Fix "INSTALL(EXPORT) given unknown export "pffft-targets" "

[G4m4]

When using this library as a cmake dependency with the option INSTALL_PFFFT set to OFF it fails during configuration time with:

INSTALL(EXPORT) given unknown export "pffft-targets"

This is due to the fact that installation is done like so:

install( TARGETS ${INSTALL_TARGETS}
  EXPORT pffft-targets
  ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
  LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
)
[...]
install(EXPORT pffft-targets
  FILE pffft-targets.cmake
  NAMESPACE PFFFT::
  DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/pffft
)

If ${INSTALL_TARGETS} is empty (which happens with INSTALL_PFFFT set to OFF) then pffft-targets does not exist and the subsequent command fails.

This is just a workaround, I guess the entire CMake file could do with some rework but at least it unblocks anyone in the same situation as me.

[marton78]

Review

Thanks for the fix! The root cause diagnosis is correct and the minimal approach is appreciated. However there are a few issues to address before merging.

⚠️ Critical — pffft-config.cmake.in unconditionally includes pffft-targets.cmake

The generated pffft-config.cmake is still installed unconditionally (via configure_package_config_file / install(FILES ... pffft-config.cmake ...) further down the file), but it always does:

include("${CMAKE_CURRENT_LIST_DIR}/pffft-targets.cmake")

When INSTALL_PFFFT=OFF, this PR correctly suppresses writing pffft-targets.cmake — but the config file still gets installed and still tries to include it. Any downstream find_package(pffft) call after such an install will immediately fail with a fatal include() error. The configure_package_config_file / write_basic_package_version_file / install(FILES pffft-config.cmake pffft-config-version.cmake ...) block should be wrapped in the same guard as the install(EXPORT ...) block.

ℹ️ Minor — empty install(FILES ${INSTALL_HEADERS} ...) (line 297)

When all INSTALL flags are OFF, INSTALL_HEADERS is empty. CMake silently creates an empty ${CMAKE_INSTALL_INCLUDEDIR}/pffft directory. Harmless, but cosmetically messy. Wrapping with if (INSTALL_HEADERS) would clean this up.

[marton78]

The guard variable here should be INSTALL_TARGETS rather than INSTALL_PFFFT.

INSTALL_TARGETS can be non-empty even when INSTALL_PFFFT=OFF — for example if INSTALL_PFDSP=ON or INSTALL_PFFASTCONV=ON. In those cases, targets are added to the pffft-targets export set (so the export exists), but this guard would suppress writing pffft-targets.cmake, breaking the install for those components.

The semantically correct condition is whether any target was actually added to the export set:

if (INSTALL_TARGETS)
  install(EXPORT pffft-targets
    FILE pffft-targets.cmake
    NAMESPACE PFFFT::
    DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/pffft
  )
endif()

This is also more robust across CMake versions, where the behaviour of install(TARGETS "" EXPORT foo) with an empty list is not consistently specified.