cmake make cannot find -lprotocol - cmake

Question

I have these files in the directory:
CMakeLists.txt datalink.c datalink.h protocol.a protocol.h
All these files were got from my teacher except CMakeLists.txt.
#CMakeLists.txt
cmake_minimum_required(VERSION 3.5)
project(DataLinkSimulator)
INCLUDE_DIRECTORIES(.)
set(SOURCE_FILES datalink.c protocol.a datalink.h protocol.h)
add_executable(DataLinkSimulator ${SOURCE_FILES})
target_link_libraries(DataLinkSimulator protocol.a)
I run 'cmake .' in this directory, and it works.
However, when I run 'make' , it gives me these errors.
Scanning dependencies of target DataLinkSimulator
[ 50%] Building C object CMakeFiles/DataLinkSimulator.dir/datalink.c.o
[100%] Linking C executable DataLinkSimulator
/usr/bin/ld: cannot find -lprotocol
collect2: 错误：ld 返回 1 #(means "error: ld return 1")
CMakeFiles/DataLinkSimulator.dir/build.make:94: recipe for target 'DataLinkSimulator' failed
make[2]: *** [DataLinkSimulator] Error 1
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/DataLinkSimulator.dir/all' failed
make[1]: *** [CMakeFiles/DataLinkSimulator.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2
The error "cannot find -lprotocol" must caused by ld's mistaking protocol as a shared lib. But my protocol is a static lib. So I am confused.
If I edit makefile by myself instead of using cmake, it works correctly.
datalink: datalink.c datalink.h
gcc datalink.c protocol.a -o datalink -lm -lstdc++ -m32
clean:
${RM} datalink
${RM} *.o
I used a "make VERBOSE=1":
/usr/local/bin/cmake -H/home/shantom/Desktop/FileRecv/Lab1-linux -B/home/shantom/Desktop/FileRecv/Lab1-linux --check-build-system CMakeFiles/Makefile.cmake 0
/usr/local/bin/cmake -E cmake_progress_start /home/shantom/Desktop/FileRecv/Lab1-linux/CMakeFiles /home/shantom/Desktop/FileRecv/Lab1-linux/CMakeFiles/progress.marks
make -f CMakeFiles/Makefile2 all
make[1]: Entering directory '/home/shantom/.wine/drive_c/Program Files/QQ/Users/1044950475/FileRecv/Lab1-linux'
make -f CMakeFiles/DataLinkSimulator.dir/build.make CMakeFiles/DataLinkSimulator.dir/depend
make[2]: Entering directory '/home/shantom/.wine/drive_c/Program Files/QQ/Users/1044950475/FileRecv/Lab1-linux'
cd /home/shantom/Desktop/FileRecv/Lab1-linux && /usr/local/bin/cmake -E cmake_depends "Unix Makefiles" /home/shantom/Desktop/FileRecv/Lab1-linux /home/shantom/Desktop/FileRecv/Lab1-linux /home/shantom/Desktop/FileRecv/Lab1-linux /home/shantom/Desktop/FileRecv/Lab1-linux /home/shantom/Desktop/FileRecv/Lab1-linux/CMakeFiles/DataLinkSimulator.dir/DependInfo.cmake --color=
make[2]: Leaving directory '/home/shantom/.wine/drive_c/Program Files/QQ/Users/1044950475/FileRecv/Lab1-linux'
make -f CMakeFiles/DataLinkSimulator.dir/build.make CMakeFiles/DataLinkSimulator.dir/build
make[2]: Entering directory '/home/shantom/.wine/drive_c/Program Files/QQ/Users/1044950475/FileRecv/Lab1-linux'
[ 50%] Linking C executable DataLinkSimulator
/usr/local/bin/cmake -E cmake_link_script CMakeFiles/DataLinkSimulator.dir/link.txt --verbose=1
/usr/bin/cc CMakeFiles/DataLinkSimulator.dir/datalink.c.o -o DataLinkSimulator -Wl,-Bstatic -lprotocol -Wl,-Bdynamic
/usr/bin/ld: cannot find -lprotocol
collect2: 错误：ld 返回 1
CMakeFiles/DataLinkSimulator.dir/build.make:94: recipe for target 'DataLinkSimulator' failed
make[2]: *** [DataLinkSimulator] Error 1
make[2]: Leaving directory '/home/shantom/.wine/drive_c/Program Files/QQ/Users/1044950475/FileRecv/Lab1-linux'
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/DataLinkSimulator.dir/all' failed
make[1]: *** [CMakeFiles/DataLinkSimulator.dir/all] Error 2
make[1]: Leaving directory '/home/shantom/.wine/drive_c/Program Files/QQ/Users/1044950475/FileRecv/Lab1-linux'
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2

Answer 1

In order to understand the error you need to launch make command from the command line using verbose mode; on Linux paltform it means launching:
make VERBOSE=1
In this manner you will have the entire command line used to compile the code.
In order to resolve your error you could:
add a link_directories(directory_where_you_have_library) CMake line adding the directory where you have protocol.a before target_link_libraries line
add protocol.a with absolute path.
Your modified Makefile works because probably you have it in the same directory of all the files while CMake creates many Makefiles in various directories starting from build root directory so it needs -L gcc flags (obtainable through link_directories) or -l with absolute path (obtainable adding protocol.a with absolute path).