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Simplify logReadFiles. NFC #26186

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11 changes: 8 additions & 3 deletions site/source/docs/porting/files/packaging_files.rst
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Expand Up @@ -113,9 +113,14 @@ Monitoring file usage

.. important:: Only package the files your app actually needs, in order to reduce download size and improve startup speed.

There is an option to log which files are actually used at runtime. To use it, define the :js:attr:`Module.logReadFiles` object. Each file that is read will be logged to stderr.

An alternative approach is to look at :js:func:`FS.readFiles` in your compiled JavaScript. This is an object with keys for all the files that were read from. You may find it easier to use than logging as it records files rather than potentially multiple file accesses.
There is an option to log which files are actually used at runtime. To use it,
define the :js:attr:`Module.logReadFiles` object. Each file that is read will be
logged to stderr. This options requires that :ref:`FS_DEBUG` is set.

An alternative approach is to look at :js:func:`FS.readFiles` in your compiled
JavaScript. This is an object with keys for all the files that were read from.
You may find it easier to use than logging as it records files rather than
potentially multiple file accesses.

.. note:: You can also modify the :js:func:`FS.readFiles` object or remove it entirely. This can be useful, say, in order to see which files are read between two points in time in your app.

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14 changes: 4 additions & 10 deletions src/lib/libfs.js
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Expand Up @@ -59,12 +59,10 @@ FS.staticInit();`;
ignorePermissions: true,
#if FS_DEBUG
trackingDelegate: {},
readFiles: {},
#endif
filesystems: null,
syncFSRequests: 0, // we warn if there are multiple in flight at once
#if expectToReceiveOnModule('logReadFiles')
readFiles: {},
#endif
#if ASSERTIONS
ErrnoError: class extends Error {
#else
Expand Down Expand Up @@ -1160,17 +1158,13 @@ FS.staticInit();`;
if (created) {
FS.chmod(node, mode & 0o777);
}
#if expectToReceiveOnModule('logReadFiles')
#if FS_DEBUG
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@kripken kripken Jan 28, 2026

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Is this PR changing the requirement of FS_DEBUG, or just documenting it? From the description I thought the latter, but the code seems to actually add it..?

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@sbc100 sbc100 Jan 28, 2026

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The log message was already wrapped in FS_DEBUG yes

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@sbc100 sbc100 Jan 28, 2026

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I think maybe there was some confusion about how this feature worked. See #7418, where the log message was changed.

The log message is the feature really. Since it was already wrapped in FS_DEBUG I think its best to document that. I seems reasonable to run in FS_DEBUG mode if want to get a list of opened files.

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@kripken kripken Jan 28, 2026

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Aside from the log there is old line 1166 though, FS.readFiles[path] = 1? I think users might have just used that, without the logging?

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@sbc100 sbc100 Jan 28, 2026

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Perhaps? Seems like we should probably keeps this as just one feature though. Bifurcating it further seems like complexity we don't need. Putting the whole feature behind FS_DEBUG seems like the most sensible thing to do.

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@kripken kripken Jan 29, 2026

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I'm sorry, but I think I disagree. IIUC, the point of the feature is to be able to read FS.readFiles to see which files were actually read. FS_DEBUG, otoh, is entirely a debug feature to add verbose logging? So I can definitely see how you might want one and not the other. This feature just happens to also have FS_DEBUG in it, to add verbose logging, but that's incidental.

With that said I'm not opposed to changing this feature - and maybe it isn't even used by anyone at this point. But perhaps we should ask on the mailing list first, before changing or removing it?

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@sbc100 sbc100 Jan 29, 2026

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According to the docs the logging on the file is the main way the feature works:

There is an option to log which files are actually used at runtime. To use it, define the :js:attr:`Module.logReadFiles` object. Each file that is read will be logged to stderr.

The option is called logReadFiles because it logs to the console.

The fact that you can also read FS.readFiles true, and but more of secordary usage, at least according to the docs:

An alternative approach is to look at :js:func:`FS.readFiles` in your compiled
JavaScript. This is an object with keys for all the files that were read from.
You may find it easier to use than logging as it records files rather than
potentially multiple file accesses.

Basically when you enable logReadFiles you get two options for how to extract the list of files, right?

Being able to turn them on and off individually seems pretty overkill.

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OK, I opened a much smaller PR instead: #26189

if (Module['logReadFiles'] && !(flags & {{{ cDefs.O_WRONLY}}})) {
if (!(path in FS.readFiles)) {
if (!FS.readFiles[path]) {
FS.readFiles[path] = 1;
#if FS_DEBUG
dbg(`FS.trackingDelegate error on read file: ${path}`);
#endif
dbg(`FS: read file: ${path}`);
}
}
#endif
#if FS_DEBUG
FS.trackingDelegate['onOpenFile']?.(path, origFlags);
#endif
return stream;
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2 changes: 1 addition & 1 deletion src/settings.js
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Expand Up @@ -996,7 +996,7 @@ var INCOMING_MODULE_JS_API = [
'buffer', 'canvas', 'doNotCaptureKeyboard', 'dynamicLibraries',
'elementPointerLock', 'extraStackTrace', 'forcedAspectRatio',
'instantiateWasm', 'keyboardListeningElement', 'freePreloadedMediaOnUse',
'loadSplitModule', 'locateFile', 'logReadFiles', 'mainScriptUrlOrBlob', 'mem',
'loadSplitModule', 'locateFile', 'mainScriptUrlOrBlob', 'mem',
'monitorRunDependencies', 'noExitRuntime', 'noInitialRun', 'onAbort',
'onExit', 'onFree', 'onFullScreen', 'onMalloc',
'onRealloc', 'onRuntimeInitialized', 'postMainLoop', 'postRun', 'preInit',
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_ctors1.json
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@@ -1,10 +1,10 @@
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"sent": [
"__cxa_throw",
"_abort_js",
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8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_ctors2.json
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@@ -1,10 +1,10 @@
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"_abort_js",
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8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_except.json
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@@ -1,10 +1,10 @@
{
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8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_except_wasm.json
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@@ -1,10 +1,10 @@
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8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_except_wasm_legacy.json
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@@ -1,10 +1,10 @@
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Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_lto.json
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@@ -1,10 +1,10 @@
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"b (_setitimer_js)",
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_mangle.json
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@@ -1,10 +1,10 @@
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Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_cxx_noexcept.json
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@@ -1,10 +1,10 @@
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Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_files_js_fs.json
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@@ -1,10 +1,10 @@
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"b (fd_read)",
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_hello_O0.json
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@@ -1,10 +1,10 @@
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"sent": [
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],
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_hello_dylink.json
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@@ -1,10 +1,10 @@
{
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"sent": [
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Expand Down
4 changes: 2 additions & 2 deletions test/codesize/test_codesize_hello_dylink_all.json
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@@ -1,7 +1,7 @@
{
"a.out.js": 244882,
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"a.out.nodebug.wasm": 577863,
"total": 822745,
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"sent": [
"IMG_Init",
"IMG_Load",
Expand Down
1 change: 0 additions & 1 deletion test/codesize/test_codesize_minimal_O0.expected.js
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Expand Up @@ -1277,7 +1277,6 @@ function checkIncomingModuleAPI() {
ignoredModuleProp('freePreloadedMediaOnUse');
ignoredModuleProp('loadSplitModule');
ignoredModuleProp('locateFile');
ignoredModuleProp('logReadFiles');
ignoredModuleProp('mainScriptUrlOrBlob');
ignoredModuleProp('mem');
ignoredModuleProp('monitorRunDependencies');
Expand Down
8 changes: 4 additions & 4 deletions test/codesize/test_codesize_minimal_O0.json
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@@ -1,10 +1,10 @@
{
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"sent": [],
"imports": [],
"exports": [
Expand Down
12 changes: 6 additions & 6 deletions test/codesize/test_unoptimized_code_size.json
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@@ -1,16 +1,16 @@
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}