Nim Compiler User Guide

Author: Andreas Rumpf
Version: 0.19.0

"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 (therefore is 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, ccompile project with default code generator (C)
docgenerate the documentation for inputfile
arguments are passed to the program being run (if --run option is selected)
-p, --path:PATHadd 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:SYMBOLundefine a conditional symbol
-f, --forceBuildforce rebuilding of all modules
--stackTrace:on|offturn stack tracing on|off
--lineTrace:on|offturn line tracing on|off
--threads:on|offturn support for multi-threading on|off
-x, --checks:on|offturn all runtime checks on|off
--objChecks:on|offturn obj conversion checks on|off
--fieldChecks:on|offturn case variant field checks on|off
--rangeChecks:on|offturn range checks on|off
--boundChecks:on|offturn bound checks on|off
--overflowChecks:on|offturn int over-/underflow checks on|off
-a, --assertions:on|offturn assertions on|off
--floatChecks:on|offturn all floating point (NaN/Inf) checks on|off
--nanChecks:on|offturn NaN checks on|off
--infChecks:on|offturn Inf checks on|off
--nilChecks:on|offturn nil checks on|off
--opt:none|speed|sizeoptimize not at all or for speed|size Note: use -d:release for a release build!
--debugger:native|endbuse native debugger (gdb) | ENDB (experimental)
--app:console|gui|lib|staticlibgenerate a console app|GUI app|DLL|static library
-r, --runrun the compiled program with given arguments
--fullhelpshow all command line switches
-h, --helpshow this help

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

Advanced command line switches are:

Advanced commands:
compileToC, cccompile project with C code generator
compileToCpp, cppcompile project to C++ code
compileToOC, objccompile project to Objective C code
jscompile project to Javascript
erun a Nimscript file
rst2htmlconvert a reStructuredText file to HTML
rst2texconvert a reStructuredText file to TeX
jsondocextract the documentation to a json file
ctagscreate a tags file
buildIndexbuild an index for the whole documentation
runrun the project (with Tiny C backend; buggy!)
genDependgenerate a DOT file containing the module dependency graph
dumpdump all defined conditionals and search paths
checkchecks the project for syntax and semantic
Advanced options:
-o:FILE, --out:FILEset the output filename
--stdoutoutput to stdout
--colors:on|offturn compiler messages coloring on|off
--listFullPathslist full paths in messages
-w:on|off|list, --warnings:on|off|listturn all warnings on|off or list all available
--warning[X]:on|offturn specific warning X on|off
--hints:on|off|listturn all hints on|off or list all available
--hint[X]:on|offturn specific hint X on|off
--lib:PATHset the system library path
--import:PATHadd an automatically imported module
--include:PATHadd an automatically included module
--nimcache:PATHset the path used for generated files
--header:FILEthe compiler should produce a .h file (FILE is optional)
-c, --compileOnlycompile Nim files only; do not assemble or link
--noLinkingcompile Nim and generated files but do not link
--noMaindo not generate a main procedure
--genScriptgenerate a compile script (in the 'nimcache' subdirectory named 'compile_$$project$$scriptext'), implies --compileOnly
--genDepsgenerate a '.deps' file containing the dependencies
--os:SYMBOLset the target operating system (cross-compilation)
--cpu:SYMBOLset the target processor (cross-compilation)
--debuginfoenables debug information
-t, --passC:OPTIONpass an option to the C compiler
-l, --passL:OPTIONpass an option to the linker
--cincludes:DIRmodify the C compiler header search path
--clibdir:DIRmodify the linker library search path
--clib:LIBNAMElink an additional C library (you should omit platform-specific extensions)
--genMappinggenerate a mapping file containing (Nim, mangled) identifier pairs
--projectdocument the whole project (doc2)
--docSeeSrcUrl:urlactivate 'see source' for doc and doc2 commands (see doc.item.seesrc in config/nimdoc.cfg)
--lineDir:on|offgeneration of #line directive on|off
--embedsrcembeds the original source code as comments in the generated output
--threadanalysis:on|offturn thread analysis on|off
--tlsEmulation:on|offturn thread local storage emulation on|off
--taintMode:on|offturn taint mode on|off
--implicitStatic:on|offturn implicit compile time evaluation on|off
--patterns:on|offturn pattern matching on|off
--memTracker:on|offturn memory tracker on|off
--hotCodeReloading:on|offturn support for hot code reloading on|off
--excessiveStackTrace:on|offstack traces use full file paths
--oldNewlines:on|offturn on|off the old behaviour of "n"
--laxStrings:on|offwhen turned on, accessing the zero terminator in strings is allowed; only for backwards compatibility
--nilseqs:on|offallow 'nil' for strings/seqs for backwards compatibility
--skipCfgdo not read the general configuration file
--skipUserCfgdo not read the user's configuration file
--skipParentCfgdo not read the parent dirs' configuration files
--skipProjCfgdo not read the project's configuration file
--gc:refc|v2|markAndSweep|boehm|go|none|regionsselect the GC to use; default is 'refc'
--index:on|offturn index file generation on|off
--putenv:key=valueset an environment variable
--NimblePath:PATHadd a path for Nimble support
--noNimblePathdeactivate the Nimble path
--noCppExceptionsuse default exception handling with C++ backend
--cppCompileToNamespace:namespaceuse the provided namespace for the generated C++ code, if no namespace is provided "Nim" will be used
--excludePath:PATHexclude a path from the list of search paths
--dynlibOverride:SYMBOLmarks SYMBOL so that dynlib:SYMBOL has no effect and can be statically linked instead; symbol matching is fuzzy so that --dynlibOverride:lua matches dynlib: ""
--dynlibOverrideAllmakes the dynlib pragma have no effect
--listCmdlist the commands used to execute external programs
--parallelBuild:0|1|...perform a parallel build value = number of processors (0 for auto-detect)
--verbosity:0|1|2|3set Nim's verbosity level (1 is default)
--experimental:$1enable experimental language feature
-v, --versionshow detailed version information

List of warnings

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

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.
ShadowIdentA local variable shadows another local variable of an outer scope.
UserSome user defined warning.

List of hints

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

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 certain safety checks are omitted 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.

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%/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

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 and so the compiler should reject it. Currently however this check is not implemented and instead the first matching file is used.

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. Files generated in this directory follow a naming logic which you can read about in the Nim Backend Integration document.

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.

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 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 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 ... --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. 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.

releaseTurns off runtime checks and turns on the optimizer.
useWinAnsiModules like os and osproc use the Ansi versions of the Windows API. The default build uses the Unicode version.
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, ableit prefixing each allocation with its size to support clearing memory on reallocation. This only works with gc:none.
useRealtimeGCEnables support of Nim's GC for soft realtime systems. See the documentation of the gc for further information.
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 (requires a C compiler with _Static_assert support, like any C11 compiler)
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.

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 an 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.

Hot code reloading

Note: At the moment hot code reloading is supported only in JavaScript projects.

The hotCodeReloading option enables special compilation mode where changes in the code can be applied automatically to a running program. The code reloading happens at the granularity of an individual module. When a module is reloaded, Nim will preserve the state of all global variables which are initialized with a standard variable declaration in the code. All other top level code will be executed repeatedly on each reload. If you want to prevent this behavior, you can guard a block of code with the once construct:

var settings = initTable[string, string]()

  for k, v in loadSettings():
    settings[k] = v

If you want to reset the state of a global variable on each reload, just re-assign a value anywhere within the top-level code:

var lastReload: Time

lastReload = now()

Known limitations: In the JavaScript target, global variables using the codegenDecl pragma will be re-initialized on each reload. Please guard the initialization with a once block to work-around this.

Usage in JavaScript projects:

Once your code is compiled for hot reloading, you can use a framework such as LiveReload <> or BrowserSync <> to implement the actual reloading behavior in your project.


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 and doc2 commands 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

The standard library can be avoided to a point where C code generation for 16bit micro controllers is feasible. Use the standalone target (--os:standalone) for a bare bones standard library that lacks any OS features.

To make the compiler output code for a 16bit target use the --cpu:avr target.

For example, to generate code for an AVR processor use this command:

nim c --cpu:avr --os:standalone --genScript x.nim

For the standalone target one needs to provide a file panicoverride.nim. See tests/manyloc/standalone/panicoverride.nim for an example implementation. Additionally, users should specify the amount of heap space to use with the -d:StandaloneHeapSize=<size> command line switch. Note that the total heap size will be <size> * sizeof(float64).

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 from 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 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))