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++), object files (produced by *emcc -c*), or LLVM
assembly files.
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"
[compile+link] 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"
[compile+link] Simple optimizations. During the compile step these
include LLVM "-O1" optimizations. During the link step this omits
various runtime assertions in JS that *-O0* would include.
"-O2"
[compile+link] 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 "EXPORTED_RUNTIME_METHODS", see
src/settings.js.
"-O3"
[compile+link] Like "-O2", but with additional optimizations that
may take longer to run and may increase code size.
Note:
This is a good setting for a release build.
"-Og"
[compile+link] Like "-O1", with an additional flag to extend the
liveness of variables for improved debugging. In future versions,
additional optimizations might also be disabled.
"-Os"
[compile+link] Like "-O3", but focuses more on code size (and may
make tradeoffs with speed). This can affect both Wasm and
JavaScript.
"-Oz"
[compile+link] 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.
"-sOPTION[=VALUE]"
[different OPTIONs affect at different stages, most at link time]
Emscripten build options. For the available options, see
src/settings.js.
Note:
If no value is specified it will default to "1".
Note:
It is possible, with boolean options, to use the "NO_" prefix to
reverse their meaning. For example, "-sEXIT_RUNTIME=0" is the
same as "-sNO_EXIT_RUNTIME=1" and vice versa. This is not
recommended in most cases.
Note:
Lists can be specified as comma separated strings:
-sEXPORTED_FUNCTIONS=foo,bar
Note:
We also support older list formats that involve more quoting.
Lists can be specified with or without quotes around each element
and with or without brackets around the list. For example, all
the following are equivalent:
-sEXPORTED_FUNCTIONS="foo","bar"
-sEXPORTED_FUNCTIONS=["foo","bar"]
-sEXPORTED_FUNCTIONS=[foo,bar]
Note:
For lists that include brackets or quote, you need quotation
marks (") around the list in most shells (to avoid errors being
raised). Two examples are shown below:
-sEXPORTED_FUNCTIONS="['liblib.so']"
-s"EXPORTED_FUNCTIONS=['liblib.so']"
You can also specify that the value of an option will be read from
a file. For example, the following will set "EXPORTED_FUNCTIONS"
based on the contents of the file at **path/to/file**.
-sEXPORTED_FUNCTIONS=@/path/to/file
Note:
* In this case the file should contain a list of symbols, one per
line. For legacy use cases JSON-formatted files are also
supported: e.g. "["_func1", "func2"]".
* The specified file path must be absolute, not relative.
* The file may contain comments where the first character of the
line is "'#'".
Note:
Options can be specified as a single argument with or without a
space between the "-s" and option name. e.g. "-sFOO" or "-s
FOO". It's highly recommended you use the notation without space.
"-g"
[compile+link] Preserve debug information.
* When compiling to object files, this is the same as in *Clang*
and *gcc*, it adds DWARF debug information to the object files.
* When linking, this is equivalent to -g3.
"-gseparate-dwarf[=FILENAME]"
[same as -g3 if passed at compile time, otherwise applies at link]
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. Instead, debug info will be present in a file on the side, in
"FILENAME" if provided, otherwise the same as the Wasm file but
with suffix ".debug.wasm". While the main file contains no debug
info, it does contain a URL to where the debug file is, so that
devtools can find it. You can use "-sSEPARATE_DWARF_URL=URL" to
customize that location (this is useful if you want to host it on a
different server, for example).
"-gsplit-dwarf"
Enable debug fission, which creates split DWARF object files
alongside the wasm object files. This option must be used together
with "-c".
"-gsource-map[=inline]"
[compile+link] [same as -g3 if passed at compile time, otherwise
applies at link] Generate a source map using LLVM debug information
(which must be present in object files, i.e., they should have been
compiled with "-g" or "-gsource-map").
When this option is provided, the **.wasm** file is updated to have
a "sourceMappingURL" section. The resulting URL will have format:
"<base-url>" + "<wasm-file-name>" + ".map". "<base-url>" defaults
to being empty (which means the source map is served from the same
directory as the Wasm file). It can be changed using --source-map-
base.
Path substitution can be applied to the referenced sources using
the "-sSOURCE_MAP_PREFIXES" (link). If "inline" is specified, the
sources content is embedded in the source map (in this case you
don't need path substitution, but it comes with the cost of having
a large source map file).
"-g<level>"
[compile+link] If used at compile time, adds progressively more
DWARF information to the object file, according to the underlying
behavior of clang. If used at link time, controls the level of
debuggability overall. Each level builds on the previous one:
* "-g0": Make no effort to keep code debuggable.
* "-g1": Preserve whitespace in JavaScript.
* "-g2": Also preserve function names in compiled code (via the
wasm name section).
* "-g3": Also keep LLVM debug info (DWARF) if there is any in
the object files (this is the same as -g).
"--profiling"
[link] Make the output suitable for profiling. This means including
function names in the wasm and JS output, and preserving whitespace
in the JS output. It does not affect optimizations (to ensure that
performance profiles reflect production builds). Currently this is
the same as "-g2".
"--profiling-funcs"
[link] Preserve wasm function names as 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"
[link] Enable the Emscripten Tracing API.
"--reproduce=<file.tar>"
[compile+link] Write tar file containing inputs and command to
reproduce invocation. When sharing this file be aware that it will
any object files, source files and libraries that that were passed
to the compiler.
"--emit-symbol-map"
[link] Save a map file between function indexes in the Wasm and
function names. By storing the names on a file on the side, you can
avoid shipping the names, and can still reconstruct meaningful
stack traces by translating the indexes back to the names. This is
a simpler format than source maps, but less detailed because it
only describes function names and not source locations.
Note:
When used with "-sWASM=2", two symbol files are created.
"[name].js.symbols" (with WASM symbols) and
"[name].wasm.js.symbols" (with ASM.js symbols)
"--emit-minification-map <file>"
[link] In cases where emscripten performs import/export
minification this option can be used to output a file that maps
minified names back to their original names. The format of this
file is single line per import/export of the form
"<minname>:<origname>".
"-flto"
[compile+link] Enables link-time optimizations (LTO).
"--closure 0|1|2"
[link] Runs the *Closure Compiler*. Possible values are:
* "0": No closure compiler (default).
* "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 "-sMODULARIZE" 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.
"--closure-args=<args>"
[link] Pass arguments to the *Closure compiler*. This is an
alternative to "EMCC_CLOSURE_ARGS".
For example, one might want to pass an externs file to avoid
minifying JS functions defined in "--pre-js" or "--post-js" files.
To pass to Closure the "externs.js" file containing those public
APIs that should not be minified, one would add the flag: "--
closure-args=--externs=path/to/externs.js"
"--pre-js <file>"
[link] 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 <file>"
[link] Like "--pre-js", but emits a file *after* the emitted code.
"--extern-pre-js <file>"
[link] 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 <file>"
[link] Like "--extern-pre-js", but appends to the end.
"--embed-file <file>"
[link] Specify a file (with path) to embed inside the generated
WebAssembly module. 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 generally more efficient than preloading as it
avoids copying the file data at runtime.
For more information about the "--embed-file" options, see
Packaging Files.
"--preload-file <name>"
[link] 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 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 <name>"
[link] Files and directories to be excluded from --embed-file and
--preload-file. Wildcards (*) are supported.
"--use-preload-plugins"
[link] 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 <path>"
[link] 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 html/shell.html and html/shell_minimal.html for examples.
* This argument is ignored if a target other than HTML is
specified using the "-o" option.
"--source-map-base <base-url>"
[link] The base URL for the location where WebAssembly source maps
will be published. Must be used with -gsource-map.
"--minify 0"
[same as -g1 if passed at compile time, otherwise applies at link]
Identical to "-g1".
"--js-transform <cmd>"
[link] Specifies a "<cmd>" 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.
"<cmd>" 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.
"<cmd>" is interpreted as a space-separated list of arguments, for
example, "<cmd>" of **python processor.py** will cause a Python
script to be run.
"--bind"
[link] Links against embind library. Deprecated: Use "-lembind"
instead.
"--embind-emit-tsd <path>"
[link] Generates TypeScript definition file. Deprecated: Use "--
emit-tsd" instead.
"--emit-tsd <path>"
[link] Generate a TypeScript definition file for the emscripten
module. The definition file will include exported Wasm functions,
runtime exports, and exported embind bindings (if used). In order
to generate bindings from embind, the program will be instrumented
and run in node.
"--ignore-dynamic-linking"
[link] 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 <lib>"
[link] A JavaScript library to use in addition to those in
Emscripten's core libraries (src/library_*).
"-v"
[general] Turns on verbose output.
This will print the internal sub-commands run by emscripten as well
as "-v" to *Clang*.
Tip:
"emcc -v" is a useful tool for diagnosing errors. It works with
or without other arguments.
"--check"
[general] Runs Emscripten's internal sanity checks and reports any
issues with the current configuration.
"--cache <directory>"
[general] 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"
[general] 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.
"--use-port=<port>"
[compile+link] Use the specified port. If you need to use more than
one port you can use this option multiple times (ex: "--use-
port=sdl2 --use-port=bzip2"). A port can have options separated by
":" (ex: "--use-port=sdl2_image:formats=png,jpg"). To use an
external port, you provide the path to the port directly (ex: "--
use-port=/path/to/my_port.py"). To get more information about a
port, use the "help" option (ex: "--use-port=sdl2_image:help"). To
get the list of available ports, use "--show-ports".
"--clear-ports"
[general] 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"
[general] Shows the list of available projects in the Emscripten
Ports repos. After this operation is complete, this process will
exit.
"-Wwarn-absolute-paths"
[compile+link] 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.
"--emrun"
[link] 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"
[link] Embeds a simple CPU profiler onto the generated page. Use
this to perform cursory interactive performance profiling.
"--memoryprofiler"
[link] Embeds a memory allocation tracker onto the generated page.
Use this to profile the application usage of the Emscripten HEAP.
"--threadprofiler"
[link] Embeds a thread activity profiler onto the generated page.
Use this to profile the application usage of pthreads when
targeting multithreaded builds (-pthread).
"--em-config <path>"
[general] 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.
"--valid-abspath <path>"
[compile+link] Note an allowed absolute path, which we should not
warn about (absolute include paths normally are warned about, since
they may refer to the local system headers etc. which we need to
avoid when cross-compiling).
"-o <target>"
[link] When linking an executable, 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> **.wasm** : WebAssembly without JavaScript support code
("standalone Wasm"; this enables "STANDALONE_WASM").
These rules only apply when linking. When compiling to object code
(See *-c* below) the name of the output file is irrelevant.
Note: Linking to a file with no extension (or a file ending in
".out", like "a.out") will cause the generated JavaScript file to
be exectuable, and include a "#!" line to make it runnable
directly.
"-c"
[compile] Tells *emcc* to emit an object file which can then be
linked with other object files to produce an executable.
"--output-eol windows|linux"
[link] 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 "\r\n" line endings in them. With "
--output-eol linux", the final generated files will be written with
Unix "\n" line endings.
"--cflags"
[other] Prints out the flags "emcc" would pass to "clang" to
compile source code to object 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" [other]
* "EMCC_AUTODEBUG" [compile+link]
* "EMCC_CFLAGS" [compile+link]
* "EMCC_CORES" [general]
* "EMCC_DEBUG" [general]
* "EMCC_DEBUG_SAVE" [general]
* "EMCC_FORCE_STDLIBS" [link]
* "EMCC_ONLY_FORCED_STDLIBS" [link]
* "EMCC_LOCAL_PORTS" [compile+link]
* "EMCC_STDERR_FILE" [general]
* "EMCC_CLOSURE_ARGS" [link] arguments to be passed to *Closure
Compiler*
* "EMCC_STRICT" [general]
* "EMCC_SKIP_SANITY_CHECK" [general]
* "EM_IGNORE_SANITY" [general]
* "EM_CONFIG" [general]
* "EM_LLVM_ROOT" [compile+link]
* "_EMCC_CCACHE" [general] Internal setting that is set to 1 by
emsdk when integrating with ccache compiler frontend
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.
