Failed to use gdb for an executive file generated by cmake

To do debug for C++ codes with cmake, I have a trick to add the following lines before the project (myProjectYY) line of the CMakeLists.txt file in the root directory of the source code.

set(CMAKE_BUILD_TYPE "Debug")set(CMAKE_CXX_FLAGS_DEBUG "$ENV{CXXFLAGS} -O0 -Wall -g -ggdb")set(CMAKE_CXX_FLAGS_RELEASE "$ENV{CXXFLAGS} -O3 -Wall")

So, after cmake and make, I have obtained the executable file mainYY, and I can simply run gdb mainYY to do the debug as I should be able to see the source codes of mainYY.cpp.

I have 2 separated projects that were already cmake and make by the others, namely a simple project (i.e. myProject1) and a complicated project (i.e. myProject2). For myProject1, the above trick works for me, i.e. after I added the above 3 lines and re-do the cmake and make again to obtain main1, I can see the source code of main1.cpp by simply executing l in gdb.

But for myProject2, I do same, i.e. I added the above 3 lines and re-do the cmake and make again to obtain main2, but in gdb there is no source code for main2.cpp. That is, gdb main2 firstly gave me Reading symbols from main2...(no debugging symbols found)...done. And then if I run l in gdb, I have No symbol table is loaded. Use the "file" command.

What are the most probable reasons for such differences between myProject1 and myProject2? How can I find out those probable reasons, and how can I do the fix such that I can debug for myProject2?

Thanks.