Understanding roles of CMake, make and GCC - gcc

1. cmake is a command from CMake software: preparation for build automation system; make and make install are commands from Make software: build automation system.
2. From reading this post, what I understand is that:
a. This "cmake and make" stuffs actually use g++ / gcc in its implementation. cmake and make stuffs are basically just tools in using g++ / gcc. Is that correct?
b. gcc / g++ are the compiler that do the actual work.
c. So I can just use gcc / g++ directly without using the make and CMake things?
3. According to this stackoverflow answer: CMake takes a CMakeList.txt file, and outputs it to a platform-specific build format, e.g., a Makefile, Visual Studio, etc.
However when I came across this openCV installation :
mkdir release
cd release
cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local ..
It executes cmake command in a directory where there is no CMakeLists.txt file. Can you explain and elaborate on this?
4. The usual steps that I've seen are: cmake, make, sudo make install.
I read this stackoverflow post, what I understand:
(i) make is for building the project.
(ii) make install is to copy the binary / executables to the installed directories.
a. So when we make, where are the result / binary files / executables stored at?
b. If we only run make without make install, does it mean that the files are not generated?
c. I came across this openCV tutorial on using openCV with GCC and CMake. It uses:
cd <DisplayImage_directory>
cmake .
make
Why doesn't it do make install as well?
5. In summary:
CMake takes CMakeList.txt file (which is cross platform) to generate a Makefile (which is specific to a platform).
I can just write Makefile manually and skip the CMake step. but it is better to do with the CMake step because it is cross platform, otherwise I have to rewrite the Makefile again if I change platform.
Make takes Makefile (which is generated by CMake or written manually) as a guide to compile and build. Make basically uses gcc / g++ or other compiler in its work. Make itself is just a tool for the compiler.
Make install put the result / executables into the install path

CMake generates files for other build systems. These can be Makefiles, Ninja files or projects files for IDEs like Visual Studio or Eclipse. The build files contain calls to compilers like GCC, Clang, or cl.exe. If you have several compilers installed, you can choose one.
All three parts are independent. The compiler, the build system and CMake.
It is easier to understand when you have the history. People used their compiler. Over time they added so many flags, that it was cumbersome to type them every time. So they put the calls in a script. From that the build systems (Make, Ninja) evolved.
The people wanted to support multiple platforms, compilers, scenarios and so on and the build system files became hard to maintain and their use was error-prone. That's the reason people invented meta build system that creates the files for the actual build system. Examples are Autotools or CMake.
Yes
CMake does not use your compiler, make does not implement it, but it calls (uses) the compiler.
The CMakeLists.txt file should be in the parent directory of release. The last argument of the CMake call indicates the path where the CMakeLists.txt file is located.
Right, make generates the file in the build directory. In your example from 3. release is the build directory. You can find all the generated files and use them. Installing is optional, especially if you want to develop the software, you are not installing it.
Try writing Makefiles for a large project and you will see how much work it is. But yes, everything in 5 is right.

Related

GCC built from source in different location is incorrectly using same shared libs as native GCC

I'm a student doing research involving extending the TM capabilities of gcc. My goal is to make changes to gcc source, build gcc from the modified source, and, use the new executable the same way I'd use my distro's vanilla gcc.
I built and installed gcc in a different location (not /usr/bin/gcc), specifically because the modified gcc will be unstable, and because our project goal is to compare transactional programs compiled with the two different versions.
Our changes to gcc source impact both /gcc and /libitm. This means we are making a change to libitm.so, one of the shared libraries that get built.
My expectation:
when compiling myprogram.cpp with /usr/bin/g++, the version of libitm.so that will get linked should be the one that came with my distro;
when compiling it with ~/project/install-dir/bin/g++, the version of libitm.so that will get linked should be the one that just got built when I built my modified gcc.
But in reality it seems both native gcc and mine are using the same libitm, /usr/lib/x86_64-linux-gnu/libitm.so.1.
I only have a rough grasp of gcc internals as they apply to our project, but this is my understanding:
Our changes tell one compiler pass to conditionally insert our own "function builtin" instead of one it would normally use, and this is / becomes a "symbol" which needs to link to libitm.
When I use the new gcc to compile my program, that pass detects those conditions and successfully inserts the symbol, but then at runtime my program gives a "relocation error" indicating the symbol is not defined in the file it is searching in: ./test: relocation error: ./test: symbol _ITM_S1RU4, version LIBITM_1.0 not defined in file libitm.so.1 with link time reference
readelf shows me that /usr/lib/x86_64-linux-gnu/libitm.so.1 does not contain our new symbols while ~/project/install-dir/lib64/libitm.so.1 does; if I re-run my program after simply copying the latter libitm over the former (backing it up first, of course), it does not produce the relocation error anymore. But naturally this is not a permanent solution.
So I want the gcc I built to use the shared libs that were built along with it when linking. And I don't want to have to tell it where they are every time - my feeling is that it should know where to look for them since I deliberately built it somewhere else to behave differently.
This sounds like the kind of problem any amateur gcc developer would have when trying to make a dev environment and still be able to use both versions of gcc, but I had difficulty finding similar questions. I am thinking this is a matter of lacking certain config options when I configure gcc before building it. What is the right configuration to do this?
My small understanding of the instructions for building and installing gcc led me to do the following:
cd ~/project/
mkdir objdir
cd objdir
../source-dir/configure --enable-languages=c,c++ --prefix=/home/myusername/project/install-dir
make -j2
make install
I only have those config options because they seemed like the ones closest related to "only building the parts I need" and "not overwriting native gcc", but I could be wrong. After the initial config step I just re-run make -j2 and make install every time I change the code. All these steps do complete without errors, and they produce the ~/project/install-dir/bin/ folder, containing the gcc and g++ which behave as described.
I use ~/project/install-dir/bin/g++ -fgnu-tm -o myprogram myprogram.cpp to compile a transactional program, possibly with other options for programs with threads.
(I am using Xubuntu 16.04.3 (64 bit), within VirtualBox on Windows. The installed /usr/bin/gcc is version 5.4.0. Our source at ~/project/source-dir/ is a modified version of 5.3.0.)
You’re running into build- versus run-time linking differences. When you build with -fgnu-tm, the compiler knows where the library it needs is found, and it tells the linker where to find it; you can see this by adding -v to your g++ command. However when you run the resulting program, the dynamic linker doesn’t know it should look somewhere special for the ITM library, so it uses the default library in /usr/lib/x86_64-linux-gnu.
Things get even more confusing with ITM on Ubuntu because the library is installed system-wide, but the link script is installed in a GCC-private directory. This doesn’t happen with the default GCC build, so your own GCC build doesn’t do this, and you’ll see libitm.so in ~/project/install-dir/lib64.
To fix this at run-time, you need to tell the dynamic linker where to find the right library. You can do this either by setting LD_LIBRARY_PATH (to /home/.../project/install-dir/lib64), or by storing the path in the binary using -Wl,-rpath=/home/.../project/install-dir/lib64 when you build it.

make without makefile after cmake

I try to use the c++ language bindings for the ev3dev lego brick: https://github.com/ddemidov/ev3dev-lang-cpp
The instruction is as follows:
mkdir build
cd build
cmake .. -DEV3DEV_PLATFORM=EV3
make
I am running windows and have cmake and mingw available. After running cmake it creates some files in the build directory. However: There is no makefile which could be picked of by make. So I am wondering how iam supposed to compile these bindings
On Windows, CMake generates a MSVC solution by default. Check for a .sln file in your build directory.
The instructions you linked are assuming a Unix-ish platform, where the default is to create Makefiles.
If you actually want Makefiles on Windows, add -G "Unix Makefiles" to the cmake line.
If you want to use MSVC as compiler but work on the command line, another option is -G "NMake Makefiles", and calling nmake after that.
Make sure to delete your build directory before trying to build a new generator target. CMake can be touchy about that.
Check cmake --help for a list of available options. (Especially the generator targets are platform-specific.)

Building Clang on Windows

I'm trying to build LLVM/Clang on Windows 7 with Microsoft C++ 2013. Building LLVM spat out a few error messages along the way but mostly seemed to be succeeding and did end up creating a folder full of exe's so that part seems to have worked. When I try to build Clang:
C:\clang>\CMake\bin\cmake.exe ..\clang-3.4 -DCLANG_PATH_TO_LLVM_BUILD=/llvm
CMake Error at CMakeLists.txt:29 (message):
Please set CLANG_PATH_TO_LLVM_BUILD to a directory containing a LLVM build.
And I get the same error message whether I omit CLANG_PATH_TO_LLVM_BUILD, define it in CMakeLists.txt or an environment variable instead of the command line, set it to possibly relevant subdirectories of /llvm etc.
What am I missing?
You're not following the instructions on this page correctly, under "Using Visual Studio". You will end up with
/
/llvm
/llvm/CMakeLists.txt
/llvm/tools/clang
/llvm/tools/clang/CMakeLists.txt
Step 4, repeated here for clarity:
Run CMake to generate the Visual Studio solution and project files:
cd ..\.. (back to where you started)
mkdir build (for building without polluting the source dir)
cd build
If you are using Visual Studio 2012: cmake -G "Visual Studio 11" ..\llvm
That last bit needs to be run from inside the VS Command Prompt, but you seem to have that sorted out. You can also generate "NMake makefiles" if you don't use the IDE to build. Anyways, the point is that you should call cmake on the toplevel CMakeLists.txt file, not on the clang one directly. Clang will be built as part of the build process. You can even add libc++ and compiler-rt to the llvm/projects directory to have these built automatically on platforms that support them.
What you are doing is building clang "out of tree". Which is possible and even supported, but only really useful in certain circumstances. You'll need a previously built build of LLVM in some directory. You then set CLANG_PATH_TO_LLVM_BUILD to the directory containing the built LLVM files (this is not the source directory). But as I said, that's making things needlessly difficult.

Building LLVM OCaml bindings on Windows under MinGW using CMake?

I'm trying to build the LLVM OCaml bindings under MinGW on Windows using CMake. I've tweaked a little bit with the LLVMBuild.txt in the bindings directory without luck. How do I build them?
Edit: To answer ygrek's question:
First attempt:
svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
cd llvm/tools
svn co http://llvm.org/svn/llvm-project/cfe/trunk clang
cd ../projects
svn co http://llvm.org/svn/llvm-project/compiler-rt/trunk compiler-rt
cd ..
mkdir build
cd build
cmake -G "MinGW Makefiles" ..
mingw32-make
I realized that llvm/bindings/ocaml wasn't being built. I then connected the LLVMBuild.txt files in many directories with the CMake build system. llvm/bindings/LLVMBuild.txt didn't contain any instructions to build subdirectories so I added them:
[common]
subdirectories = ocaml
llvm/bindings/ocaml/LLVMBuild.txt didn't have an LLVMBuild.txt either so I added one:
[component_0]
type = Group
name = OCamlBindings
parent = Bindings
I tried building this, but the OCaml binding weren't built. I think it's because there are no LLVMBuild.txt in any of the ocaml subdirectories. I had considered adding LLVMBuild.txt files to all of the ocaml subdirectories but realized I didn't know what their dependencies were or how to specify them.
I didn't want to pursue this aimlessly if it isn't possible or if someone had already done the work. It seems like there's a disconnect between the CMake build system & the bindings directories.
Not a real answer, but still.
The note at the beginning of llvm/bindings/Makefile.ocaml reading
An ocaml library is a unique project type in the context of LLVM, so rules are here rather than in Makefile.rules.
seems to suggest that building ocaml bindings with cmake is not supported..

cmake & gcc compiles every file every time

I'm a learning c++ developer writing a game initially on the Mac platform using XCode, but now moving to cross platform by leveraging CMake. So far I can get it compiled on my ickle linux netbook and I'm putting together a dev environment on this machine for on the go coding. However I'm finding that gcc recompiles every file whenever I make a change. Clearly I need some additional configuration to the CMakeLists.txt . My current one is very simple. Like so;
cmake_minimum_required (VERSION 2.8)
set (source
Creature.cpp
DisplayManager.cpp
Engine.cpp
EngineState.cpp
Entity.cpp
GameWorld.cpp
GfxSFML.cpp
Item.cpp
Map.cpp
Position.cpp
Projectile.cpp
ScreenTile.cpp
SquadAI.cpp
Terrain.cpp
UIButton.cpp
UICharPanel.cpp
UIView.cpp
Utility.cpp
Weapon.cpp
fov.cpp
main.cpp
)
find_package (OpenAL)
find_package (OpenGL)
find_package (SFML)
set(CMAKE_CXX_FLAGS "-g -Wall -pg")
add_executable (tractionedge ${source})
target_link_libraries(tractionedge ${SFML_LIBRARY} ${OPENGL_LIBRARY} ${OPENAL_LIBRARY})
I've concentrated so far on C++ as a language rather than build systems by sticking with XCode for everything. My knowledge of Autotools (make?) and Gcc is very limited. How do I have gcc only recompile the changed source?
Are you rerunning cmake every time? If you just modify one source file, you should be able to simply rerun make, and it should rebuild just the one object file before linking. If you rerun cmake, it might mark all of the source files dirty and rebuild everything.
Only rerun cmake if you change the actual list of source files being used, or other major changes like that.
Rebuilding only the modified sources SHOULD be the default behavior. Of course if you change a central header included by nearly all dependent cpp files it'll trigger a nearly complete rebuild. Look at what happens if you only modify one cpp file (adding a comment or alike), if more than that compilation unit is compiling then I propose you to invest more time investigating it eventually giving you my EMail to have a deeper look at the configuration.
Another possibility is that you are compiling under windows and using a 2.8 cmake that has a bug regarding this. Look at a 2.9 version to see if that defect is away then: http://www.mail-archive.com/cmake#cmake.org/msg24876.html
I would rewrite your CMakeLists.txt using glob (maybe move the files in a "src" directory if you have other *.cpp files around) and give your project a name (this sets some important variables):
cmake_minimum_required (VERSION 2.8)
project(TRACTION)
file (GLOB TRACTION_SOURCES *.cpp)
find_package (OpenAL)
find_package (OpenGL)
find_package (SFML)
set(CMAKE_CXX_FLAGS "-g -Wall -pg")
add_executable (tractionedge ${TRACTION_SOURCES})
target_link_libraries(tractionedge ${SFML_LIBRARY} ${OPENGL_LIBRARY} ${OPENAL_LIBRARY})
I also experienced unnecessary rebuilds using cmake and visual studio. The problem is related to an inappropriate x64 configuration parameter: Visual Studio 2008 Unnecessary Project Building
A simple solution in many of these cases is to completely wipe the build tree and regenerate it (and I mean something along the lines of rm -rf build && mkdir build && cd build && cmake -G "Unix Makefiles" ../src, not just make clean)

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