Keep per-test coverage with --coverage (roadmap step 4)feature/per-test-coverage

--coverage=DIR runs every attempt in a process of its own and keeps the
coverage of each test, for binaries built with LLVM source-based
coverage (Clang: -fprofile-instr-generate -fcoverage-mapping) or gcov
(GCC: --coverage).

How it works:

- The scheduler gives each attempt a fresh worker, as --isolate does,
  and sets LLVM_PROFILE_FILE and GCOV_PREFIX for it: the profile the
  process writes at exit is then the coverage of that one test, in
  DIR/profiles/t<index>-a<attempt>.profraw or a .gcov directory. Each
  runtime reads its own variable, so no symbol of either runtime is
  looked up: weak references would not pull them out of their static
  archives, and Mach-O does not allow undefined weak symbols.
- A process that runs other tests first (bisection) writes beside the
  profiles, never into one. The attempt records its profile, also in
  the JSON results.
- The profiles of an earlier run are removed when the run starts,
  because gcov adds to the .gcda files it finds.
- After the run, CoverageReport reads the last profile of every test:
  llvm-profdata and llvm-cov (PATH, xcrun on macOS, or
  CATCH3_LLVM_PROFDATA / CATCH3_LLVM_COV) export LCOV, gcov (or
  CATCH3_GCOV) writes JSON next to a copy of the .gcno. The result is
  DIR/coverage.json, the files, lines and branches each test reached,
  and DIR/coverage.lcov for the whole run. Without the tools the
  profiles stay and the run says so.
- --coverage with --in-process or --break is refused.

Tests: option parsing, LCOV reading and writing, gcov JSON, and an
integration test on CoverageFixture, a binary CMake builds with the
coverage flags of Clang or GCC: two tests reach different lines, each
map holds only its own, the LCOV report holds both, and a stale profile
of an earlier run is gone. Locally all 84 CTest tests pass with Clang,
and the coverage tests pass with GCC (built with
CATCH3_ENABLE_WERROR=OFF: Catch3 does not build with GCC's warnings as
errors, before and after this change).

CI: the Linux jobs install LLVM's coverage tools, a new job checks the
gcov path with GCC, and CATCH3_COVERAGE_REQUIRE_MAP turns a missing map
into a failure on Linux and macOS instead of a skip. MSVC has no
source-based coverage, so its builds skip those tests; the roadmap
lists step 4 as in progress until Windows with clang-cl is tested.

8 files changed, 760 insertions, 3 deletions

diff --git a/src/Progmasoft/Runner/CoverageReport.cpp b/src/Progmasoft/Runner/CoverageReport.cpp
new file mode 100644
index 00000000..c443c158
--- /dev/null
+++ b/src/Progmasoft/Runner/CoverageReport.cpp
@@ -0,0 +1,594 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+#include <algorithm>
+#include <cstdlib>
+#include <optional>
+#include <string_view>
+#include <system_error>
+
+#include "CoverageReport.hpp"
+#include "Files.hpp"
+#include "Json.hpp"
+#include "Process.hpp"
+
+namespace Progmasoft::Catch3::Runner
+{
+    namespace
+    {
+        /// Reads a non-negative decimal number; nothing for anything else,
+        /// such as the `-` LCOV writes for a branch never reached.
+        std::optional<std::uint64_t>
+        Number(std::string_view text)
+        {
+            if (text.empty() || text.size() > 19)
+            {
+                return std::nullopt;
+            }
+            std::uint64_t value = 0;
+            for (const char character : text)
+            {
+                if (character < '0' || character > '9')
+                {
+                    return std::nullopt;
+                }
+                value
+                    = value * 10 + static_cast<std::uint64_t>(character - '0');
+            }
+            return value;
+        }
+
+        std::vector<std::string_view>
+        Split(std::string_view text, char separator)
+        {
+            std::vector<std::string_view> parts;
+            std::size_t start = 0;
+            while (true)
+            {
+                const auto end = text.find(separator, start);
+                parts.push_back(text.substr(start, end - start));
+                if (end == std::string_view::npos)
+                {
+                    return parts;
+                }
+                start = end + 1;
+            }
+        }
+
+        /// A tool of a coverage format: the program, or the command that
+        /// runs it, such as `xcrun llvm-cov` on macOS.
+        using Tool = std::vector<std::string>;
+
+        std::optional<Tool>
+        FindTool(const char *variable, const std::string &name)
+        {
+            std::vector<Tool> candidates;
+            if (auto configured = ReadEnvironment(variable);
+                !configured.empty())
+            {
+                candidates.push_back({ std::move(configured) });
+            }
+            candidates.push_back({ name });
+#if defined(__APPLE__)
+            candidates.push_back({ "xcrun", name });
+#endif
+            for (auto &candidate : candidates)
+            {
+                ChildOptions options;
+                options.arguments = candidate;
+                options.arguments.emplace_back("--version");
+                options.searchPath = true;
+                if (auto run = RunAndCapture(options);
+                    run && run->status.Succeeded())
+                {
+                    return std::move(candidate);
+                }
+            }
+            return std::nullopt;
+        }
+
+        /// Runs `tool` with `arguments`; its output, or why it failed.
+        Expected<std::string>
+        RunTool(const Tool &tool,
+                const std::vector<std::string> &arguments,
+                const std::filesystem::path &directory = {})
+        {
+            ChildOptions options;
+            options.arguments = tool;
+            options.arguments.insert(options.arguments.end(),
+                                     arguments.begin(),
+                                     arguments.end());
+            options.searchPath = true;
+            options.workingDirectory = directory;
+            auto run = RunAndCapture(options);
+            if (!run)
+            {
+                return run.GetError();
+            }
+            if (!run->status.Succeeded())
+            {
+                std::string message = run->errors;
+                while (!message.empty()
+                       && (message.back() == '\n' || message.back() == '\r'))
+                {
+                    message.pop_back();
+                }
+                return Error{ tool.back() + " failed: " + message };
+            }
+            return std::move(run->output);
+        }
+
+        void
+        Add(CoverageMap &total, const CoverageMap &part)
+        {
+            for (const auto &[file, coverage] : part)
+            {
+                FileCoverage &into = total[file];
+                for (const auto &[line, count] : coverage.lines)
+                {
+                    into.lines[line] += count;
+                }
+                for (const auto &[branch, count] : coverage.branches)
+                {
+                    into.branches[branch] += count;
+                }
+            }
+        }
+
+        /// What one test reached: the lines it ran and, per line, how many
+        /// of its branches it took.
+        JsonValue
+        ReachedJson(const CoverageMap &map)
+        {
+            JsonValue files = JsonValue::Array();
+            for (const auto &[file, coverage] : map)
+            {
+                JsonValue lines = JsonValue::Array();
+                for (const auto &[line, count] : coverage.lines)
+                {
+                    if (count > 0)
+                    {
+                        lines.Append(line);
+                    }
+                }
+                JsonValue branches = JsonValue::Array();
+                std::map<std::size_t, std::pair<std::size_t, std::size_t>>
+                    perLine;
+                for (const auto &[branch, count] : coverage.branches)
+                {
+                    auto &[taken, total] = perLine[branch.first];
+                    ++total;
+                    if (count > 0)
+                    {
+                        ++taken;
+                    }
+                }
+                for (const auto &[line, counts] : perLine)
+                {
+                    if (counts.first == 0)
+                    {
+                        continue;
+                    }
+                    branches.Append(
+                        JsonValue::Object({ { "line", line },
+                                            { "taken", counts.first },
+                                            { "total", counts.second } }));
+                }
+                if (lines.Items().empty())
+                {
+                    continue;
+                }
+                files.Append(
+                    JsonValue::Object({ { "file", file },
+                                        { "lines", std::move(lines) },
+                                        { "branches", std::move(branches) } }));
+            }
+            return files;
+        }
+
+        /// The final attempt of a test that left a profile.
+        const Attempt *
+        ProfiledAttempt(const TestResult &test)
+        {
+            for (auto attempt = test.attempts.rbegin();
+                 attempt != test.attempts.rend();
+                 ++attempt)
+            {
+                if (!attempt->coverageProfile.empty())
+                {
+                    return &*attempt;
+                }
+            }
+            return nullptr;
+        }
+
+        /// The .gcda files gcov wrote below `prefix`, each with the path the
+        /// object was built at, where its .gcno is.
+        std::vector<std::pair<std::filesystem::path, std::filesystem::path>>
+        GcdaFiles(const std::filesystem::path &prefix)
+        {
+            std::vector<std::pair<std::filesystem::path, std::filesystem::path>>
+                files;
+            std::error_code error;
+            for (auto entry
+                 = std::filesystem::recursive_directory_iterator(prefix, error);
+                 !error
+                 && entry != std::filesystem::recursive_directory_iterator();
+                 entry.increment(error))
+            {
+                if (!entry->is_regular_file(error)
+                    || entry->path().extension() != ".gcda")
+                {
+                    continue;
+                }
+                // GCOV_PREFIX is put before the absolute path of the object.
+                const auto relative
+                    = std::filesystem::relative(entry->path(), prefix, error);
+                std::filesystem::path original
+                    = std::filesystem::path{ "/" } / relative;
+                files.emplace_back(entry->path(), original);
+            }
+            std::sort(files.begin(), files.end());
+            return files;
+        }
+
+        Expected<CoverageMap>
+        GcovCoverage(const Tool &gcov, const std::filesystem::path &prefix)
+        {
+            CoverageMap map;
+            for (const auto &[gcda, original] : GcdaFiles(prefix))
+            {
+                auto gcno = original;
+                gcno.replace_extension(".gcno");
+                auto beside = gcda;
+                beside.replace_extension(".gcno");
+                std::error_code error;
+                std::filesystem::copy_file(
+                    gcno,
+                    beside,
+                    std::filesystem::copy_options::overwrite_existing,
+                    error);
+                if (error)
+                {
+                    return Error{ "the notes file " + gcno.string()
+                                  + " could not be read: " + error.message() };
+                }
+                auto json = RunTool(
+                    gcov,
+                    { "--json-format", "--stdout", gcda.filename().string() },
+                    gcda.parent_path());
+                if (!json)
+                {
+                    return json.GetError();
+                }
+                // One JSON document per data file, one per line.
+                for (const auto line : Split(*json, '\n'))
+                {
+                    if (!line.empty())
+                    {
+                        AddGcovJson(std::string{ line }, map);
+                    }
+                }
+            }
+            return map;
+        }
+    } // namespace
+
+    CoverageMap
+    ParseLcov(const std::string &text)
+    {
+        CoverageMap map;
+        FileCoverage *current = nullptr;
+        for (auto line : Split(text, '\n'))
+        {
+            if (!line.empty() && line.back() == '\r')
+            {
+                line.remove_suffix(1);
+            }
+            if (line.substr(0, 3) == "SF:")
+            {
+                current = &map[std::string{ line.substr(3) }];
+            }
+            else if (line == "end_of_record")
+            {
+                current = nullptr;
+            }
+            else if (current != nullptr && line.substr(0, 3) == "DA:")
+            {
+                const auto fields = Split(line.substr(3), ',');
+                const auto number = Number(fields[0]);
+                const auto count
+                    = fields.size() > 1 ? Number(fields[1]) : std::nullopt;
+                if (number && count)
+                {
+                    current->lines[*number] += *count;
+                }
+            }
+            else if (current != nullptr && line.substr(0, 5) == "BRDA:")
+            {
+                const auto fields = Split(line.substr(5), ',');
+                if (fields.size() != 4)
+                {
+                    continue;
+                }
+                const auto number = Number(fields[0]);
+                if (!number)
+                {
+                    continue;
+                }
+                // The branches of a line are numbered in the order they come;
+                // `-` counts as never taken.
+                std::size_t index = 0;
+                for (auto it = current->branches.lower_bound({ *number, 0 });
+                     it != current->branches.end()
+                     && it->first.first == *number;
+                     ++it)
+                {
+                    ++index;
+                }
+                current->branches[{ *number, index }]
+                    += Number(fields[3]).value_or(0);
+            }
+        }
+        return map;
+    }
+
+    void
+    AddGcovJson(const std::string &text, CoverageMap &map)
+    {
+        auto document = ParseJson(text);
+        if (!document)
+        {
+            return;
+        }
+        for (const auto &file : (*document)["files"].Items())
+        {
+            FileCoverage &coverage = map[file["file"].AsString()];
+            for (const auto &line : file["lines"].Items())
+            {
+                const auto number = static_cast<std::size_t>(
+                    line["line_number"].AsUnsigned());
+                coverage.lines[number] += line["count"].AsUnsigned();
+                std::size_t index = 0;
+                for (const auto &branch : line["branches"].Items())
+                {
+                    coverage.branches[{ number, index++ }]
+                        += branch["count"].AsUnsigned();
+                }
+            }
+        }
+    }
+
+    std::string
+    ToLcov(const CoverageMap &map)
+    {
+        std::string text;
+        for (const auto &[file, coverage] : map)
+        {
+            text += "SF:" + file + "\n";
+            std::size_t linesHit = 0;
+            for (const auto &[line, count] : coverage.lines)
+            {
+                text += "DA:" + std::to_string(line) + ","
+                        + std::to_string(count) + "\n";
+                linesHit += count > 0 ? 1 : 0;
+            }
+            std::size_t branchesHit = 0;
+            for (const auto &[branch, count] : coverage.branches)
+            {
+                text += "BRDA:" + std::to_string(branch.first) + ",0,"
+                        + std::to_string(branch.second) + ","
+                        + (count > 0 ? std::to_string(count) : "-") + "\n";
+                branchesHit += count > 0 ? 1 : 0;
+            }
+            text += "LF:" + std::to_string(coverage.lines.size()) + "\n";
+            text += "LH:" + std::to_string(linesHit) + "\n";
+            text += "BRF:" + std::to_string(coverage.branches.size()) + "\n";
+            text += "BRH:" + std::to_string(branchesHit) + "\n";
+            text += "end_of_record\n";
+        }
+        return text;
+    }
+
+    Status
+    PrepareCoverageDirectory(const std::filesystem::path &directory)
+    {
+        std::error_code error;
+        std::filesystem::remove_all(directory / "profiles", error);
+        if (error)
+        {
+            return Error{ "the profiles of an earlier run in "
+                          + directory.string()
+                          + " could not be removed: " + error.message() };
+        }
+        std::filesystem::create_directories(directory / "profiles", error);
+        if (error)
+        {
+            return Error{ "the coverage directory " + directory.string()
+                          + " could not be created: " + error.message() };
+        }
+        return Status::Ok();
+    }
+
+    std::vector<std::string>
+    WriteCoverageReports(const std::filesystem::path &directory,
+                         const RunResult &run,
+                         const std::string &program)
+    {
+        std::vector<std::string> notes;
+        std::size_t llvmProfiles = 0;
+        std::size_t gcovProfiles = 0;
+        for (const auto &test : run.tests)
+        {
+            if (const Attempt *attempt = ProfiledAttempt(test))
+            {
+                std::error_code error;
+                llvmProfiles += std::filesystem::exists(attempt->coverageProfile
+                                                            + ".profraw",
+                                                        error)
+                                    ? 1
+                                    : 0;
+                gcovProfiles += std::filesystem::exists(attempt->coverageProfile
+                                                            + ".gcov",
+                                                        error)
+                                    ? 1
+                                    : 0;
+            }
+        }
+        if (llvmProfiles == 0 && gcovProfiles == 0)
+        {
+            notes.emplace_back(
+                "--coverage: the tests wrote no coverage; build them with "
+                "-fprofile-instr-generate -fcoverage-mapping (Clang) or "
+                "--coverage (GCC)");
+            return notes;
+        }
+        const bool llvm = llvmProfiles >= gcovProfiles;
+        std::optional<Tool> profdata;
+        std::optional<Tool> cov;
+        std::optional<Tool> gcov;
+        if (llvm)
+        {
+            profdata = FindTool("CATCH3_LLVM_PROFDATA", "llvm-profdata");
+            cov = FindTool("CATCH3_LLVM_COV", "llvm-cov");
+        }
+        else
+        {
+            gcov = FindTool("CATCH3_GCOV", "gcov");
+        }
+        if (llvm ? (!profdata || !cov) : !gcov)
+        {
+            notes.push_back(
+                "--coverage: "
+                + std::to_string(llvm ? llvmProfiles : gcovProfiles)
+                + " profile(s) in " + (directory / "profiles").string() + "; "
+                + (llvm ? "llvm-profdata and llvm-cov were" : "gcov was")
+                + " not found, so there is no map of what each test reached");
+            return notes;
+        }
+
+        CoverageMap combined;
+        std::vector<std::string> raw;
+        JsonValue tests = JsonValue::Array();
+        std::vector<std::string> failures;
+        for (const auto &test : run.tests)
+        {
+            const Attempt *attempt = ProfiledAttempt(test);
+            if (attempt == nullptr)
+            {
+                continue;
+            }
+            const std::string base = attempt->coverageProfile;
+            Expected<CoverageMap> reached = CoverageMap{};
+            std::string profile;
+            if (llvm)
+            {
+                profile = base + ".profraw";
+                std::error_code error;
+                if (!std::filesystem::exists(profile, error))
+                {
+                    continue;
+                }
+                raw.push_back(profile);
+                auto merged = RunTool(
+                    *profdata,
+                    { "merge", "-sparse", profile, "-o", base + ".profdata" });
+                if (!merged)
+                {
+                    failures.push_back(test.name + ": "
+                                       + merged.GetError().Message());
+                    continue;
+                }
+                auto lcov = RunTool(*cov,
+                                    { "export",
+                                      "-format=lcov",
+                                      "-instr-profile=" + base + ".profdata",
+                                      program });
+                if (!lcov)
+                {
+                    failures.push_back(test.name + ": "
+                                       + lcov.GetError().Message());
+                    continue;
+                }
+                reached = ParseLcov(*lcov);
+            }
+            else
+            {
+                profile = base + ".gcov";
+                std::error_code error;
+                if (!std::filesystem::exists(profile, error))
+                {
+                    continue;
+                }
+                reached = GcovCoverage(*gcov, profile);
+                if (!reached)
+                {
+                    failures.push_back(test.name + ": "
+                                       + reached.GetError().Message());
+                    continue;
+                }
+                Add(combined, *reached);
+            }
+            tests.Append(
+                JsonValue::Object({ { "name", test.name },
+                                    { "outcome", OutcomeName(test.outcome) },
+                                    { "profile", profile },
+                                    { "files", ReachedJson(*reached) } }));
+        }
+
+        std::string lcovText;
+        if (llvm && !raw.empty())
+        {
+            std::vector<std::string> arguments{ "merge", "-sparse" };
+            arguments.insert(arguments.end(), raw.begin(), raw.end());
+            const std::string all = (directory / "coverage.profdata").string();
+            arguments.emplace_back("-o");
+            arguments.push_back(all);
+            auto merged = RunTool(*profdata, arguments);
+            auto exported = merged ? RunTool(*cov,
+                                             { "export",
+                                               "-format=lcov",
+                                               "-instr-profile=" + all,
+                                               program })
+                                   : Expected<std::string>{ merged.GetError() };
+            if (exported)
+            {
+                lcovText = *exported;
+            }
+            else
+            {
+                failures.push_back("the combined report: "
+                                   + exported.GetError().Message());
+            }
+        }
+        else
+        {
+            lcovText = ToLcov(combined);
+        }
+
+        JsonValue report
+            = JsonValue::Object({ { "version", 1 },
+                                  { "format", llvm ? "llvm" : "gcov" },
+                                  { "binary", program },
+                                  { "tests", std::move(tests) } });
+        const auto json = directory / "coverage.json";
+        const auto lcov = directory / "coverage.lcov";
+        if (auto status = WriteFileAtomically(json, report.ToPrettyString());
+            !status)
+        {
+            failures.push_back(status.GetError().Message());
+        }
+        if (auto status = WriteFileAtomically(lcov, lcovText); !status)
+        {
+            failures.push_back(status.GetError().Message());
+        }
+        notes.push_back(
+            "--coverage: " + std::to_string(llvm ? llvmProfiles : gcovProfiles)
+            + " test profile(s); what each test reached is in " + json.string()
+            + ", the run combined in " + lcov.string());
+        for (const auto &failure : failures)
+        {
+            notes.push_back("--coverage: " + failure);
+        }
+        return notes;
+    }
+} // namespace Progmasoft::Catch3::Runner
diff --git a/src/Progmasoft/Runner/CoverageReport.hpp b/src/Progmasoft/Runner/CoverageReport.hpp
new file mode 100644
index 00000000..35f9dc75
--- /dev/null
+++ b/src/Progmasoft/Runner/CoverageReport.hpp
@@ -0,0 +1,58 @@
+// Copyright (c) 2026 Progmasoft.
+// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+#pragma once
+
+#include <Progmasoft/Catch3/Results.hpp>
+#include <cstdint>
+#include <filesystem>
+#include <map>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "Expected.hpp"
+
+namespace Progmasoft::Catch3::Runner
+{
+    /// What one test, or the whole run, reached in one source file.
+    struct FileCoverage final
+    {
+        /// Execution count of each instrumented line.
+        std::map<std::size_t, std::uint64_t> lines;
+        /// Count of each branch, by line and by its position on the line.
+        std::map<std::pair<std::size_t, std::size_t>, std::uint64_t> branches;
+    };
+
+    /// Coverage by source file.
+    using CoverageMap = std::map<std::string, FileCoverage>;
+
+    /// Reads an LCOV tracefile: `SF`, `DA` and `BRDA` records.
+    [[nodiscard]] CoverageMap
+    ParseLcov(const std::string &text);
+
+    /// Reads the JSON that `gcov --json-format` writes, adding to `map`.
+    void
+    AddGcovJson(const std::string &text, CoverageMap &map);
+
+    /// Writes `map` as an LCOV tracefile.
+    [[nodiscard]] std::string
+    ToLcov(const CoverageMap &map);
+
+    /// Prepares the directory of a coverage run: the profiles of an earlier
+    /// run below it are removed, because gcov adds to the .gcda files it
+    /// finds instead of replacing them.
+    Status
+    PrepareCoverageDirectory(const std::filesystem::path &directory);
+
+    /// Turns the profiles a coverage run left in `directory` into
+    /// `directory/coverage.json`, the lines and branches each test reached,
+    /// and `directory/coverage.lcov`, the run combined. LLVM profiles are
+    /// read with `llvm-profdata` and `llvm-cov`, gcov data with `gcov`,
+    /// found on PATH or named by CATCH3_LLVM_PROFDATA, CATCH3_LLVM_COV and
+    /// CATCH3_GCOV. Returns what the run should be told: how many profiles
+    /// there were and where the reports are, or why there are none.
+    [[nodiscard]] std::vector<std::string>
+    WriteCoverageReports(const std::filesystem::path &directory,
+                         const RunResult &run,
+                         const std::string &program);
+} // namespace Progmasoft::Catch3::Runner
diff --git a/src/Progmasoft/Runner/Options.cpp b/src/Progmasoft/Runner/Options.cpp
index 13cf4be7..aef0df6c 100644
--- a/src/Progmasoft/Runner/Options.cpp
+++ b/src/Progmasoft/Runner/Options.cpp
@@ -830,6 +830,21 @@ namespace Progmasoft::Catch3::Runner
                      options.tracePath = *value;
                      return Status::Ok();
                  }},
+                {"coverage",
+                 '\0',
+                 Takes::kValue,
+                 "DIR",
+                 "Output",
+                 "run every test in a process of its own and keep its "
+                 "coverage in DIR, with a map of what each test reached",
+                 [](Options &options, const std::optional<std::string> &value) {
+                     if (value->empty())
+                     {
+                         return Status{ Error{ "--coverage needs a directory" } };
+                     }
+                     options.coverageDirectory = *value;
+                     return Status::Ok();
+                 }},
                 {"premature-exit-guard-file",
                  '\0',
                  Takes::kValue,
@@ -1105,6 +1120,11 @@ namespace Progmasoft::Catch3::Runner
                 return status.GetError();
             }
         }
+        if (!options.coverageDirectory.empty() && options.inProcess)
+        {
+            return Error{ "--coverage runs every test in a process of its own, "
+                          "so it cannot be used with --in-process or --break" };
+        }
         return options;
     }
 
diff --git a/src/Progmasoft/Runner/Options.hpp b/src/Progmasoft/Runner/Options.hpp
index 256f5ac7..3d53eb42 100644
--- a/src/Progmasoft/Runner/Options.hpp
+++ b/src/Progmasoft/Runner/Options.hpp
@@ -190,6 +190,10 @@ namespace Progmasoft::Catch3::Runner
         std::string junitPath;
         std::string jsonPath;
         std::string tracePath;
+        /// Per-test coverage: every attempt runs in a process of its own and
+        /// leaves its profile below this directory, with the map of what
+        /// each test reached and the combined report; see CoverageReport.
+        std::string coverageDirectory;
         unsigned slowest = 0;
         /// Created when the run starts and removed when it ends normally, so
         /// a file left behind shows that the run ended early.
diff --git a/src/Progmasoft/Runner/ResultsJson.cpp b/src/Progmasoft/Runner/ResultsJson.cpp
index ab892cf7..c82475ec 100644
--- a/src/Progmasoft/Runner/ResultsJson.cpp
+++ b/src/Progmasoft/Runner/ResultsJson.cpp
@@ -203,6 +203,7 @@ namespace Progmasoft::Catch3::Runner
         SetText(value, "exitDescription", attempt.exitDescription);
         SetText(value, "lastLocation", attempt.lastLocation);
         SetText(value, "temporaryDirectory", attempt.temporaryDirectory);
+        SetText(value, "coverageProfile", attempt.coverageProfile);
         return value;
     }
 
@@ -242,6 +243,7 @@ namespace Progmasoft::Catch3::Runner
         attempt.exitDescription = value["exitDescription"].AsString();
         attempt.lastLocation = value["lastLocation"].AsString();
         attempt.temporaryDirectory = value["temporaryDirectory"].AsString();
+        attempt.coverageProfile = value["coverageProfile"].AsString();
         return attempt;
     }
 
diff --git a/src/Progmasoft/Runner/Runner.cpp b/src/Progmasoft/Runner/Runner.cpp
index 77011322..61f8f2b7 100644
--- a/src/Progmasoft/Runner/Runner.cpp
+++ b/src/Progmasoft/Runner/Runner.cpp
@@ -22,6 +22,7 @@
 #include <random>
 #include <system_error>
 
+#include "CoverageReport.hpp"
 #include "Analysis.hpp"
 #include "Catalog.hpp"
 #include "Console.hpp"
@@ -949,6 +950,17 @@ namespace Progmasoft::Catch3
                 }
             }
 
+            if (!options.coverageDirectory.empty())
+            {
+                if (auto status = Runner::PrepareCoverageDirectory(
+                        options.coverageDirectory);
+                    !status)
+                {
+                    PrintError(status.GetError().Message());
+                    return Catch::UnspecifiedErrorExitCode;
+                }
+            }
+
             const auto started = std::chrono::steady_clock::now();
             RunResult run;
             run.binary = programName;
@@ -983,6 +995,14 @@ namespace Progmasoft::Catch3
                 run.tests.push_back(std::move(kept));
             }
             run.notes = std::move(outcome.notes);
+            if (!options.coverageDirectory.empty())
+            {
+                for (auto &note : Runner::WriteCoverageReports(
+                         options.coverageDirectory, run, program.string()))
+                {
+                    run.notes.push_back(std::move(note));
+                }
+            }
 
             const bool anyFailure
                 = std::any_of(run.tests.begin(),
diff --git a/src/Progmasoft/Runner/Scheduler.cpp b/src/Progmasoft/Runner/Scheduler.cpp
index 56a2aa36..263257f7 100644
--- a/src/Progmasoft/Runner/Scheduler.cpp
+++ b/src/Progmasoft/Runner/Scheduler.cpp
@@ -346,6 +346,10 @@ namespace Progmasoft::Catch3::Runner
                 }
                 attempt.temporaryDirectory
                     = TemporaryFor(catalogIndex, attempt.number);
+                if (target)
+                {
+                    attempt.coverageProfile = worker.coverageProfile;
+                }
                 worker.running = std::move(running);
                 if (options_.inProcess)
                 {
@@ -973,8 +977,33 @@ namespace Progmasoft::Catch3::Runner
 
             // Worker processes.
 
+            /// Where the coverage profile of the next attempt of `job` goes,
+            /// without its extension. Only an attempt that runs alone in its
+            /// process gets one: a job that runs other tests first would
+            //