Question

(Running MingW on 64-bit Windows 7 and the GCC on Kubuntu)

This may possibly be just a MingW problem, but it’s failed on at least one Kubuntu installation as well, so I’m doubtful.

I have a short, simple C program, which is supposed to call an assembly function. I compile the assembler using nasm and the c program using MingW’s implementation of the gcc. The two are linked together with a makefile – bog-simple. And yet, linkage fails on the claim the claim that the external function is an ‘undefined reference’

Relevant part of the makefile:

assign0: ass0.o main.o 
gcc -v -m32 -g -Wall -o assign0 ass0.o main.o 


    main.o: main.c 
         gcc -g -c -Wall -m32 -o main.o main.c 

    ass0.o: ass0.s
     nasm -g -f elf -w+all -o ass0.o ass0.s

The beginning of the assembly file:

section .data                       ; data section, read-write
    an:    DD 0                 ; this is a temporary var

section .text                       ; our code is always in the .text section
    global do_str               ; makes the function appear in global scope
    extern printf               

do_str:                             ; functions are defined as labels
[Just Code]

And the c file’s declaration:

extern int do_str(char* a);

This has worked on at least one Kubuntu installation, failed on another, and failed on MingW. Does anyone have an idea?

Answer 1

C compilers may call the actual “function” differently, e.g. _do_str instead of do_str. Name mangling not happening always could depends on the system (and of course on the compiler). Try calling the asm function _do_str. Using proper attributes (in gcc) could also fix the problem. Also read this.