
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.

"-O1"
   Simple optimizations. These include using **asm.js**, LLVM "-O1"
   optimizations, relooping, removing runtime assertions and C++
   exception catching, and enabling "-s ALIASING_FUNCTION_POINTERS=1".
   This is the recommended setting when you want a reasonably
   optimized build that is generated as quickly as possible (it builds
   much faster than "-O2").

   Note: * For details on the effects of different optimization
     levels,

       see "apply_opt_level()" in tools/shared.py and also
       src/settings.js.

     * To re-enable C++ exception catching, use -s
       DISABLE_EXCEPTION_CATCHING=0.

"-O2"
   Like "-O1", but with various JavaScript-level optimizations and
   LLVM "-O3" optimizations.

   Note: This is the recommended setting for a release build,
     offering slower compilation time in return for the smallest and
     fastest output.

"-Os"
   Like "-O2", but with extra optimizations that reduce code size at
   the expense of performance. This applies only for bitcode
   optimization ("-O2" is used for JavaScript optimizations).

"-Oz"
   Like "-Os", but reduces code size even further. This applies only
   for bitcode optimization ("-O2" is used for JavaScript
   optimizations).

"-O3"
   Like "-O2", but with additional JavaScript optimizations that can
   take a significant amount of compilation time and/or are relatively
   new.

   Note: This differs from "-O2" only during the bitcode to
     JavaScript (final link and JavaScript generation) stage. It is
     JavaScript- specific, so you can run "-Os" on your source files
     for example, and "-O3" during JavaScript generation if you want.

   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: 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 bitcode, this is the same as in *Clang* and
     *gcc* (it adds debug information to the object files).

   * When compiling from source to JavaScript or bitcode to
     JavaScript, it is equivalent to -g3 (discards LLVM debug info
     including C/C++ line numbers, but otherwise keeps as much debug
     information as possible). Use -g4 to get line number debugging
     information in JavaScript.

"-g<level>"
   Controls how much debug information is kept when compiling from
   bitcode to JavaScript. Each level builds on the previous level:

      * "-g0": Make no effort to keep code debuggable. Will discard
        LLVM debug information (this is done by default in -01 and
        higher).

      * "-g1": Preserve whitespace (do not minify).

              function a(a, b) {
                      a = a | 0;
                      b = b | 0;
                      f(a + b | 0);
              }

      * "-g2": Preserve function names.

              function _addAndPrint(a, b) {
                      a = a | 0;
                      b = b | 0;
                      _printAnInteger(a + b | 0); // _printAnInteger is human readable.
              }

      * "-g3": Preserve variable names (this is the same as -g).

              function _addAndPrint($left, $right) {
                      $left = $left | 0;
                      $right = $right | 0;
                      _printAnInteger($left + $right | 0);
              }

           Note: Variable names in the output will not necessarily
             match the original variable names in source. They are,
             however, usually similar enough to infer the purpose of
             the variables.

      * "-g4": Preserve LLVM debug information. This is the highest
        level of debuggability. If "-g" was used when compiling the
        C/C++ sources, this shows line number debug comments, and
        generates source maps.

           Note: * This debugging level may make compilation at

               optimization level -O1 and above significantly slower,
               because JavaScript optimization will be limited to one
               core (the default in "-O0").

             * Source maps allow you to view and debug the *C/C++
               source code* in your browser's debugger! This works in
               Firefox, Chrome and Safari.

"--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 <level>"
   Enables JavaScript optimizations. Possible "level" values are:

      * "0": Prevent JavaScript optimizer from running.

      * "1": Use JavaScript optimizer (default).

"--llvm-opts <level>"
   Enables LLVM optimizations. 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']"

"--llvm-lto <level>"
   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 <on>"
   Runs the *Closure Compiler*. Possible "on" values are:

      * "0": No closure compiler (default in "-O2" and below).

      * "1": Run closure compiler. This greatly reduces code size
        and may in some cases increase runtime speed (although the
        opposite can also occur). Note that it takes time to run, and
        may require some changes to the code. In **asm.js** mode,
        closure will only be used on the 'shell' code around the
        compiled code (the compiled code will be processed by the
        custom **asm.js** minifier).

      * "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 <file>"
   Specify a file whose contents are added before the generated code.
   This is done *before* optimization, so it will be minified properly
   if the *Closure Compiler* is run.

"--post-js <file>"
   Specify a file whose contents are added after the generated code.
   This is done *before* optimization, so it will be minified properly
   if the *Closure Compiler* is run.

"--embed-file <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 <name>"
   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 <name>"
   Files and directories to be excluded from --embed-file and
   --preload-file. Wildcards (*) are supported.

"--shell-file <path>"
   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.

"--minify 0"
   Identical to "-g1".

"--js-transform <cmd>"
   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"
   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 <lib>"
   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 being located in the path name
   stored in the "EM_CACHE" environment variable or
   "~/.emscripten_cache".

"--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.

"--clear-ports"
   Manually clears the local copies and builds of projects from the
   Emscripten Ports repos (sdl2, etc.)

   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 <on>"
   Specifies whether to emit a separate memory initialization file.
   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*.

"--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.

"--em-config"
   Specifies the location of the **.emscripten** configuration file
   for the current compiler run. If not specified, the environment
   variable "EM_CONFIG" is first read for this location. If neither
   are specified, the default location **~/.emscripten** is used.

"--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 <target>"
   The "target" file name extension defines the output type to be
   generated:

      * <name> **.js** : JavaScript.

      * <name> **.html** : HTML + separate JavaScript file
        (**<name>.js**). Having the separate JavaScript file improves
        page load time.

      * <name> **.bc** : LLVM bitcode (default).

      * <name> **.o** : LLVM bitcode (same as .bc).

   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.

"-fsanitize=<check>"
   Instructs the compiler to turn on runtime checks for various forms
   of undefined or suspicious behavior. For more information, refer
   to the appropriate compiler documentation. Only the following have
   been tested:

     * "cfi": Enable [control flow integrity](http://clang.llvm.org/docs/ControlFlowIntegrity.html)
       checks. Note that this is only supported by Clang/LLVM, and
       indirect function call checking requires Clang/LLVM 3.9+.

"--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 \r\n line endings in them. With
   "--output-eol linux", the final generated files will be written
   with Unix \n line endings.

Environment variables
=====================

*emcc* is affected by several environment variables, as listed below:

   * "EMMAKEN_JUST_CONFIGURE"

   * "EMMAKEN_JUST_CONFIGURE_RECURSE"

   * "EMCONFIGURE_JS"

   * "CONFIGURE_CC"

   * "EMMAKEN_CXX"

   * "EMMAKEN_CXX"

   * "EMMAKEN_COMPILER"

   * "EMMAKEN_CFLAGS"

   * "EMCC_DEBUG"

Search for 'os.environ' in emcc 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.
