Emscripten Compiler Frontend (emcc)
The Emscripten Compiler Frontend (“emcc”) is used to call the
Emscripten compiler from the command line. It is effectively a drop-in
replacement for a standard compiler like gcc or clang.
Command line syntax
emcc [options] file…
(Note that you will need “./emcc” if you want to run emcc from your
current directory.)
The input file(s) can be either source code files that Clang can
handle (C or C++), LLVM bitcode in binary form, or LLVM assembly files
in human-readable form.
Arguments
Most clang options will work, as will gcc options, for example:
Display this information
emcc --help
Display compiler version information
emcc --version
To see the full list of Clang options supported on the version of
Clang used by Emscripten, run “clang --help”.
Options that are modified or new in emcc are listed below:
“-O0”
No optimizations (default). This is the recommended setting for
starting to port a project, as it includes various assertions.
This and other optimization settings are meaningful both during
compile and during link. During compile it affects LLVM
optimizations, and during link it affects final optimization of the
code in Binaryen as well as optimization of the JS. (For fast
incremental builds “-O0” is best, while for release you should link
with something higher.)
“-O1”
Simple optimizations. During the compile step these include LLVM
“-O1” optimizations. During the link step this removes various
runtime assertions in JS and also runs the Binaryen optimizer (that
makes link slower, so even if you compiled with a higher
optimization level, you may want to link with “-O0” for fast
incremental builds).
“-O2”
Like “-O1”, but enables more optimizations. During link this will
also enable various JavaScript optimizations.
Note: These JavaScript optimizations can reduce code size by
removing things that the compiler does not see being used, in
particular, parts of the runtime may be stripped if they are not
exported on the “Module” object. The compiler is aware of code in
–pre-js and --post-js, so you can safely use the runtime from
there. Alternatively, you can use
“EXTRA_EXPORTED_RUNTIME_METHODS”, see src/settings.js.
“-O3”
Like “-O2”, but with additional optimizations that may take longer
to run.
Note: This is a good setting for a release build.
“-Os”
Like “-O3”, but focuses more on code size (and may make tradeoffs
with speed). This can affect both wasm and JavaScript.
“-Oz”
Like “-Os”, but reduces code size even further, and may take longer
to run. This can affect both wasm and JavaScript.
Note: For more tips on optimizing your code, see Optimizing Code.
“-s OPTION[=VALUE]”
JavaScript code generation option passed into the Emscripten
compiler. For the available options, see src/settings.js.
Note: You can prefix boolean options with “NO_” to reverse them.
For example, “-s EXIT_RUNTIME=1” is the same as “-s
NO_EXIT_RUNTIME=0”.
Note: If no value is specifed it will default to “1”.
Note: For options that are lists, you need quotation marks (")
around the list in most shells (to avoid errors being raised).
Two examples are shown below:
-s RUNTIME_LINKED_LIBS="['liblib.so']"
-s "RUNTIME_LINKED_LIBS=['liblib.so']"
You can also specify that the value of an option will be read from
a specified JSON-formatted file. For example, the following option
sets the “DEAD_FUNCTIONS” option with the contents of the file at
path/to/file.
-s DEAD_FUNCTIONS=@/path/to/file
Note:
* In this case the file might contain a JSON-formatted list of
functions: "["_func1", "func2"]".
* The specified file path must be absolute, not relative.
“-g”
Preserve debug information.
-
When compiling to object files, this is the same as in Clang
and gcc, it adds debug information to the object files. -
When linking, this is equivalent to -g3.
“-gseparate-dwarf[=FILENAME]”
Preserve debug information, but in a separate file on the side.
This is the same as “-g”, but the main file will contain no debug
info, while debug info will be present in a file on the side
(“FILENAME” if provided, otherwise the same as the wasm file but
with suffix “.debug.wasm”).
“-g”
Controls the level of debuggability. Each level builds on the
previous one:
* "-g0": Make no effort to keep code debuggable.
* "-g1": When linking, preserve whitespace in JavaScript.
* "-g2": When linking, preserve function names in compiled
code.
* "-g3": When compiling to object files, keep debug info,
including JS whitespace, function names, and LLVM debug info
if any (this is the same as -g).
* "-g4": When linking, generate a source map using LLVM debug
information (which must be present in object files, i.e., they
should have been compiled with "-g").
Note:
* Source maps allow you to view and debug the *C/C++
source code* in your browser's debugger!
* This debugging level may make compilation significantly
slower (this is why we only do it on "-g4").
“–profiling”
Use reasonable defaults when emitting JavaScript to make the build
readable but still useful for profiling. This sets “-g2” (preserve
whitespace and function names) and may also enable optimizations
that affect performance and otherwise might not be performed in
“-g2”.
“–profiling-funcs”
Preserve function names in profiling, but otherwise minify
whitespace and names as we normally do in optimized builds. This is
useful if you want to look at profiler results based on function
names, but do not intend to read the emitted code.
“–tracing”
Enable the Emscripten Tracing API.
“–emit-symbol-map”
Save a map file between the minified global names and the original
function names. This allows you, for example, to reconstruct
meaningful stack traces.
Note: This is only relevant when minifying global names, which
happens in “-O2” and above, and when no “-g” option was specified
to prevent minification.
"–js-opts "
Enables JavaScript optimizations, relevant when we generate
JavaScript. Possible “level” values are:
* "0": Prevent JavaScript optimizer from running.
* "1": Use JavaScript optimizer (default).
You normally don’t need to specify this option, as “-O” with an
optimization level will set a good value.
Note: Some options might override this flag (e.g.
“DEAD_FUNCTIONS”, “SAFE_HEAP” and “SPLIT_MEMORY” override the
value with “js- opts=1”), because they depend on the js-
optimizer.
"–llvm-opts "
Enables LLVM optimizations, relevant when we call the LLVM
optimizer (which is done when building source files to object files
/ bitcode). Possible “level” values are:
* "0": No LLVM optimizations (default in -O0).
* "1": LLVM "-O1" optimizations (default in -O1).
* "2": LLVM "-O2" optimizations.
* "3": LLVM "-O3" optimizations (default in -O2+).
You can also specify arbitrary LLVM options, e.g.:
--llvm-opts "['-O3', '-somethingelse']"
You normally don’t need to specify this option, as “-O” with an
optimization level will set a good value.
"–llvm-lto "
Enables LLVM link-time optimizations (LTO). Possible “level” values
are:
* "0": No LLVM LTO (default).
* "1": LLVM LTO is performed.
* "2": Combine all the bitcode and run LLVM opt on it using
the specified "--llvm-opts". This optimizes across modules,
but is not the same as normal LTO.
* "3": Does level "2" and then level "1".
Note:
* If LLVM optimizations are not run (see "--llvm-opts"), this
setting has no effect.
* LLVM LTO is not perfectly stable yet, and can cause code to
behave incorrectly.
"–closure "
Runs the Closure Compiler. Possible “on” values are:
* "0": No closure compiler (default in "-O2" and below).
* "1": Run closure compiler. This greatly reduces the size of
the support JavaScript code (everything but the WebAssembly or
asm.js). Note that this increases compile time significantly.
* "2": Run closure compiler on *all* the emitted code, even on
**asm.js** output in **asm.js** mode. This can further reduce
code size, but does prevent a significant amount of **asm.js**
optimizations, so it is not recommended unless you want to
reduce code size at all costs.
Note:
* Consider using "-s MODULARIZE=1" when using closure, as it
minifies globals to names that might conflict with others in
the global scope. "MODULARIZE" puts all the output into a
function (see "src/settings.js").
* Closure will minify the name of *Module* itself, by default!
Using "MODULARIZE" will solve that as well. Another solution is
to make sure a global variable called *Module* already exists
before the closure-compiled code runs, because then it will
reuse that variable.
* If closure compiler hits an out-of-memory, try adjusting
"JAVA_HEAP_SIZE" in the environment (for example, to 4096m for
4GB).
* Closure is only run if JavaScript opts are being done ("-O2"
or above, or "--js-opts 1").
"–pre-js "
Specify a file whose contents are added before the emitted code and
optimized together with it. Note that this might not literally be
the very first thing in the JS output, for example if “MODULARIZE”
is used (see “src/settings.js”). If you want that, you can just
prepend to the output from emscripten; the benefit of “–pre-js” is
that it optimizes the code with the rest of the emscripten output,
which allows better dead code elimination and minification, and it
should only be used for that purpose. In particular, “–pre-js”
code should not alter the main output from emscripten in ways that
could confuse the optimizer, such as using “–pre-js” + “–post-js”
to put all the output in an inner function scope (see “MODULARIZE”
for that).
–pre-js (but not –post-js) is also useful for specifying
things on the “Module” object, as it appears before the JS looks at
“Module” (for example, you can define “Module[‘print’]” there).
"–post-js "
Like “–pre-js”, but emits a file after the emitted code.
"–extern-pre-js "
Specify a file whose contents are prepended to the JavaScript
output. This file is prepended to the final JavaScript output,
after all other work has been done, including optimization,
optional “MODULARIZE”-ation, instrumentation like “SAFE_HEAP”, etc.
This is the same as prepending this file after “emcc” finishes
running, and is just a convenient way to do that. (For comparison,
“–pre-js” and “–post-js” optimize the code together with
everything else, keep it in the same scope if running MODULARIZE,
etc.).
"–extern-post-js "
Like “–extern-pre-js”, but appends to the end.
"–embed-file "
Specify a file (with path) to embed inside the generated
JavaScript. The path is relative to the current directory at
compile time. If a directory is passed here, its entire contents
will be embedded.
For example, if the command includes “–embed-file dir/file.dat”,
then “dir/file.dat” must exist relative to the directory where you
run emcc.
Note: Embedding files is much less efficient than preloading
them. You should only use it for small files, in small numbers.
Instead use “–preload-file”, which emits efficient binary data.
For more information about the “–embed-file” options, see
Packaging Files.
"–preload-file "
Specify a file to preload before running the compiled code
asynchronously. The path is relative to the current directory at
compile time. If a directory is passed here, its entire contents
will be embedded.
Preloaded files are stored in filename.data, where
filename.html is the main file you are compiling to. To run
your code, you will need both the .html and the .data.
Note: This option is similar to --embed-file, except that it is
only relevant when generating HTML (it uses asynchronous binary
XHRs), or JavaScript that will be used in a web page.
emcc runs tools/file_packager.py to do the actual packaging of
embedded and preloaded files. You can run the file packager
yourself if you want (see Packaging using the file packager tool).
You should then put the output of the file packager in an emcc “–
pre-js”, so that it executes before your main compiled code.
For more information about the “–preload-file” options, see
Packaging Files.
"–exclude-file "
Files and directories to be excluded from --embed-file and
–preload-file. Wildcards (*) are supported.
“–use-preload-plugins”
Tells the file packager to run preload plugins on the files as they
are loaded. This performs tasks like decoding images and audio
using the browser’s codecs.
"–shell-file
The path name to a skeleton HTML file used when generating HTML
output. The shell file used needs to have this token inside it:
“{{{ SCRIPT }}}”.
Note:
* See src/shell.html and src/shell_minimal.html for examples.
* This argument is ignored if a target other than HTML is
specified using the "-o" option.
"–source-map-base "
The URL for the location where WebAssembly source maps will be
published. When this option is provided, the .wasm file is
updated to have a “sourceMappingURL” section. The resulting URL
will have format: “” + “” + “.map”.
“–minify 0”
Identical to “-g1”.
"–js-transform "
Specifies a “” to be called on the generated code before it is
optimized. This lets you modify the JavaScript, for example adding
or removing some code, in a way that those modifications will be
optimized together with the generated code.
“” will be called with the file name of the generated code as
a parameter. To modify the code, you can read the original data and
then append to it or overwrite it with the modified data.
“” is interpreted as a space-separated list of arguments, for
example, “” of python processor.py will cause a Python
script to be run.
“–bind”
Compiles the source code using the Embind bindings to connect C/C++
and JavaScript.
“–ignore-dynamic-linking”
Tells the compiler to ignore dynamic linking (the user will need to
manually link to the shared libraries later on).
Normally emcc will simply link in code from the dynamic library
as though it were statically linked, which will fail if the same
dynamic library is linked more than once. With this option, dynamic
linking is ignored, which allows the build system to proceed
without errors.
"–js-library "
A JavaScript library to use in addition to those in Emscripten’s
core libraries (src/library_*).
“-v”
Turns on verbose output.
This will pass “-v” to Clang, and also enable “EMCC_DEBUG” to
generate intermediate files for the compiler’s various stages. It
will also run Emscripten’s internal sanity checks on the toolchain,
etc.
Tip: “emcc -v” is a useful tool for diagnosing errors. It works
with or without other arguments.
“–cache”
Sets the directory to use as the Emscripten cache. The Emscripten
cache is used to store pre-built versions of “libc”, “libcxx” and
other libraries.
If using this in combination with “–clear-cache”, be sure to
specify this argument first.
The Emscripten cache defaults to “emscripten/cache” but can be
overridden using the “EM_CACHE” environment variable or “CACHE”
config setting.
“–clear-cache”
Manually clears the cache of compiled Emscripten system libraries
(libc++, libc++abi, libc).
This is normally handled automatically, but if you update LLVM in-
place (instead of having a different directory for a new version),
the caching mechanism can get confused. Clearing the cache can fix
weird problems related to cache incompatibilities, like Clang
failing to link with library files. This also clears other cached
data. After the cache is cleared, this process will exit.
By default this will also clear any download ports since the ports
directory is usually within the cache directory.
“–clear-ports”
Manually clears the local copies of ports from the Emscripten Ports
repos (sdl2, etc.). This also clears the cache, to remove their
builds.
You should only need to do this if a problem happens and you want
all ports that you use to be downloaded and built from scratch.
After this operation is complete, this process will exit.
“–show-ports”
Shows the list of available projects in the Emscripten Ports repos.
After this operation is complete, this process will exit.
“–save-bc PATH”
When compiling to JavaScript or HTML, this option will save a copy
of the bitcode to the specified path. The bitcode will include all
files being linked after link-time optimizations have been
performed (if any), including standard libraries.
"–memory-init-file "
Specifies whether to emit a separate memory initialization file.
Note: Note that this is only relevant when *not* emitting
wasm, as wasm embeds the memory init data in the wasm binary.
Possible “on” values are:
* "0": Do not emit a separate memory initialization file.
Instead keep the static initialization inside the generated
JavaScript as text. This is the default setting if compiling
with -O0 or -O1 link-time optimization flags.
* "1": Emit a separate memory initialization file in binary
format. This is more efficient than storing it as text inside
JavaScript, but does mean you have another file to publish.
The binary file will also be loaded asynchronously, which
means "main()" will not be called until the file is downloaded
and applied; you cannot call any C functions until it arrives.
This is the default setting when compiling with -O2 or higher.
Note: The safest way to ensure that it is safe to call C
functions (the initialisation file has loaded) is to call a
notifier function from "main()".
Note: If you assign a network request to
"Module.memoryInitializerRequest" (before the script runs),
then it will use that request instead of automatically
starting a download for you. This is beneficial in that you
can, in your HTML, fire off a request for the memory init
file before the script actually arrives. For this to work,
the network request should be an XMLHttpRequest with
responseType set to "'arraybuffer'". (You can also put any
other object here, all it must provide is a ".response"
property containing an ArrayBuffer.)
“-Wwarn-absolute-paths”
Enables warnings about the use of absolute paths in “-I” and “-L”
command line directives. This is used to warn against unintentional
use of absolute paths, which is sometimes dangerous when referring
to nonportable local system headers.
“–proxy-to-worker”
Runs the main application code in a worker, proxying events to it
and output from it. If emitting HTML, this emits a .html file,
and a separate .js file containing the JavaScript to be run in
a worker. If emitting JavaScript, the target file name contains the
part to be run on the main thread, while a second .js file with
suffix “.worker.js” will contain the worker portion.
“–emrun”
Enables the generated output to be aware of the emrun command line
tool. This allows “stdout”, “stderr” and “exit(returncode)” capture
when running the generated application through emrun. (This
enables EXIT_RUNTIME=1, allowing normal runtime exiting with
return code passing.)
“–cpuprofiler”
Embeds a simple CPU profiler onto the generated page. Use this to
perform cursory interactive performance profiling.
“–memoryprofiler”
Embeds a memory allocation tracker onto the generated page. Use
this to profile the application usage of the Emscripten HEAP.
“–threadprofiler”
Embeds a thread activity profiler onto the generated page. Use this
to profile the application usage of pthreads when targeting
multithreaded builds (-s USE_PTHREADS=1/2).
“–em-config”
Specifies the location of the .emscripten configuration file.
If not specified emscripten will search for “.emscripten” first in
the emscripten directory itself, and then in the user’s home
directory ("~/.emscripten"). This can be overridden using the
“EM_CONFIG” environment variable.
“–default-obj-ext .ext”
Specifies the file suffix to generate if the location of a
directory name is passed to the “-o” directive.
For example, consider the following command, which will by default
generate an output name dir/a.o. With “–default-obj-ext .ext”
the generated file has the custom suffix dir/a.ext.
emcc -c a.c -o dir/
“–valid-abspath path”
Whitelist an absolute path to prevent warnings about absolute
include paths.
"-o "
The “target” file name extension defines the output type to be
generated:
* <name> **.js** : JavaScript (+ separate **<name>.wasm** file
if emitting WebAssembly). (default)
* <name> **.mjs** : ES6 JavaScript module (+ separate
**<name>.wasm** file if emitting WebAssembly).
* <name> **.html** : HTML + separate JavaScript file
(**<name>.js**; + separate **<name>.wasm** file if emitting
WebAssembly).
* <name> **.bc** : LLVM bitcode.
* <name> **.o** : WebAssembly object file (unless fastcomp or
-flto is used in which case it will be in LLVM bitcode
format).
* <name> **.wasm** : WebAssembly without JavaScript support
code ("standalone wasm"; this enables "STANDALONE_WASM").
Note: If “–memory-init-file” is used, a .mem file will be
created in addition to the generated .js and/or .html
file.
“-c”
Tells emcc to generate LLVM bitcode (which can then be linked
with other bitcode files), instead of compiling all the way to
JavaScript.
“–separate-asm”
Emits asm.js in one file, and the rest of the code in another, and
emits HTML that loads the asm.js first, in order to reduce memory
load during startup. See Avoid memory spikes by separating out
asm.js.
“–output_eol windows|linux”
Specifies the line ending to generate for the text files that are
outputted. If “–output_eol windows” is passed, the final output
files will have Windows rn line endings in them. With “–output_eol
linux”, the final generated files will be written with Unix n line
endings.
“–cflags”
Prints out the flags “emcc” would pass to “clang” to compile source
code to object/bitcode form. You can use this to invoke clang
yourself, and then run “emcc” on those outputs just for the final
linking+conversion to JS.
Environment variables
emcc is affected by several environment variables, as listed below:
-
“EMMAKEN_JUST_CONFIGURE”
-
“EMMAKEN_COMPILER”
-
“EMMAKEN_CFLAGS”
-
“EMCC_DEBUG”
-
“EMCC_CLOSURE_ARGS” : arguments to be passed to Closure
Compiler
Search for ‘os.environ’ in emcc.py to see how these are used. The most
interesting is possibly “EMCC_DEBUG”, which forces the compiler to
dump its build and temporary files to a temporary directory where they
can be reviewed.
emcc: supported targets: llvm bitcode, javascript, NOT elf
(autoconf likes to see elf above to enable shared object support)