How to configure CMake to build a library with -fP

Question

This question already has an answer here:

• How to link against PIC version of a library's dependency in CMake? 1 answer

I'm trying to create a static library with -fPIC specified.

add_library(cpp-netlib_pic
    STATIC
    ${SRCS})
set_property(TARGET cpp-netlib_pic PROPERTY POSITION_INDEPENDENT_CODE)

This doesn't work.

The library (cpp-netlib_pic) is built without the -fPIC flags.

Other targets which link against cpp-netlib_pic do however, have -fPIC added to their compiler flags, but the linking fails because cpp-netlib_pic didn't.

Here foo will have -fPIC added:

add_library(foo
    SHARED
    ${SRCS})
target_link_libraries(foo cpp-netlib_pic)

I've proved this to myself with make VERBOSE=1

[ 87%] Building CXX object third_party/cpp-netlib/CMakeFiles/cpp-netlib_pic.dir/src/server_request_parsers_impl.cpp.o
/usr/bin/c++   ... -std=c++14 -Werror -Wall -Wextra ... \
    -o CMakeFiles/cpp-netlib_pic.dir/src/server_request_parsers_impl.cpp.o \
    -c .../third_party/cpp-netlib/src/server_request_parsers_impl.cpp

Note no -fPIC here.

When building target foo which uses cpp-netlib_pic, -fPIC appears:

[ 93%] Building CXX object foo.cc.o
/usr/bin/c++  ... -std=c++14 -Werror -Wall -Wextra ... -fPIC ... \
    -o CMakeFiles/foo_shared_lib.dir/foo.cc.o \
    -c .../foo/foo.cc

How can I configure CMake to build the 1st library (cpp-netlib_pic) with -fPIC?

Answer 1

You seem to have forgotten the ON:

set_property(TARGET cpp-netlib_pic PROPERTY POSITION_INDEPENDENT_CODE ON)