CMake protobuf_generate is missing cpp_out directory

[scharri]

What version of protobuf and what language are you using?
v22.2

What operating system (Linux, Windows, ...) and version?
Debian 11

What runtime / compiler are you using (e.g., python version or gcc version)
GCC 12.2.0

What did you do?
Steps to reproduce the behavior:

1. Clone repository, checkout v22.2, build and install it.
2. Go into protobuf/example folder, create dir build and go into it.
3. Call cmake ../ and make
4. See error

What did you expect to see
I expect that addressbook.proto is compiled by the protoc and resulted .pb.c and .pb.h are available to build add_person.cc

What did you see instead?
When protoc is running on addressbook.proto you get the output: cpp_out: No such file or directory and off course building add_preson.cc fails because addressbook.pb.h is not available.

Make sure you include information that can help us debug (full error message, exception listing, stack trace, logs).

cmake ../
-- The C compiler identification is GNU 12.2.0
-- The CXX compiler identification is GNU 12.2.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found ZLIB: /usr/lib/x86_64-linux-gnu/libz.so (found version "1.2.13")
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
-- Found Threads: TRUE
-- Configuring done
-- Generating done
-- Build files have been written to: /home/stefan/sources/motionConnect/protobuf/examples/build
❯ make
[ 12%] Running cpp protocol buffer compiler on addressbook.proto
make[2]: cpp_out: No such file or directory
[ 25%] Building CXX object CMakeFiles/add_person_cpp.dir/add_person.cc.o
/home/stefan/sources/motionConnect/protobuf/examples/add_person.cc:9:10: fatal error: addressbook.pb.h: No such file or directory
9 | #include "addressbook.pb.h"
| ^~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [CMakeFiles/add_person_cpp.dir/build.make:83: CMakeFiles/add_person_cpp.dir/add_person.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:85: CMakeFiles/add_person_cpp.dir/all] Error 2
make: *** [Makefile:91: all] Error 2

Anything else we should know about your project / environment

[scharri]

I tried to investigate this issue further and in my opinion there is an error in the custom command of protobuf-generate.cmake starting at line 138.

add_custom_command(
      OUTPUT ${_generated_srcs}
      COMMAND ${protobuf_PROTOC_EXE}
      ARGS ${protobuf_generate_PROTOC_OPTIONS} --${protobuf_generate_LANGUAGE}_out ${_plugin_options}:${protobuf_generate_PROTOC_OUT_DIR} ${_plugin} ${_protobuf_include_path} ${_abs_file}
      DEPENDS ${_abs_file} ${protobuf_PROTOC_EXE} ${protobuf_generate_DEPENDENCIES}
      COMMENT ${_comment}
      VERBATIM )

In my opinion --${protobuf_generate_LANGUAGE}_out has to be followed by = and the ${protobuf_generate_PROTOC_OUT_DIR}.
I found also that ${_protobuf_include_path} contains a semicolon after -I before PATH.

[deannagarcia]

Thanks for looking into this! Would you be willing to send us a PR with this fix?

[hector-pelletier]

I believe the problem is actually the variable protobuf_PROTOC_EXEC not being set when running add_custom_command. Hence the command becomes cpp_out which is of course not recognized.

For what it is worth, I made it work for myself by

• running cmake with the flag protobuf_MODULE_COMPATIBLE set, which ensures that Protobuf_PROTOC_EXECUTABLE contains the path to the protobuf compiler
• in protobuf-generate.cmake, replacing the occurences of protobuf_PROTOC_EXEC by the variable mentioned above
  This allowed me to compile the examples and my own projects.

I have not investigated further to understand why the protobuf compiler variable was not set, as I am not extremely competent on cmake. But I thought this might help someone.

[acozzette]

This should be fixed in b302597.