build error using CMake - c

Question

I get a build error when trying to build a flex and lemon project with CMake. Can you help me find what is wrong?
$ make
/usr/bin/cmake -H/home/dac/ClionProjects/openshell -B/home/dac/ClionProjects/openshell --check-build-system CMakeFiles/Makefile.cmake 0
/usr/bin/cmake -E cmake_progress_start /home/dac/ClionProjects/openshell/CMakeFiles /home/dac/ClionProjects/openshell/CMakeFiles/progress.marks
make -f CMakeFiles/Makefile2 all
make[1]: Entering directory '/home/dac/ClionProjects/openshell'
make -f CMakeFiles/lemon.dir/build.make CMakeFiles/lemon.dir/depend
make[2]: Entering directory '/home/dac/ClionProjects/openshell'
cd /home/dac/ClionProjects/openshell && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /home/dac/ClionProjects/openshell /home/dac/ClionProjects/openshell /home/dac/ClionProjects/openshell /home/dac/ClionProjects/openshell /home/dac/ClionProjects/openshell/CMakeFiles/lemon.dir/DependInfo.cmake --color=
make[2]: Leaving directory '/home/dac/ClionProjects/openshell'
make -f CMakeFiles/lemon.dir/build.make CMakeFiles/lemon.dir/build
make[2]: Entering directory '/home/dac/ClionProjects/openshell'
make[2]: Nothing to be done for 'CMakeFiles/lemon.dir/build'.
make[2]: Leaving directory '/home/dac/ClionProjects/openshell'
/usr/bin/cmake -E cmake_progress_report /home/dac/ClionProjects/openshell/CMakeFiles 1
[ 16%] Built target lemon
make -f CMakeFiles/openshell.dir/build.make CMakeFiles/openshell.dir/depend
make[2]: Entering directory '/home/dac/ClionProjects/openshell'
cd /home/dac/ClionProjects/openshell && /usr/bin/cmake -E cmake_depends "Unix Makefiles" /home/dac/ClionProjects/openshell /home/dac/ClionProjects/openshell /home/dac/ClionProjects/openshell /home/dac/ClionProjects/openshell /home/dac/ClionProjects/openshell/CMakeFiles/openshell.dir/DependInfo.cmake --color=
Scanning dependencies of target openshell
make[2]: Leaving directory '/home/dac/ClionProjects/openshell'
make -f CMakeFiles/openshell.dir/build.make CMakeFiles/openshell.dir/build
make[2]: Entering directory '/home/dac/ClionProjects/openshell'
/usr/bin/cmake -E cmake_progress_report /home/dac/ClionProjects/openshell/CMakeFiles 2
[ 33%] Building C object CMakeFiles/openshell.dir/main.c.o
/usr/bin/cc -Wall -Werror -O3 -std=c99 -I/usr/include/readline -o CMakeFiles/openshell.dir/main.c.o -c /home/dac/ClionProjects/openshell/main.c
Linking C executable openshell
/usr/bin/cmake -E cmake_link_script CMakeFiles/openshell.dir/link.txt --verbose=1
/usr/bin/cc -Wall -Werror -O3 -std=c99 CMakeFiles/openshell.dir/main.c.o CMakeFiles/openshell.dir/errors.c.o CMakeFiles/openshell.dir/util.c.o CMakeFiles/openshell.dir/stack.c.o CMakeFiles/openshell.dir/flex/shellparser.c.o -o openshell -rdynamic -lreadline
CMakeFiles/openshell.dir/main.c.o: In function `main':
main.c:(.text.startup+0xef): undefined reference to `yylex_init'
main.c:(.text.startup+0x100): undefined reference to `yyset_in'
main.c:(.text.startup+0x12e): undefined reference to `yylex'
main.c:(.text.startup+0x13a): undefined reference to `yyget_text'
main.c:(.text.startup+0x178): undefined reference to `yylex_destroy'
collect2: error: ld returned 1 exit status
CMakeFiles/openshell.dir/build.make:188: recipe for target 'openshell' failed
make[2]: *** [openshell] Error 1
make[2]: Leaving directory '/home/dac/ClionProjects/openshell'
CMakeFiles/Makefile2:98: recipe for target 'CMakeFiles/openshell.dir/all' failed
make[1]: *** [CMakeFiles/openshell.dir/all] Error 2
make[1]: Leaving directory '/home/dac/ClionProjects/openshell'
Makefile:78: recipe for target 'all' failed
make: *** [all] Error 2
My CMake build file is
cmake_minimum_required (VERSION 2.6)
project (openshell)
set(CMAKE_VERBOSE_MAKEFILE on)
include_directories(/usr/include/readline)
#### Lemon bootstrap ####
ADD_EXECUTABLE(lemon lemon.c)
file(GLOB SOURCES "./*.c")
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Werror -O3 -std=c99")
add_executable(openshell main.c openshell.h errors.c errors.h util.c util.h stack.c stack.h flex/shellparser.c flex/shellscanner.l flex/shellscanner.h)
target_link_libraries(openshell readline)

Answer 1

You're not linking the Flex library, so you will get linker errors for any Flex functions you use, such as yyset_in.
You can use find_package(FLEX) and after using FLEX_TARGET to define your Flex targets, you can then use something like the following:
# Note: I use "ShellScanner" below as in FLEX_TARGET(ShellScanner flex/shellscanner.l ...)
add_executable(openshell
...
flex/shellparser.c
${FLEX_ShellScanner_OUTPUTS}
)
target_link_libraries(openshell ${READLINE_LIBRARY} ${FLEX_LIBRARIES})
See FindFLEX for more information.
However, I don't see a FindReadline.cmake module with the other modules that CMake provides by default, so you might have to create one yourself or use an existing one such as the one I just linked. There's also the Editline library, which provides similar functionality, and you can use an existing FindEditline.cmake file for that too.