Cmake position-indepentent code

[jvillasante]

In Chapter 7 we get introduced to the following CMake function to compile with position-independent code enabled:

function(add_test_cpp_target name)
    add_executable(${name} "${name}.cpp")
    target_compile_options(${name} PRIVATE -g -O0 -pie)
    add_dependencies(tests ${name})
endfunction()

However, that didn't work for me in Fedora as shown here after compiling with that function:

readelf -h build/test/targets/hello_sdb | grep 'Type:'
  Type:                              EXEC (Executable file)

When I changed the previous function to the followng:

function(add_test_cpp_target name)
    add_executable(${name} "${name}.cpp")

    # Add -fPIC for the compiler
    target_compile_options(${name} PRIVATE -g -O0 -fPIC)

    # Add -pie for the linker
    target_link_options(${name} PRIVATE -pie)

    # target_compile_options(${name} PRIVATE -g -O0)
    add_dependencies(tests ${name})
endfunction()

It appears to work again as shown below. The explanation is in the comment (apparently you need one flag for the compiler and another for the linker):

readelf -h build/test/targets/hello_sdb | grep 'Type:'
  Type:                              DYN (Position-Independent Executable file)

[gazeddyw]

Great find, thanks for raising it! I had been failing to get this working for some time before I saw this issue.

I also came up against this problem on Fedora, but it didn't show itself until the end of chapter 16.

The code will not compile with -fPIC so I've had to use -fPIE instead:

function(add_test_cpp_target name)
    add_executable(${name} "${name}.cpp")
    target_compile_options(${name} PRIVATE -g -O0 -fPIE -gdwarf-4)
    target_link_options(${name} PRIVATE -pie)
    add_dependencies(tests ${name})
endfunction()

The error I get with -fPIC is:

test/targets/force_inline.cpp:3:37: error: inlining failed in call to ‘always_inline’ ‘void a()’: function body can be overwritten at link time

It works fine and the test suite passes with -fPIE however.