Nim Compiler User Guide¶

Author:	Andreas Rumpf
Version:	\|nimversion\|

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 (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:

Usage:

nim command [options] [projectfile] [arguments]

Command:

//compile, c compile project with default code generator (C) //doc generate the documentation for inputfile

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)

Note: use -d:release for a release build!

generate a console app|GUI app|DLL|static library

`-r, --run`	run the compiled program with given arguments
`--advanced`	show advanced command line switches
`-h, --help`	show 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, 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 //rst2tex convert a reStructuredText file to TeX //jsondoc extract the documentation to a json file //jsondoc2 extract the documentation to a json file (uses doc2) //ctags create a tags file //buildIndex build an index for the whole documentation //run run the project (with Tiny C backend; buggy!) //genDepend generate a DOT file containing the

module dependency graph

//dump dump all defined conditionals and search paths //check checks the project for syntax and semantic

Advanced options:

turn all warnings on|off or list all available

–warning[X]:on|off turn specific warning X on|off –hints:on|off|list turn all hints on|off or list all available –hint[X]:on|off turn specific hint X on|off –lib:PATH set the system library path –import:PATH add an automatically imported module –include:PATH add an automatically included module –nimcache:PATH set the path used for generated files –header:FILE the compiler should produce a .h file (FILE

is optional)

`-c, --compileOnly`
	compile only; do not assemble or link
`--noLinking`	compile but do not link
`--noMain`	do not generate a main procedure
`--genScript`	generate a compile script (in the ‘nimcache’ subdirectory named ‘compile_$project$scriptext’)
`--genDeps`	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 enables debug information -t, –passC:OPTION pass an option to the C compiler -l, –passL:OPTION pass an option to the linker –cincludes:DIR modify the C compiler header search path –clibdir:DIR modify the linker library search path –clib:LIBNAME link an additional C library

(you should omit platform-specific extensions)

`--genMapping`	generate a mapping file containing (Nim, mangled) identifier pairs
`--project`	document the whole project (doc2)

–docSeeSrcUrl:url activate ‘see source’ for doc and doc2 commands: (see doc.item.seesrc in config/nimdoc.cfg)

–lineDir:on|off generation of #line directive on|off –embedsrc embeds the original source code as comments

in the generated output

stack traces use full file paths

select the GC to use; default is ‘refc’

–index:on|off turn index file generation on|off –putenv:key=value set an environment variable –NimblePath:PATH add a path for Nimble support –noNimblePath deactivate the Nimble path –noCppExceptions use default exception handling with C++ backend –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`
	makes the dynlib pragma have no effect
`--listCmd`	list 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|3 set Nim’s verbosity level (1 is default) –experimental enable experimental language features -v, –version show detailed version information

List of warnings¶

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

Name	Description
CannotOpenFile	Some file not essential for the compiler’s working could not be opened.
OctalEscape	The code contains an unsupported octal sequence.
Deprecated	The code uses a deprecated symbol.
ConfigDeprecated	The project makes use of a deprecated config file.
SmallLshouldNotBeUsed	The letter ‘l’ should not be used as an identifier.
EachIdentIsTuple	The code contains a confusing `var` declaration.
ShadowIdent	A local variable shadows another local variable of an outer scope.
User	Some user defined warning.

Verbosity levels¶

Level	Description
0	Minimal output level for the compiler.
1	Displays compilation of all the compiled files, including those imported by other modules or through the `compile pragma<#compile-pragma>`_. This is the default level.
2	Displays 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.
3	In 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<manual.html#implementation-specific-pragmas-compile-time-define-pragmas>`_ to override symbols during build time.

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):

$nim/config/nim.cfg, /etc/nim.cfg (UNIX) or %NIMROD%/config/nim.cfg (Windows). This file can be skipped with the --skipCfg command line option.
/home/$user/.config/nim.cfg (UNIX) or %APPDATA%/nim.cfg (Windows). This file can be skipped with the --skipUserCfg command line option.
$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.
$projectDir/nim.cfg where $projectDir stands for the project file’s path. This file can be skipped with the --skipProjCfg command line option.
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`:idx:. To compile a `release build`:idx: 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:

$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 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 in your project directory. 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.

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.

Compilation cache¶

Warning: The compilation cache is still highly experimental!

The nimcache directory may also contain so called `rod`:idx: or `symbol files`:idx:. These files are pre-compiled modules that are used by the compiler to perform `incremental compilation`:idx:. This means that only modules that have changed since the last compilation (or the modules depending on them etc.) are re-compiled. However, per default no symbol files are generated; use the --symbolFiles:on command line switch to activate them.

Unfortunately due to technical reasons the --symbolFiles:on needs to aggregate some generated C code. This means that the resulting executable might contain some cruft even when dead code elimination is turned on. So the final release build should be done with --symbolFiles:off.

Due to the aggregation of C code it is also recommended that each project resides in its own directory so that the generated nimcache directory is not shared between different projects.

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 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"

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.

Define	Effect
`release`	Turns off runtime checks and turns on the optimizer.
`useWinAnsi`	Modules like `os` and `osproc` use the Ansi versions of the Windows API. The default build uses the Unicode version.
`useFork`	Makes `osproc` use `fork` instead of `posix_spawn`.
`useNimRtl`	Compile and link against `nimrtl.dll`.
`useMalloc`	Makes Nim use C’s `malloc`:idx: 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`.
`useRealtimeGC`	Enables support of Nim’s GC for soft realtime systems. See the documentation of the gc for further information.
`nodejs`	The JS target is actually `node.js`.
`ssl`	Enables OpenSSL support for the sockets module.
`memProfiler`	Enables memory profiling for the native GC.
`uClibc`	Use uClibc instead of libc. (Relevant for Unix-like OSes)
`checkAbi`	When 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)
`tempDir`	This 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)
`useShPath`	This 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`

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.

Debugger option¶

The debugger option enables or disables the Embedded Nim Debugger. See the documentation of endb for further information.

Breakpoint pragma¶

The breakpoint pragma was specially added for the sake of debugging with ENDB. See the documentation of endb for further information.

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`:idx: and `doc2`:idx: 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`:idx: 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`:idx: processor use this command:

nim c --cpu:avr --os:standalone --deadCodeElim:on --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.

Debugging with Nim¶

Nim comes with its own Embedded Nim Debugger. See the documentation of endb for further information.

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:

switch off the embedded debugger (it is slow!)
turn on the optimizer and turn off runtime checks
profile your code to find where the bottlenecks are
try to find a better algorithm
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:

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`:idx::

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

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: