Why `make install` is deleting library files *.so generated by `make`?

Question

I am compiling C++ Poco libraries. I do

cmake -DCMAKE_INSTALL_PREFIX=./ -DCMAKE_BUILD_TYPE=Debug ../
make
make install

At make install I get an error

 "path/poco/instDir/lib/libPocoEncodingsd.so.60".
Call Stack (most recent call first):
  cmake_install.cmake:50 (include)


Makefile:85: recipe for target 'install' failed
make: *** [install] Error 

Basically the file libPocoEncodingsd.so.60 is created with make but then make install deletes it.

Why is that?

If I do not run make install the folder inside the installation path is not created and all the *.h files are not copied there.

Answer 1

This happens because:

-DCMAKE_INSTALL_PREFIX=./

means that you are installing over the top of the build directory itself, with the result that for each filename in the cmake command:

  file(INSTALL DESTINATION "${CMAKE_INSTALL_PREFIX}/lib" TYPE SHARED_LIBRARY FILES
    "path/poco/instDir/lib/libPocoEncodingsd.so.60"
    "path/poco/instDir/lib/libPocoEncodingsd.so"
    )

the source and destination files are the same. Cmake preemptively deletes the destination file to replace it with the source file. Which means it has deleted the source file. Hence your failure.

This behaviour may be a bug in cmake. On the face of it, if it checks and finds that the destination file exists and has the same timestamp as the source file, it should consider the destination file up to date and not attempt to replace it. But I haven't delved into that. Installing on top of your build directory is reasonably classified under Don't do that.

Conventionally on Unix-like OSes, locally built packages shall be installed to /usr/local. That's what /usr/local is for. Cmake, like other source package deployment tools, respects this convention by default. So if you simply run:

cmake -DCMAKE_BUILD_TYPE=Debug ../
make
sudo make install  # Or however you run `make install` as root.

then the poco libraries will be installed in /usr/local/lib and the headers in /usr/local/include/Poco. /usr/local/lib is a default library search path for the linker and /usr/local/include is a default header search path for the compiler.

If for some reason you don't want to install to the default prefix, then choose some:

-DCMAKE_INSTALL_PREFIX=/not/in/the/build/dir

and you will avoid this problem.