This directory holds meson.build, included from the top-level build with subdir('meson'). It resolves every external dependency GEMC links against and configures the bundled CMake subprojects (assimp, yaml-cpp). This README documents how the three shared-with-CLAS12 dependencies — Qt6, CLHEP, and Geant4 — are loaded, and how this repository acts as the producer of the Geant4 pkg-config files that downstream GEMC projects consume.

The companion consumer is clas12-systems/meson/README.md: read both together, because the Geant4 handling only makes sense as a producer/consumer pair.

The top-level project() sets the build defaults for both direct meson setup and CI. The former core.ini native file is removed. The active settings are preserved: C++17, static GEMC libraries, static selection from both-library targets, prefer_static=true, b_lundef=false, and b_asneeded=false. The existing libdir=lib default is retained. The commented LTO, static PIC, and executable PIE settings are not added as defaults.

Builds that previously omitted the native file now use these settings too. prefer_static=true prefers archives for dependencies without an explicit static choice; explicit shared requests such as Qt, Linux SQLite, and zlib still take precedence. b_lundef=false permits unresolved plugin symbols at link time, and b_asneeded=false retains shared dependencies even when the linker sees no direct symbol reference.

Override any default with a command-line -D option. CI still selects shared GEMC libraries for sanitizer and profile builds. Existing build directories configured with --native-file=core.ini retain that path and need a fresh setup after removal. Use a new build directory and repeat the desired prefix, build type, and feature options, omitting --native-file=core.ini.

GEMC (src) is the root of the dependency graph. It is built and installed against a specific Geant4 / CLHEP, and it records that choice in pkg-config files it installs into its own prefix:

• src generates geant4.pc and geant4_core.pc (Geant4 ships no pkg-config file of its own) and installs them into <prefix>/lib/pkgconfig.
• src also installs gemc.pc describing the GEMC libraries and their private static-link needs.
• Downstream projects (e.g. clas12-systems, which builds dlopen-able digitization plugins) do not re-derive Geant4 from geant4-config. They resolve geant4_core straight from the installed GEMC's lib/pkgconfig, so the plugins are always built against the exact Geant4 GEMC itself uses. See the consumer README for how that lookup finds the GEMC prefix from the gemc binary on PATH.

This is why the Geant4 code in the two repositories looks different: they play different roles. src is the only place that runs geant4-config and builds the full -lG4* link line; clas12-systems only consumes the resulting .pc.

Loaded with the qt6 meson module. Qt is requested as shared (static : false) because supported package managers ship Qt only as shared runtime libraries; asking for static archives just produces fallback warnings and does not actually make Qt static. Includes are marked include_type : 'system' so Qt headers stay warning-free.

• Required modules: Core, Gui, Widgets, OpenGLWidgets — the GUI cannot build without them.
• Svg is probed optionally. No GEMC source includes a QtSvg header; it is used only at run time to rasterize the toolbar button icons through the "SVG" image-format plugin. When missing, the GUI still builds and buttons fall back to empty icons — so it only warns.
• Charts is a real compile/link dependency (gAnalysisView.cc includes <QtCharts/...>). It is probed optionally: when present, -DGEMC_HAS_QTCHARTS is defined and the analysis view is built; when absent, GEMC still builds and the GUI analysis page shows a placeholder.

Qt6 exists only in src. clas12-systems has no GUI and therefore no Qt dependency at all.

clhep_deps = dependency('clhep', version : '>=2.4.7.1', include_type : 'system')

Resolved through pkg-config (module name clhep), minimum version 2.4.7.1, includes treated as system. It is attached to essentially every GEMC module library and to the gemc binary.

CLHEP is loaded shared here — no static : true. This is deliberate and is the one intentional difference from clas12-systems, which forces static : true:

• In src, GEMC and Geant4 must share a single copy of CLHEP's stateful globals — most importantly the HepRandom engine singleton that Geant4 uses for all random numbers. One shared libCLHEP guarantees exactly one RNG engine for the whole process.
• Forcing CLHEP static would be actively harmful in the shared / sanitizer / profile build (use_sharedl, see below): each of the ~30 GEMC module .sos would embed its own copy of CLHEP's globals, giving multiple diverging RNG states and triggering ASan/UBSan ODR-violation reports. That reintroduces exactly the duplicate-symbol class of problem use_sharedl exists to avoid.
• clas12-systems forces static for an unrelated, loader-specific reason: its plugins must carry no @rpath/libCLHEP* dependency or the dlopen fails. GEMC's executable has no such problem — it resolves libCLHEP at run time via Geant4's rpath.

Conclusion from the analysis: do not make CLHEP static in src. If a self-contained default binary were ever an explicit goal, the only safe form would be conditional — static : not use_sharedl — keeping the shared and sanitizer builds on shared CLHEP. The payoff is marginal, so the current shared setup stands.

gemc.pc still records --libs --static clhep under libraries_private regardless of this flag, so downstream static-link information is present either way.

Geant4 ships no pkg-config file, so src builds its own and installs them for downstream use.

1. Find geant4-config. $G4INSTALL/bin/geant4-config is preferred when $G4INSTALL is set (the module layout), otherwise a geant4-config on PATH. Missing → hard error.
2. Generate the .pc files. meson/g4_pkgconfig.py writes geant4.pc and geant4_core.pc into the build tree (<build>/pkgconfig). They live in the build tree, not the install prefix, so a fresh rm -rf build regenerates them cleanly — important when switching Geant4 versions.
3. Resolve them via pkg-config with an augmented PKG_CONFIG_PATH (meson.build cannot mutate the pkg_config_path option or dependency()'s environment, so pkg-config is run directly and the result wrapped in declare_dependency). Both .pc are checked for >=11.3.2 and for agreement with the active geant4-config version.
4. Turn each -lG4* into a cached cpp.find_library object. A shared cache makes the full set (geant4) and the core set (geant4_core) point at the same library objects. This lets meson deduplicate the Geant4 archives across the whole link graph — raw declare_dependency link_args are never deduplicated, so with dozens of static modules each carrying the Geant4 link line, GNU ld would reprocess every large archive on each repeat and the link would crawl.
5. Two dependency objects. geant4_dep (full, -isystem includes) drives the GUI/vis targets; geant4_core_dep (core, plain -I) drives everything else. On Linux, geant4_deps also pulls in the X11/GLX stack for G4OpenGL.
6. Install the .pc files into <prefix>/lib/pkgconfig via an install script (they are generated build-tree files). This is the step that makes the producer/consumer model work: clas12-systems reads geant4_core.pc from here.

Static Geant4 and Assimp both bundle pugixml. Upcoming in the next release, the Assimp target compiles with -Dpugi=gemc_assimp_pugi, giving its bundled XML parser a distinct namespace and preventing duplicate symbols when both archives are linked into GEMC. The definition applies to all Assimp sources, including XML importers, and is private to that target; GEMC, yaml-cpp, and Geant4 do not receive it.

use_sharedl = (sanitize != 'none') or (get_option('default_library') == 'shared')

• Default build: GEMC modules are static .a archives, linked once into the gemc executable. A static link_with library forwards its dependencies' link args, so plugins/examples receive only the compile/include part of this_deps to avoid duplicate -l flags.
• use_sharedl build (sanitizer or default_library=shared, the profile/CI path): each module is a self-contained shared library that does not forward those link args, so the full this_deps must be passed to plugins and examples instead.

Static linking of CLHEP and the bundled CMake subprojects interacts badly with the use_sharedl shape (duplicate symbols / duplicated global state across shared objects), which is the core reason CLHEP is kept shared — see the CLHEP section above.

• opengl, sqlite3, expat (Geant4's GIDI/XML parsing needs it under static Geant4), zlib (shared, to avoid linking the static system archive), threads.
• ROOT is optional: probed via root-config, wrapped in declare_dependency; disabled features degrade gracefully unless -Droot=enabled forces it.
• CMake subprojects assimp and yaml-cpp are configured through cmake.subproject_options(). yaml-cpp headers are part of GEMC's public API (goption.h includes yaml-cpp/yaml.h), so they are installed alongside the GEMC headers.