GNAT generates Position Independent Executable (PIE) code by default. PIE binaries are loaded into random memory locations, introducing an additional layer of protection against attacks.
Building PIE binaries requires that all of their dependencies also be built as Position Independent. If the link of your project fails with an error like:
/[...]/ld: /path/to/object/file: relocation R_X86_64_32S against symbol `symbol name' can not be used when making a PIE object; recompile with -fPIE
it means the identified object file has not been built as Position Independent.
If you are not interested in building PIE binaries, you can simply
turn this feature off by first compiling your code with -fno-pie
and then by linking with -no-pie (note the subtle but important
difference in the names of the switches – the linker switch does ‘not’
have an f after the dash!). When using gprbuild, you do this
by updating the ‘Required_Switches’ attribute in package Compiler
and, depending on your type of project, either attribute ‘Switches’
or attribute ‘Library_Options’ in package Linker.
On the other hand, if you would like to build PIE binaries and you are
getting the error above, a quick and easy workaround to allow linking
to succeed again is to disable PIE during the link, thus temporarily
lifting the requirement that all dependencies also be Position
Independent code. To do so, you simply need to add -no-pie to
the list of switches passed to the linker. As part of this workaround,
there is no need to adjust the compiler switches.
From there, to be able to link your binaries with PIE and therefore
drop the -no-pie workaround, you’ll need to get the identified
dependencies rebuilt with PIE enabled (compiled with -fPIE
and linked with -pie).