warning: label alone on a line without a colon might be in error

Question

I've got this simple assembly program asm1.asm, but I get an error when trying to compile it. This is the code

;--- constant variables ---
SECTION .data

    msg: db "Hello World!", 10, 0; 10: carriage return, 0: NULL end of msg (stop)

;--- dynamic variables ---
; SECTION .bss

;--- assembler code ---
SECTION .text

extern _printf
global _main                    ;
_main:                          ; void main() {
    push ebp ;basepointer       ;   /* creation of the stack */
    mov ebp, esp ;stackpointer  ;

    push msg                    ;   /* pushing memory address */
    call _printf                    ;   /* call printf */

    mov esp, ebp                ;   /* function body */
    pop ebp                     ;
    return;                     ; }

I get this error

C:\Users\David\Desktop>nasm -f elf asm1.asm
asm1.asm:23: warning: label alone on a line without a colon might be in error

I'm new to assembly so I guess it's just something minor but could someone please tell me what is causing the warning?

Answer 1

What's with the return; line? That's not an x86 instruction name, so the assembler treats it as equivalent to return: and warns you about it in case that's not what you meant.

x86's return instruction is called ret.

In NASM syntax, a label without a : is allowed as a way to define a symbol, but it's deprecated because of the ambiguity with typoed instructions / directives, or new instruction mnemonics that this version of NASM doesn't recognize yet. (With a :, you can even use instruction names as labels, like loop:) -w+orphan-labels is on by default.

https://nasm.us/doc/nasmdoc3.html#section-3.1 Layout of a NASM Source Line documents this.