（二）emcc 的 help 內容：
C:\WINDOWS\system32>emcc --help
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 a reasonable setting for a release build.
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 JavaScript optimizations that can
take a significant amount of compilation time.
Note: This is a good setting for a release build.
"-Os"
Like "-O3", but with extra optimizations that reduce code size at
the expense of performance. This can affect both bitcode generation
and JavaScript.
"-Oz"
Like "-Os", but reduces code size even further. This can affect
both bitcode generation and JavaScript.
For JavaScript, this turns on some code size reduction
optimizations that can take a significant amount of compilation
time.
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 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, 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. "EMTERPRETIFY",
"DEAD_FUNCTIONS", "SAFE_HEAP" and
"SPLIT_MEMORY" override the value with "js-opts=1"), because they
depend on the js-optimizer.
"--llvm-opts <level>"
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 <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 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").
"--closure-args <args>"
Specifies extra arguments to pass to Closure compiler. Use quotes
to escape multiple arguments to be passed, e.g.
--closure-args "--externs myExterns.js"
Multiple passed --closure-args directives will be concatenated
left to right.
"--pre-js <file>"
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>"
Like *--pre-js`*, but emits a file *after* the emitted code.
"--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.
"--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 <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.
"--source-map-base <base-url>"
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: "<base-url>" + "<wasm-file-name>" + ".map".
"--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 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 <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*. (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
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 (+ 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** : LLVM bitcode (same as .bc), unless in
*WASM_OBJECT_FILES* mode, in which case it will contain a
WebAssembly object.
* <name> **.wasm** : WebAssembly without JavaScript support
code ("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_JUST_CONFIGURE_RECURSE"
* "EMCONFIGURE_JS"
* "CONFIGURE_CC"
* "EMMAKEN_CXX"
* "EMMAKEN_CXX"
* "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)