mpgen: Change Cap'n Proto link order to fix undefined references in static builds #367

mutatrum commented at 1:16 PM on September 17, 2026: contributor

When building against static Cap'n Proto libraries, mpgen compilation fails at link time as libcapnpc.a depends on symbols defined in libcapnp.a.

Flipping the order fixes the undefined references.

I ran into this issue when compiling Bitcoin Core with IPC enabled in a Ubuntu/Debian container to produce self-contained binaries that can be extracted to a host.

Change CapnProto library linkage for mpgen

When building against static Cap'n Proto libraries, `mpgen` compilation fails at link time as `libcapnpc.a` depends on symbols defined in `libcapnp.a`.

Flipping the order fixes the undefined references.

56cfdaa884

DrahtBot commented at 1:16 PM on September 17, 2026: none

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mutatrum renamed this:
~~Change Cap'n Proto library linkage order for mpgen~~
mpgen: Change Cap'n Proto library linkage order to fix undefined references on static li
on Sep 17, 2026

mutatrum renamed this:
~~mpgen: Change Cap'n Proto library linkage order to fix undefined references on static li~~
mpgen: Change Cap'n Proto link order to fix undefined references in static builds
on Sep 17, 2026

hebasto commented at 1:20 PM on September 17, 2026: member

When building against static Cap'n Proto libraries, mpgen compilation fails at link time as libcapnpc.a depends on symbols defined in libcapnp.a.

Could you please provide the exact steps to reproduce the issue, including the toolchain details?

mutatrum commented at 2:00 PM on September 17, 2026: contributor

Use the following Dockerfile:

FROM ubuntu:22.04

ENV DEBIAN_FRONTEND=noninteractive
RUN apt-get update -qq && apt-get install -y --no-install-recommends \
    build-essential cmake git curl ca-certificates pkg-config

WORKDIR /tmp
RUN curl -sSL https://capnproto.org/capnproto-c++-1.1.0.tar.gz | tar -xz && \
    cd capnproto-c++-1.1.0 && \
    ./configure --prefix=/usr --disable-shared --enable-static CXXFLAGS="-fPIC" CFLAGS="-fPIC" && \
    make -j$(nproc) && \
    make install

WORKDIR /src
RUN git clone https://github.com/bitcoin-core/libmultiprocess.git && \
    cd libmultiprocess && \
    cmake -B build && \
    cmake --build build --target mpgen

Run docker build . results in:

5.397 schema-parser.c++:(.text+0x117d): undefined reference to `capnp::SchemaLoader::get(unsigned long, capnp::schema::Brand::Reader, capnp::Schema) const'
5.397 schema-parser.c++:(.text+0x1538): undefined reference to `capnp::SchemaLoader::get(unsigned long, capnp::schema::Brand::Reader, capnp::Schema) const'
5.397 schema-parser.c++:(.text+0x1ba4): undefined reference to `capnp::SchemaLoader::get(unsigned long, capnp::schema::Brand::Reader, capnp::Schema) const'
...
5.487 collect2: error: ld returned 1 exit status
5.488 gmake[3]: *** [CMakeFiles/mpgen.dir/build.make:104: mpgen] Error 1
5.488 gmake[2]: *** [CMakeFiles/Makefile2:969: CMakeFiles/mpgen.dir/all] Error 2
5.488 gmake[1]: *** [CMakeFiles/Makefile2:976: CMakeFiles/mpgen.dir/rule] Error 2
5.488 gmake: *** [Makefile:582: mpgen] Error 2

in CMakeLists.txt:208 in 56cfdaa884

 203 | @@ -204,8 +204,8 @@ add_executable(mpgen src/mp/gen.cpp $<TARGET_OBJECTS:mputil>)
 204 |  add_executable(Libmultiprocess::mpgen ALIAS mpgen)
 205 |  target_include_directories(mpgen PRIVATE $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/include>)
 206 |  target_include_directories(mpgen PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include> $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
 207 | -target_link_libraries(mpgen PRIVATE CapnProto::capnp)
 208 |  target_link_libraries(mpgen PRIVATE CapnProto::capnpc)
 209 | +target_link_libraries(mpgen PRIVATE CapnProto::capnp)

ryanofsky commented at 2:04 PM on October 1, 2026:

In commit "Change CapnProto library linkage for mpgen" (56cfdaa884e710a2ddf6d9636141c36269a034c1)

Note: This is a Cap'n Proto packaging bug, not a libmultiprocess one. In autotools installs, capnpc.pc has Requires: kj but leaves out capnp, so the CapnProto::capnpc target that CapnProtoTargets.cmake generates from it doesn't know it depends on capnp. With static libraries, capnp then has to come after capnpc on the link line, which is what this PR does.

CMake installs of Cap'n Proto declare the dependency correctly, and with shared libraries the order doesn't matter, so CI never hit this. The other Cap'n Proto targets we link list their dependencies correctly, so this swap is a complete fix.

ryanofsky approved

ryanofsky commented at 2:29 PM on October 1, 2026: collaborator

Code review ACK 56cfdaa884e710a2ddf6d9636141c36269a034c1. Thanks for the fix and the clear reproduction steps! The cause is a bug in Cap'n Proto's autotools packaging (details in the inline comment).

I'm planning to merge this as-is, but two follow-ups could be worth doing:

Upstream fix: capnpc.pc.in has been missing Requires: capnp since it was added in capnproto/capnproto#1047 in 2020, and it is still missing in v1.5.0. Fixing it would help other projects using CapnProto::capnpc. We would still need the order here for older versions.
CI coverage: this fix depends on line order that nothing tests, so a later cleanup could undo it. ci/scripts/ci.sh already builds Cap'n Proto from source when a config sets CAPNP_CHECKOUT, but always with CMake. A CAPNP_BUILD=autotools-static setting next to it could make that step build with autotools instead (autoreconf -i, ./configure --disable-shared --enable-static, make install), so newdeps or a new config could turn it on with one line.

ryanofsky merged this on Oct 1, 2026

ryanofsky closed this on Oct 1, 2026

mutatrum deleted the branch on Oct 1, 2026