Compile shared library from asm code with current sources

I just did some basic functions in asm that I compile in a shared library. Like : BITS 64 global foo section .text foo: mov rax, 1 ret I compiled with : nasm -f elf64 foo.S -o foo.o && gcc -shared foo.o -o libfoo.so I have a main of test : #include int foo(); int main() { printf("%d\n", foo()); return (0); } If I compiled it with the foo.o directly, everything works well. But if I compiled like this : gcc main.c -L. -lfoo I would get this error : /usr/.../bin/ld: warning: type and size of dynamic symbol `foo' are not defined I thought it was because the prototype is not declared, but I recompiled foo.o with a lib.h file containing the prototype, and the same problem occurs. Is that I must complete another section of the elf file? Thank you.