simple Linux and use Termux to connect and run the Nim compiler directly on android as if it was Linux. These programs are console-only programs that can't be distributed in the Play Store.

Use regular nim c inside termux to make Android terminal programs.

Normal Android apps are written in Java, to use Nim inside an Android app you need a small Java stub that calls out to a native library written in Nim using the NDK. You can also use native-activity to have the Java stub be auto-generated for you.

Use nim c -c --cpu:arm --os:android -d:androidNDK --noMain:on to generate the C source files you need to include in your Android Studio project. Add the generated C files to CMake build script in your Android project. Then do the final compile with Android Studio which uses Gradle to call CMake to compile the project.

Because Nim is part of a library it can't have its own c style main() so you would need to define your own android_main and init the Java environment, or use a library like SDL2 or GLFM to do it. After the Android stuff is done, it's very important to call NimMain() in order to initialize Nim's garbage collector and to run the top level statements of your program.

proc NimMain() {.importc.}
proc glfmMain*(display: ptr GLFMDisplay) {.exportc.} =
  NimMain() # initialize garbage collector memory, types and stack

Cross-compilation for iOS

To cross-compile for iOS you need to be on a macOS computer and use XCode. Normal languages for iOS development are Swift and Objective C. Both of these use LLVM and can be compiled into object files linked together with C, C++ or Objective C code produced by Nim.

Use nim c -c --os:ios --noMain:on to generate C files and include them in your XCode project. Then you can use XCode to compile, link, package and sign everything.

Because Nim is part of a library it can't have its own c style main() so you would need to define main that calls autoreleasepool and UIApplicationMain to do it, or use a library like SDL2 or GLFM. After the iOS setup is done, it's very important to call NimMain() to initialize Nim's garbage collector and to run the top-level statements of your program.

proc NimMain() {.importc.}
proc glfmMain*(display: ptr GLFMDisplay) {.exportc.} =
  NimMain() # initialize garbage collector memory, types and stack

Note: XCode's "make clean" gets confused about the generated nim.c files, so you need to clean those files manually to do a clean build.

Cross-compilation for Nintendo Switch

Simply add --os:nintendoswitch to your usual nim c or nim cpp command and set the passC and passL command line switches to something like:

or setup a nim.cfg file like so:

The DevkitPro setup must be the same as the default with their new installer here for Mac/Linux or here for Windows.

For example, with the above-mentioned config:

nim c --os:nintendoswitch switchhomebrew.nim

This will generate a file called switchhomebrew.elf which can then be turned into an nro file with the elf2nro tool in the DevkitPro release. Examples can be found at the nim-libnx github repo.

There are a few things that don't work because the DevkitPro libraries don't support them. They are:

1. Waiting for a subprocess to finish. A subprocess can be started, but right now it can't be waited on, which sort of makes subprocesses a bit hard to use
2. Dynamic calls. DevkitPro libraries have no dlopen/dlclose functions.
3. Command line parameters. It doesn't make sense to have these for a console anyways, so no big deal here.
4. mqueue. Sadly there are no mqueue headers.
5. ucontext. No headers for these either. No coroutines for now :(
6. nl_types. No headers for this.

DLL generation

Nim supports the generation of DLLs. However, there must be only one instance of the GC per process/address space. This instance is contained in nimrtl.dll. This means that every generated Nim DLL depends on nimrtl.dll. To generate the "nimrtl.dll" file, use the command:

nim c -d:release lib/nimrtl.nim

To link against nimrtl.dll use the command:

nim c -d:useNimRtl myprog.nim

Note: Currently the creation of nimrtl.dll with thread support has never been tested and is unlikely to work!

Additional compilation switches

The standard library supports a growing number of useX conditional defines affecting how some features are implemented. This section tries to give a complete list.

Additional Features

This section describes Nim's additional features that are not listed in the Nim manual. Some of the features here only make sense for the C code generator and are subject to change.

LineDir option

The lineDir option can be turned on or off. If turned on the generated C code contains #line directives. This may be helpful for debugging with GDB.

StackTrace option

If the stackTrace option is turned on, the generated C contains code to ensure that proper stack traces are given if the program crashes or some uncaught exception is raised.

LineTrace option

The lineTrace option implies the stackTrace option. If turned on, the generated C contains code to ensure that proper stack traces with line number information are given if the program crashes or an uncaught exception is raised.

DynlibOverride

By default Nim's dynlib pragma causes the compiler to generate GetProcAddress (or their Unix counterparts) calls to bind to a DLL. With the dynlibOverride command line switch this can be prevented and then via --passL the static library can be linked against. For instance, to link statically against Lua this command might work on Linux:

nim c --dynlibOverride:lua --passL:liblua.lib program.nim

Backend language options

The typical compiler usage involves 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. More details can be read in the Nim Backend Integration document.

Nim documentation tools

Nim provides the doc command to generate HTML documentation from .nim source files. Only exported symbols will appear in the output. For more details see the docgen documentation.

Nim idetools integration

Nim provides language integration with external IDEs through the idetools command. See the documentation of idetools for further information.

Nim for embedded systems

While the default Nim configuration is targeted for optimal performance on modern PC hardware and operating systems with ample memory, it is very well possible to run Nim code and a good part of the Nim standard libraries on small embedded microprocessors with only a few kilobytes of memory.

A good start is to use the any operating target together with the malloc memory allocator and the arc garbage collector. For example:

nim c --os:any --gc:arc -d:useMalloc [...] x.nim

• --gc:arc will enable the reference counting memory management instead of the default garbage collector. This enables Nim to use heap memory which is required for strings and seqs, for example.
• The --os:any target makes sure Nim does not depend on any specific operating system primitives. Your platform should support only some basic ANSI C library stdlib and stdio functions which should be available on almost any platform.
• The -d:useMalloc option configures Nim to use only the standard C memory manage primitives malloc(), free(), realloc().

If your platform does not provide these functions it should be trivial to provide an implementation for them and link these to your program.

For targets with very restricted memory, it might be beneficial to pass some additional flags to both the Nim compiler and the C compiler and/or linker to optimize the build for size. For example, the following flags can be used when targeting a gcc compiler:

--opt:size --passC:-flto --passL:-flto

The --opt:size flag instructs Nim to optimize code generation for small size (with the help of the C compiler), the flto flags enable link-time optimization in the compiler and linker.

Check the Cross-compilation section for instructions on how to compile the program for your target.

Nim for realtime systems

See the documentation of Nim's soft realtime GC for further information.

Signal handling in Nim

The Nim programming language has no concept of Posix's signal handling mechanisms. However, the standard library offers some rudimentary support for signal handling, in particular, segmentation faults are turned into fatal errors that produce a stack trace. This can be disabled with the -d:noSignalHandler switch.

Optimizing for Nim

Nim has no separate optimizer, but the C code that is produced is very efficient. Most C compilers have excellent optimizers, so usually it is not needed to optimize one's code. Nim has been designed to encourage efficient code: The most readable code in Nim is often the most efficient too.

However, sometimes one has to optimize. Do it in the following order:

1. switch off the embedded debugger (it is slow!)
2. turn on the optimizer and turn off runtime checks
3. profile your code to find where the bottlenecks are
4. try to find a better algorithm
5. do low-level optimizations

This section can only help you with the last item.

Optimizing string handling

String assignments are sometimes expensive in Nim: They are required to copy the whole string. However, the compiler is often smart enough to not copy strings. Due to the argument passing semantics, strings are never copied when passed to subroutines. The compiler does not copy strings that are a result of a procedure call, because the callee returns a new string anyway. Thus it is efficient to do:

However, it is not efficient to do:

For let symbols a copy is not always necessary:

If you know what you're doing, you can also mark single-string (or sequence) objects as shallow:

Usage of shallow is always safe once you know the string won't be modified anymore, similar to Ruby's freeze.

The compiler optimizes string case statements: A hashing scheme is used for them if several different string constants are used. So code like this is reasonably efficient: