pkg-config: Fix Install Directories, Add C++17 & MPI Flags

[ax3l]

The generated openPMD.pc did not follow the actual install layout and lacked the public usage requirements of openPMD.

Install directories

• use openPMD_INSTALL_{BIN,LIB,INCLUDE}DIR, keep absolute dirs (e.g., Nix) as they are
• substitute prefix at install time: honors cmake --install --prefix (also relative) and DESTDIR
• exec_prefix=${prefix} (was ${prefix}/bin), add bindir
• derive openPMD_INSTALL_CMAKEDIR and the default openPMD_INSTALL_PYTHONDIR from openPMD_INSTALL_LIBDIR

Public usage requirements

• MPI builds: MPI include dirs, definitions and compile options in Cflags; MPI link flags and libraries in Libs (public headers include mpi.h and call MPI functions)
• new variable cxxstd with the C++ standard flag: pkg-config --variable=cxxstd openPMD (e.g., -std=c++17)
• escape paths and arguments (spaces, #, quotes)

Details: behavior changes, design, testing, known limitations

Behavior changes (changelog):

• pkg-config --variable=exec_prefix openPMD now returns the prefix (GNU/pkg-config convention); use the new bindir for the CLI tools.
• New cxxstd variable. The flag is intentionally not in Cflags: it would override newer standards requested by consumers, e.g., CMake's pkg_check_modules(IMPORTED_TARGET) puts it after the target's own -std=c++20, and it breaks C sources.

Bugs fixed (reproduced before the change):

• -DopenPMD_INSTALL_LIBDIR=lib64 -DopenPMD_INSTALL_INCLUDEDIR=include/openpmd installed there, but the .pc pointed to lib and include.
• Absolute CMAKE_INSTALL_LIBDIR produced libdir=${prefix}//abs/lib.
• On Windows, GNUInstallDirs is not included, so CMAKE_INSTALL_LIBDIR is typically empty and libdir=${prefix}/ resulted (code inspection, not tested on Windows). Same for the default Python install dir (/site-packages).
• With openPMD_INSTALL_LIBDIR set, the CMake package config still went to ${CMAKE_INSTALL_LIBDIR}/cmake/openPMD.
• MPI builds: compiling against the .pc flags alone failed with mpi.h: No such file or directory.

Design:

• The prefix placeholder is replaced via string(REPLACE) in install(CODE) rather than a second configure_file, so dependency paths are not rescanned for @VAR@. The install code sets cmake_policy(VERSION 3.22), since install scripts run with policies unset (CMP0053 would expand @VAR@).
• Absolute prefix instead of a relocatable ${pcfiledir}-relative one: the latter breaks when only the .pc file is symlinked elsewhere. Package managers (conda, Spack) rewrite absolute prefixes when relocating.
• Paths are backslash-escaped rather than quoted: pkg-config's define-prefix escapes the guessed prefix itself, which breaks inside quotes.
• MPI flags come from the FindMPI result variables: the MPI::MPI_<lang> target properties contain generator expressions ($<HOST_LINK:SHELL:...>, $<COMPILE_LANG_AND_ID:...>). Shared MPI libraries are written as -L<dir> -l<name>, since FindPkgConfig places plain paths before the object files, which fails to link with --as-needed. The MPICH_SKIP_MPICXX/OMPI_SKIP_MPICXX/_MPICC_H defines are only for building openPMD itself and are filtered, as for CMake consumers.
• The MSVC PUBLIC options (/bigobj, /Zc:__cplusplus) are not added: public headers do not use __cplusplus.

Tested (Linux, real build + install; pkgconf 1.8.1 and pkg-config 0.29.2, each with and without define-prefix; pkgconf --validate ok):

• Layouts: default dirs, --prefix <other>, relative --prefix from another directory, trailing slash, prefix with a space and #, DESTDIR, lib64 + custom includedir, absolute lib/include dirs, unchanged re-install is Up-to-date. A consumer compiled, linked and ran using only the .pc flags.
• MPI (MPICH), shared and static: a consumer calling MPI and openPMD::Series(..., MPI_COMM_WORLD) compiled with plain g++ (not mpicxx), -std=c++14 followed by cxxstd, linked and ran on 2 ranks. Controls fail as expected: previous flags (mpi.h not found), -std=c++14 without cxxstd.
• CMake pkg_check_modules(IMPORTED_TARGET) consumer with CXX_STANDARD 20 and -Wl,--as-needed: compiles as C++20 only, links and runs on 2 ranks; control with plain MPI library paths fails with undefined reference to symbol 'MPI_Init'.
• openPMD_MPI_LINK_C=ON and --static: no duplicate MPI flags. Unit tests for the escape/link helpers (including -framework X kept as is).

Known limitations, not addressed here:

• If openPMD-api is built with an MPI compiler wrapper as C++ compiler (mpicxx, Cray CC), FindMPI reports no MPI flags; consumers need the same wrapper (documented).
• --define-prefix (default in e.g. conda-forge's pkg-config) guesses the prefix as two levels above the .pc file, which is wrong for multiarch libdirs such as lib/x86_64-linux-gnu. This affects every package (verified with Debian's zlib.pc).
• MPI link flags can contain -Wl,-rpath,<MPI prefix>/lib, as with mpicxx and CMake's MPI::MPI_CXX: another libopenPMD in that prefix can shadow the intended one. Seen in testing with a conda env shipping openPMD-api 0.17.1 (also via conda's LDFLAGS).
• With special characters in the prefix, --variable=prefix/libdir/... show the escaping backslashes (as with define-prefix). ${ and $$ in paths are not escaped. pkgconf 1.8.1 does not re-escape spaces on output for arguments other than -I/-L.
• Pre-existing in static builds: Libs.private does not list the libraries of imported targets (e.g., ADIOS2, HDF5) themselves, and FindHDF5's debug/optimized keywords can leak into it.

🤖 Generated with Claude Code