HTML2LCOV(1)

📦 version 2.6, in arch rolling

🌸 NAME 🌸

This is html2lcov(1) from the LCOV manual — manual section 1, manual group LCOV Tools. The docs say html2lcov is html2lcov documentation, and bestie, it's here to recover lcov .info coverage data and a source diff from an HTML report. That's the whole assignment. 💅

✨ SYNOPSIS ✨

html2lcov [--output filename] [--source-directory dir]+ [--current-file file]+ [options] report_directory+

💖 DESCRIPTION 💖

Okay so html2lcov screen-scrapes a genhtml-generated HTML coverage report to recover these two things:

  • The LCOV .info coverage data that was used to produce the report. genhtml copies its input .info files into the top level of the report directory when it is run with --save; html2lcov finds and aggregates those files, using the same layout definition that genhtml --save uses to write them (so the reader and writer cannot drift). It is an error if the report does not contain any such .info file. 🚨
  • The source code that was embedded in the per-file HTML pages. This is compared against the source found under --source-directory to produce a universal diff and a human-readable difference report. Note that html2lcov needs to see all of the original source code: the input HTML report must not be a subset report containing only part of the source code (i.e., not a report generated using a --select-script callback). No half-report shenanigans, bestie. 🙈

The source directory may contain new files that were not in the report, and may omit files that were in the report; these differences are documented in the difference report rather than being treated as errors. It is an error if the source directory contains no file that matches any file in the report. So like, zero matches = nope. 💀

Because genhtml expands tabs and normalizes some whitespace when it renders source into HTML, the comparison is run with diff -b, which ignores differences in the amount of whitespace (a tab and any run of spaces compare equal), so re-indented or tab-vs-space-only changes do not appear as spurious differences. Slay for not being fake-dramatic about indentation. ✨

🦋 USE CASES 🦋

The common use case for html2lcov is when the user has code and/or test changes in their sandbox, has a coverage report from earlier in the day, and now wants to see whether subsequent changes are adequately tested. Unfortunately: the user isn’t entirely sure what the code looked like when the report was generated — and does not have a corresponding label. In that case: file diffs and baseline coverage data can be extracted from the report, and then used as the genhtml --diff-file and --baseline-file inputs. Iconic little rescue mission, honestly. 🫶

Another common case is when some or all of the source code is generated and not under revision control — but the user wants to verify that it is properly tested after configuration and/or generator changes. Very "trust me, the generator made it, now prove it's tested" energy. 🎀

🎀 OPTIONS 🎀

html2lcov supports the options that are common to the other tools in the LCOV suite, and honours the corresponding lcovrc(5) settings. In particular:

  • file selection and path manipulation: --include, --exclude, --erase-functions, --substitute, --filter, --omit-lines, --demangle-cpp. These apply to the coverage data recovered from the report exactly as they do when the same data is read by lcov — so a file which is excluded is also absent from the universal diff and from the difference report. Exclusion goes hard, no cap. 🚫
  • parallelism and resource limits: --parallel (-j), --memory, --tempdir, --preserve. html2lcov forks children both to read the report’s saved .info file(s) and to scrape and diff the report’s source pages, so --parallel speeds up both phases of a large report; see PARALLELISM. Multi-core moment. 🔥
  • message control and diagnostics: --ignore-errors, --keep-going, --expect-message-count, --msg-log, --quiet, --verbose, --debug.
  • configuration and provenance: --config-file, --rc, --comment, --version, --help.

The tool-specific options are:

  • --output filename, -o filename — Name a base for the output files. Any extension is stripped from filename to form base, and each artifact then appends its own extension: the aggregated coverage is written to base.info, the human-readable difference report to base.rpt, the universal diff to base.udiff (unless --diff-file redirects it), and the profile (if --profile is used) to base.json. When --output is omitted base is html2lcov and the universal diff is written to standard output. Cute little naming system, tbh. 🎀
  • --source-directory dir — Search dir for the current source files to diff against the report’s embedded source. May be used more than once. Files are matched by their path relative to the report’s recovered source root. When --source-directory is not specified it defaults to . (the current directory). Bestie, it's literally right there. 📁
  • --current-file filename — An lcov .info file whose SF: records name the current source files. May be specified more than once, and each value may itself be a list separated by the configured list separator (see lcovrc(5)).

When any --current-file is given, its union of SF: paths becomes an authoritative whitelist of the current source files:

  • Only files named in a SF: record may appear in the universal diff (as changed or added). Every other file found under --source-directory is ignored.
  • A file present in the report (baseline) but not in the current set is treated as removed.
  • A file present in the current set but not in the report is treated as added.

html2lcov uses the .info only as a file list; its coverage data is not merged into any output. The expected workflow is to feed the html2lcov universal diff to genhtml --diff-file and the aggregated .info output to genhtml --baseline-file, using this same --current-file .info as the genhtml current input. Workflow slay. 💅

A current SF: path is matched to a report file when it equals either the report file’s recovered path or its report-root-relative path (after any --substitute rules are applied); relative and absolute path styles are preserved on output. If a file appears in both sets under names that differ only in path (a shared basename or path tail), html2lcov emits an ignorable mismatch warning. Use --substitute (or an external tool such as sed(1)) to reconcile path differences the tool cannot resolve on its own. Path drama is real, but manageable. 😭

If a SF: path names a file that cannot be read from disk under --source-directory, html2lcov reports an ignorable source error; when that error is ignored, the file is dropped from the universal diff and its record is removed from the aggregated .info output.

A missing --current-file is a fatal error; an empty file raises an ignorable empty error; a non-empty file that contains no SF: record is not an LCOV file and is a fatal error. So like, don't hand it a random file and call it a day. 🚨

  • --diff-file filename — Write the universal diff to filename instead of the default base.udiff (or standard output when --output is omitted). Redirect that diff, bestie. 🌀
  • --profile [filename] — Write timing and statistics data as JSON to filename (default is base.json). Stats girlies stay winning. 📊

See lcov(1) and lcovrc(5) for details of other supported options and configuration settings. iykyk. ✨

🌈 OUTPUTS 🌈

html2lcov produces up to four distinct artifacts:

  • universal diff (base.udiff) — written to base.udiff, or to standard output when --output is omitted; --diff-file redirects it elsewhere. Standard diff -u format, so it can be consumed by genhtml --diff. Files only under --source-directory appear as additions (versus /dev/null); files only in the report appear as removals. May be empty when there are no code changes. For a changed file both diff headers (--- and +++) name the same path – the source path recovered from the report, which is exactly the SF: record in the aggregated .info – so genhtml can associate the diff with the recovered baseline coverage. Matching headers = the pipeline understands each other. 🔥
  • aggregated coverage (base.info) — the current coverage recovered from the report’s saved .info file(s). The main character artifact, honestly. 👑
  • difference report (base.rpt) — a human-readable table listing every changed, added, removed, and unchanged file, with added/deleted line counts. Receipts in table form. 🧾
  • profile (base.json) — timing/statistics data, when --profile is used.

🔥 PARALLELISM 🔥

Both of the expensive phases of html2lcov are parallelized, and both are governed by --parallel (-j) and --memory:

  • reading the .info file(s) saved in the report — the same parallel parse that lcov --add-tracefile uses.
  • scraping the source out of the report’s HTML pages and diffing it against --source-directory. Each child scrapes and diffs a group of files and returns only the classification and the udiff text, so the recovered source never accumulates in the parent. Files are grouped by the size of the HTML page each is scraped from, so that the groups cost roughly the same; a report with only a handful of files is processed in this process rather than paying for a fork. Smart grouping, no wasted forks. 🧚

The output does not depend on --parallel: the .info, .udiff and .rpt artifacts are identical for any value. So you can crank the parallel knob and the receipts stay the same. Periodt. 💅

🍓 EXAMPLES 🍓

Recover coverage and diff current source against a saved report:

# Generate coverage and an HTML report, saving the input .info files
$ lcov --capture -d . -o cov.info
$ genhtml cov.info --save -o html_report

# make code and/or test changes
...

# capture new coverage data
$ lcov --capture -d . -o current.info

# Recover the .info data and diff the current source tree
$ html2lcov -o changes html_report --source-directory ./src --current-file current.info

# 'changes.udiff' is the universal diff (diff -u format)
# 'changes.info'  is the recovered coverage data
# 'changes.rpt'   is the human-readable difference report

# now generate a differential report - to see what has changed:
$ genhtml -o differential --baseline-file changes.info --diff-file changes.udiff current.info

Also see the example_html2lcov in the example directory |ToolName|_HOME/share/lcov/example:

$ cd |ToolName|_HOME/share/lcov/example
$ make example_html2lcov

🌸 NOTES 🌸

Okay bestie, this part is important, so pay attention. 👀

Coverage literally cannot be recovered from a report that was built without --save, because the .info files just aren't there in the report directory. Like, they're not hiding, they're not in the group chat, they're gone. 💀 And html2lcov treats this as a full-on error rather than attempting a lossy scrape of the coverage counts from the HTML. No sketchy workarounds here — she said no cap or nothing. 🚫

Now for the second vibe check: if the source under --source-directory is identical to the source embedded in the report, there are no differences to emit. That's not a bug, bestie, that's a normal, expected outcome — for example, when the current source has not changed since the report was built. ✨ So html2lcov reports it as a non-fatal empty warning (No source code differences found) and still exits successfully. Slay, she's being gentle. 🫶

Want her to fully stop talking about it? Use --ignore-errors empty to suppress the warning entirely. Peace and quiet, periodt. 🤫

👑 AUTHOR 👑

Henry Cox <henry.cox@mediatek.com>

(Yes, a real human made this. Give him his flowers. 💐)

🦋 SEE ALSO 🦋

Go read the whole friend group, iykyk. 📚✨

2024-2026, LCOV Project

And that's the whole page, bestie — this is HTML2LCOV(1) from LCOV version 2.6, dated 2026-10-03. Bookmark it, screenshot it, send it to the group chat, whatever you gotta do. 💅✨

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