Nim Compiler User Guide

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Author:Andreas Rumpf

"Look at you, hacker. A pathetic creature of meat and bone, panting and sweating as you run through my corridors. How can you challenge a perfect, immortal machine?"


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:


nim command [options] [projectfile] [arguments]
compile, c
compile project with default code generator (C)
compile to $nimcache/projname, run with [arguments] using backend specified by --backend (default: c)
generate the documentation for inputfile for backend specified by --backend (default: c)
arguments are passed to the program being run (if --run option is selected)
-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:on|off
force rebuilding of all modules
turn stack tracing on|off
turn line tracing on|off
turn support for multi-threading on|off
-x, --checks:on|off
turn all runtime checks on|off
-a, --assertions:on|off
turn assertions on|off
optimize not at all or for speed|size Note: use -d:release for a release build!
Use native debugger (gdb)
generate a console app|GUI app|DLL|static library
-r, --run
run the compiled program with given arguments
evaluates nim code directly; e.g.: nim --eval:"echo 1" defaults to e (nimscript) but customizable: nim r --eval:'for a in stdin.lines: echo a'
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
compile project to Javascript
run a Nimscript file
convert a reStructuredText file to HTML use --docCmd:skip to skip compiling snippets
convert a reStructuredText file to LaTeX
extract the documentation to a LaTeX file
extract the documentation to a json file
create a tags file
build an index for the whole documentation
generate a DOT file containing the module dependency graph
dump all defined conditionals and search paths see also: --dump.format:json (useful with: | jq)
checks the project for syntax and semantics (can be combined with --defusages)
Runtime checks (see -x):
turn obj conversion checks on|off
turn case variant field checks on|off
turn range checks on|off
turn bound checks on|off
turn int over-/underflow checks on|off
turn all floating point (NaN/Inf) checks on|off
turn NaN checks on|off
turn Inf checks on|off
Advanced options:
find the definition and all usages of a symbol
-o:FILE, --out:FILE
set the output filename
set the path where the output file will be written
will use outdir=$$nimcache, whichever it resolves to after all options have been processed
output to stdout
turn compiler messages coloring on|off
customize how filenames are rendered in compiler messages, defaults to abs (absolute)
show files as they're being processed by nim compiler
use the ASCII unit separator (31) between error messages, useful for IDE-like tooling
show declaration locations in messages
show at most num >= 0 spelling suggestions on typos. if num is not specified (or auto), return an implementation defined set of suggestions.
on|off enables or disables hints. list reports which hints are selected.
turn specific hint X on|off. hint:X means hint:X:on, as with similar flags. all is the set of all hints (only all:off is supported).
turn specific hint X into an error on|off
-w:on|off|list, --warnings:on|off|list
same as --hints but for warnings.
--warningAsError:X:on|off ditto
produce hints or errors for Nim identifiers that do not adhere to Nim's official style guide
only enforce consistent spellings of identifiers, do not enforce the style on declarations
show all mismatching candidates in overloading resolution
set the system library path
add an automatically imported module see also patchFile in nimscript which offers more flexibility.
add an automatically included module
-c, --compileOnly:on|off
compile Nim files only; do not assemble or link
compile Nim and generated files but do not link
do not generate a main procedure
generate a compile script (in the 'nimcache' subdirectory named 'compile_$$project$$scriptext'), and a '.deps' file containing the dependencies; implies --compileOnly
set the target operating system (cross-compilation)
set the target processor (cross-compilation)
enables debug information
-t, --passC:OPTION
pass an option to the C compiler
-l, --passL:OPTION
pass an option to the linker
specify the C compiler
modify the C compiler header search path
modify the linker library search path
link an additional C library (you should omit platform-specific extensions)
document the whole project (doc)
nim doc --docRoot:/foo --project --outdir:docs /foo/sub/main.nim generates: docs/sub/main.html if path == @pkg, will use nimble file enclosing dir if path == @path, will use first matching dir in --path if path == @default (the default and most useful), will use best match among @pkg,@path. if these are nonexistent, will use project path
-b, --backend:c|cpp|js|objc
sets backend to use with commands like nim doc or nim r
if cmd == skip, skips runnableExamples else, runs runnableExamples with given options, e.g.: --docCmd:"-d:foo --threads:on"
activate 'see source' for doc command (see doc.item.seesrc in config/nimdoc.cfg)
also generate documentation for non-exported symbols
generation of #line directive on|off
embeds the original source code as comments in the generated output
turn thread local storage emulation on|off
turn implicit compile time evaluation on|off
turn term rewriting macros on|off
turn multi-methods on|off
turn support for hot code reloading on|off
stack traces use full file paths
enable user defined stack frame msgs via setFrameMsg
do not read the nim installation's configuration file
do not read the user's configuration file
do not read the parent dirs' configuration files
do not read the project's configuration file
select the GC to use; default is 'refc'
select the exception handling implementation
turn index file generation on|off
set an environment variable
add a path for Nimble support
deactivate the Nimble path
empty the list of Nimble package search paths
use the provided namespace for the generated C++ code, if no namespace is provided "Nim" will be used
dump every generated AST from MACRO
show how PROCNAME looks like after diverse optimizations before the final backend phase (mostly ARC/ORC specific)
exclude a path from the list of search paths
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: ""
disables the effects of the dynlib pragma
list the compilation commands; can be combined with --hint:exec:on and --hint:link:on
produce assembler code
perform a parallel build value = number of processors (0 for auto-detect)
only recompile the changed modules (experimental!)
set Nim's verbosity level (1 is default)
stop compilation after N errors; 0 means unlimited
set max iterations for all VM loops
enable experimental language feature
enable obsolete/legacy language feature
emulate Nim version X of the Nim compiler, for testing
turn benchmarking of VM code with cpuTime() on|off
turn compile time VM profiler on|off
turn sink parameter inference on|off (default: on)
turn panics into process terminations (default: off)
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 with {.warning[NAME]:on|off.}.

CannotOpenFileSome file not essential for the compiler's working could not be opened.
OctalEscapeThe code contains an unsupported octal sequence.
DeprecatedThe code uses a deprecated symbol.
ConfigDeprecatedThe project makes use of a deprecated config file.
SmallLshouldNotBeUsedThe letter 'l' should not be used as an identifier.
EachIdentIsTupleThe code contains a confusing var declaration.
CStringConvWarn about dangerous implicit conversions to cstring.
EnumConvWarn about conversions from enum to enum.
AnyEnumConvWarn about any conversions to an enum type.
HoleEnumConvWarn about conversion to an enum with holes. These conversions are unsafe.
ResultUsedWarn about the usage of the built-in result variable.
UserSome user-defined warning.

List of hints

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

CCShows when the C compiler is called.
ConfA config file was loaded.
ExecProgram is executed.
GCStatsDumps statistics about the Garbage Collector.
GlobalVarShows global variables declarations.
LineTooLongLine exceeds the maximum length.
LinkLinking phase.
PathSearch paths modifications.
ProcessingArtifact being compiled.
SourceThe source line that triggered a diagnostic message.
Success, SuccessXSuccessful compilation of a library or a binary.
XDeclaredButNotUsedUnused symbols in the code.

Verbosity levels

0Minimal output level for the compiler.
1Displays compilation of all the compiled files, including those imported by other modules or through the compile pragma. This is the default level.
2Displays compilation statistics, enumerates the dynamic libraries that will be loaded by the final binary, and dumps to standard output the result of applying a filter to the source code if any filter was used during compilation.
3In addition to the previous levels dumps a debug stack trace for compiler developers.

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 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:


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 llvm_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.


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 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
# `nim r` also works, running the binary via `wine` or `wine64`:
nim r -d:mingw --eval:'import os; echo "a" / "b"'

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

The MinGW-w64 toolchain can be installed as follows:

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

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:

nim c ... --d:nimAllocPagesViaMalloc --gc:orc --passC="-I$DEVKITPRO/libnx/include" ...
--passL="-specs=$DEVKITPRO/libnx/switch.specs -L$DEVKITPRO/libnx/lib -lnx"

or setup a nim.cfg file like so:

--passL="-specs=$DEVKITPRO/libnx/switch.specs -L$DEVKITPRO/libnx/lib -lnx"

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. Switch OS (Horizon) doesn't support dynamic libraries, so dlopen/dlclose are not available.
  3. mqueue. Sadly there are no mqueue headers.
  4. ucontext. No headers for these either. No coroutines for now :(
  5. nl_types. No headers for this.
  6. As mmap is not supported, the nimAllocPagesViaMalloc option has to be used.

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.

releaseTurns on the optimizer. More aggressive optimizations are possible, e.g.: --passC:-ffast-math (but see issue #10305)
dangerTurns off all runtime checks and turns on the optimizer.
useForkMakes osproc use fork instead of posix_spawn.
useNimRtlCompile and link against nimrtl.dll.
useMallocMakes Nim use C's malloc instead of Nim's own memory manager, albeit prefixing each allocation with its size to support clearing memory on reallocation. This only works with --gc:none, --gc:arc and --gc:orc.
useRealtimeGCEnables support of Nim's GC for soft realtime systems. See the documentation of the gc for further information.
logGCEnable GC logging to stdout.
nodejsThe JS target is actually node.js.
sslEnables OpenSSL support for the sockets module.
memProfilerEnables memory profiling for the native GC.
uClibcUse uClibc instead of libc. (Relevant for Unix-like OSes)
checkAbiWhen using types from C headers, add checks that compare what's in the Nim file with what's in the C header. This may become enabled by default in the future.
tempDirThis symbol takes a string as its value, like --define:tempDir:/some/temp/path to override the temporary directory returned by os.getTempDir(). The value should end with a directory separator character. (Relevant for the Android platform)
useShPathThis symbol takes a string as its value, like --define:useShPath:/opt/sh/bin/sh to override the path for the sh binary, in cases where it is not located in the default location /bin/sh.
noSignalHandlerDisable the crash handler from system.nim.
globalSymbolsLoad all {.dynlib.} libraries with the RTLD_GLOBAL flag on Posix systems to resolve symbols in subsequently loaded libraries.

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.


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's default allocator is based on TLSF, this algorithm was designed for embedded devices. This allocator gets blocks/pages of memory via a currently undocumented osalloc API which usually uses POSIX's mmap call. On many environments mmap is not available but C's malloc is. You can use the nimAllocPagesViaMalloc define to use malloc instead of mmap. nimAllocPagesViaMalloc is currently only supported with --gc:arc or --gc:orc. (Since version 1.6)

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:

var s = procA() # assignment will not copy the string; procA allocates a new
                # string already

However, it is not efficient to do:

var s = varA    # assignment has to copy the whole string into a new buffer!

For let symbols a copy is not always necessary:

let s = varA    # may only copy a pointer if it safe to do so

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

var s = "abc"
shallow(s) # mark 's' as a shallow string
var x = s  # now might not copy the string!

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:

case normalize(k.key)
of "name": = v
of "displayname": c.displayName = v
of "version": c.version = v
of "os": c.oses = split(v, {';'})
of "cpu": c.cpus = split(v, {';'})
of "authors": c.authors = split(v, {';'})
of "description": c.description = v
of "app":
  case normalize(v)
  of "console": = appConsole
  of "gui": = appGUI
  else: quit(errorStr(p, "expected: console or gui"))
of "license": c.license = UnixToNativePath(k.value)
else: quit(errorStr(p, "unknown variable: " & k.key))