[CMake] How to build only when file contents are actually changed?

[J Decker]

Pretty much every build system will rebuild if you touch the file.
This is a way to force it to compile.... it's a feature not a bug :)

On Mon, Aug 13, 2012 at 10:56 PM, Michael Wild <themiwi at gmail.com> wrote:
> CMake really leaves the decision when to recompile something to the
> backend, i.e. GNU Make, Xcode, Visual Studio, ninja etc. It merely
> defines dependencies and then lets the actual build tool handle the
> rest, and most of them choose to use simple time-stamps instead of
> hashes. Also note that computing the hash might also incur considerable
> overhead, in particular for very large projects with thousands of files.
>
> You can alleviate the situation a bit by using e.g. ccache to speed up
> unnecessary recompilations. E.g. on a system that has ccache installed
> in /usr/lib/ccache (Ubuntu, Debian,...)
>
> mkdir build
> cd build
> rm -rf *
> PATH=/usr/lib/cchace:$PATH cmake ..
>
> will build using ccache instead the normal compilers.
>
> HTH
>
> Michael
>
> On 08/14/2012 05:22 AM, Peng Yu wrote:
>> Hi,
>>
>> The following command output shows that when I touch a source without
>> changing the content, the source are compiled and linked, which is a
>> waste.
>>
>> This post shows how to use the checksum to decide whether a file is
>> changed or not, if changed then update target. This feature seems to
>> be missing in cmake. In case that I miss something in the document,
>> could anybody let me know if this is the case?
>>
>> http://blog.jgc.org/2006/04/rebuilding-when-hash-has-changed-not.html
>>
>> ~/linux/test/cmake/lang/command/target_link_libraries/src$ cat.sh  *
>> ==> CMakeLists.txt <==
>> cmake_minimum_required(VERSION 2.8)
>> add_library(print print.cpp)
>> #include_directories(${HELLO_SOURCE_DIR})
>> add_executable(main main.cpp)
>> target_link_libraries(main print)
>>
>> ==> main.cpp <==
>> #include "print.hpp"
>> int main() {
>>   print();
>> }
>>
>> ==> print.cpp <==
>> #include "print.hpp"
>> #include <iostream>
>> using namespace std;
>> void print() {
>>   cout << "Hello, World!" << endl;
>> }
>>
>> ==> print.hpp <==
>> #ifndef _hello_h
>> #define _hello_h
>> void print();
>> #endif
>>
>> ~/linux/test/cmake/lang/command/target_link_libraries/build$ cmake ../src/
>> -- The C compiler identification is GNU 4.2.1
>> -- The CXX compiler identification is GNU 4.2.1
>> -- Checking whether C compiler has -isysroot
>> -- Checking whether C compiler has -isysroot - yes
>> -- Checking whether C compiler supports OSX deployment target flag
>> -- Checking whether C compiler supports OSX deployment target flag - yes
>> -- Check for working C compiler: /usr/bin/gcc
>> -- Check for working C compiler: /usr/bin/gcc -- works
>> -- Detecting C compiler ABI info
>> -- Detecting C compiler ABI info - done
>> -- Checking whether CXX compiler has -isysroot
>> -- Checking whether CXX compiler has -isysroot - yes
>> -- Checking whether CXX compiler supports OSX deployment target flag
>> -- Checking whether CXX compiler supports OSX deployment target flag - yes
>> -- Check for working CXX compiler: /usr/bin/c++
>> -- Check for working CXX compiler: /usr/bin/c++ -- works
>> -- Detecting CXX compiler ABI info
>> -- Detecting CXX compiler ABI info - done
>> -- Configuring done
>> -- Generating done
>> -- Build files have been written to:
>> /Users/pengy/linux/test/cmake/lang/command/target_link_libraries/build
>> ~/linux/test/cmake/lang/command/target_link_libraries/build$ make
>> Scanning dependencies of target print
>> [ 50%] Building CXX object CMakeFiles/print.dir/print.cpp.o
>> Linking CXX static library libprint.a
>> [ 50%] Built target print
>> Scanning dependencies of target main
>> [100%] Building CXX object CMakeFiles/main.dir/main.cpp.o
>> Linking CXX executable main
>> [100%] Built target main
>> ~/linux/test/cmake/lang/command/target_link_libraries/build$ touch
>> ../src/print.hpp
>> ~/linux/test/cmake/lang/command/target_link_libraries/build$ make
>> Scanning dependencies of target print
>> [ 50%] Building CXX object CMakeFiles/print.dir/print.cpp.o
>> Linking CXX static library libprint.a
>> [ 50%] Built target print
>> Scanning dependencies of target main
>> [100%] Building CXX object CMakeFiles/main.dir/main.cpp.o
>> Linking CXX executable main
>> [100%] Built target main
>>
>>
>>
>>
>
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