The pkg-config file was generated at *configure* time with
`prefix=${CMAKE_INSTALL_PREFIX}`, which captured whatever
CMAKE_INSTALL_PREFIX happened to be set to at `cmake -B build`
time — typically the default `/usr/local`. `cmake --install
build --prefix /foo` then put the files under /foo but the .pc
still pointed pkg-config at /usr/local/include and /usr/local/lib,
which broke downstream consumers configuring against the install
tree.
Concrete bite encountered today on hertz: the daedalus-v4l2 daemon
CMake configure on a /tmp/df-prefix install tree resolved
DAEDALUS_FOURIER_INCLUDE_DIRS to /usr/local/include (empty path on
the test host), so main.c failed to find <daedalus.h>.
Fix: write the .pc with `prefix=${pcfiledir}/<rel>` where <rel> is
the configure-time-computed relative path from
<prefix>/<libdir>/pkgconfig back to <prefix>. pkg-config
substitutes ${pcfiledir} with the actual on-disk location of the
.pc at lookup time, so the resolved prefix tracks wherever the
install tree is moved to — including DESTDIR-staged builds, apt
package installs, and ad-hoc `cmake --install --prefix /tmp/foo`
test installs.
The relative-path computation handles GNUInstallDirs layouts that
add multiarch tuples (Debian's lib/aarch64-linux-gnu) without
hard-coding `../..`. Tested on hertz (Debian trixie, libdir=lib):
prefix=${pcfiledir}/../../
...
$ pkg-config --variable=prefix daedalus-fourier
/tmp/df-prefix-test/lib/pkgconfig/../../
# mv preserves relocation:
$ mv /tmp/df-prefix-test /tmp/df-prefix-moved
$ pkg-config --variable=prefix daedalus-fourier
/tmp/df-prefix-moved/lib/pkgconfig/../../
This unblocks the daedalus-v4l2 daemon out-of-tree builds against
local daedalus-fourier installs and is a prerequisite for tidy
test-rig deployments (per the hertz reload session 2026-05-23).
claude-noether