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I try to make functions in assembly language and put them in a dynamic library so I create .o with .S with this command:
nasm -f elf64 hello.S -o hello.o
but when I want to create the lib with gcc:
gcc -fPIC -shared hello.o -o libasm.so
and it displays me this error:
/usr/bin/ld: hello.o: relocation R_X86_64_PC32 against undefined symbol printf@@GLIBC_2.2.5 can not be used when making a shared object; recompile with -fPIC

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1 Answer 1

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From http://www.nasm.us/xdoc/2.10rc8/html/nasmdoc9.html#section-9.2.5:

To call an external routine, you must use another special PIC relocation type, WRT ..plt. This is much easier than the GOT-based ones: you simply replace calls such as CALL printf with the PLT-relative version CALL printf WRT ..plt.

so instead of

; ...
call     printf

use

; ...
call     printf WRT ..plt

and compile and link as normal.

BTW, "WRT" means "With Respect To ...", i.e, "in the context of ..."

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  • 2
    Or better, call [rel printf wrt ..got] like gcc -fno-plt uses. Full details in Can't call C standard library function on 64-bit Linux from assembly (yasm) code
    – Peter Cordes
    Commented Apr 8, 2021 at 20:06
  • maybe this should be closed as duplicate of that if the justification for ..got appears there?
    – cat
    Commented Apr 9, 2021 at 12:32
  • I already did close it as a duplicate, right after updating my answer on the linked question to include a TL:DR at the top :P And yes, my answer there describes the fact that using the GOT pointer directly removes the layer of indirection through the PLT.
    – Peter Cordes
    Commented Apr 9, 2021 at 19:52
  • ah, way ahead of me
    – cat
    Commented Apr 9, 2021 at 21:37

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