why does ld finds an undefined reference in another function than the one having the reference

Question

as a hobby I'm trying to learn ARM assembly after briefly seeing it years ago in College.

I modified a little pseudo Hello World program (found in a book) this way :

 .global _start          

_start:         ldr     R1,=msgtxt      
                mov     R2,#11          
                bl      v_asc           
                mov     R0,#0           
                mov     R7,#1           
                svc     0               


                .text
v_asc:          mov    R0,#1           
                mov    R7,#4           
                svc    0               
                bx      LR              
                .end


                .data
msgtxt:         .ascii  "Yeah Baby!\n"

LD throws the following error :

prog.o: In function v_asc': (.text+0x1c): undefined reference tomsgtxt'

Simply putting the .data section above the .text one makes it work like a charm. But then, _start is still above .data :

 .global _start          

_start:         ldr     R1,=msgtxt      
                mov     R2,#11          
                bl      v_asc           
                mov     R0,#0           
                mov     R7,#1           
                svc     0               

                .data
msgtxt:         .ascii  "Yeah Baby!\n"

                .text
v_asc:          mov    R0,#1           
                mov    R7,#4           
                svc    0               
                bx      LR              
                .end

But this confuses me :

Why is LD pretending the reference is in v_asc while it is in _start? How come the line "ldr R1,=msgtxt" does not throw an undefined reference?

Thanks by advance.

Don't put stuff after .end as that will be ignored. The order of your blocks does not matter. As for the misleading message, that's just because = instructs the assembler to place the constant into a literal pool and that happens to be located after v_asc. — Jester

Ykon O'Clast Ykon O'Clast · Accepted Answer · 2019-03-11T05:51:20

I'm answering my question since Jester did it in comment (thanks!).

I made a rookie mistake not understanding the .end was for the whole program.

As for the main question about the undefined reference, it is due to the semantics of '=' which places the constant in the literal pool which, in the object file, is after v_asc (I found out more with this page : http://benno.id.au/blog/2009/01/02/literal-pools).

why does ld finds an undefined reference in another function than the one having the reference

1 Answers