C makefile issue, undefined reference to function

Question

Im trying to do a simple excercise in compilation. I have 1 c file 1 assembly file and a makefile. when I run the 'make' command I get the following error:

gcc -g -m32 -Wall -o mainAssignment0.o mainAssignment0.c
/tmp/ccXfVxtg.o: In function `main':
/home/caspl202/Desktop/tasks/Assignment0/mainAssignment0.c:12: undefined reference to `do_Str'
collect2: error: ld returned 1 exit status
makefile:10: recipe for target 'mainAssignment0.o' failed
make: * [mainAssignment0.o] Error 1

Meaning that for some reason the c program doesnt recognize the external ASM function. Whats even weirder is that when I run the same makefile on the same files on a different machine it works like a charm. I would really like someone to shed some light on this thing. C code:

#include <stdio.h>
#define MAX_LEN 100         
extern int do_Str(char*);
int main(int argc, char** argv) {
  char str_buf[MAX_LEN];   
  int counter = 0;
  fgets(str_buf, MAX_LEN, stdin);   
  counter = do_Str (str_buf);       
  printf("%s%d\n",str_buf,counter); 
  return 0;
}

ASM code:

section .data                       
        an: dd 0                    
section .text                       
        global do_Str               
do_Str:                             
        push ebp                    
        mov ebp, esp                 
        pushad                      
        mov ecx, dword [ebp+8]              
    loop:           
        cmp byte [ecx], 32
        jnz noS
        inc dword [an] 
    noS:
        cmp byte [ecx], 65
        jl noC
        cmp byte [ecx], 90
        jg noC
        add byte [ecx], 32
    noC:
        inc ecx                 
        cmp byte [ecx], 0       
        jnz loop
        popad                       
        mov eax,[an]                
        mov esp, ebp            
        pop ebp              
        ret

Makefile:

all: exec
libs: asm-lib
asm-lib: asmAssignment0.s
    nasm -g -f elf -o asmAssignment0.o asmAssignment0.s
exec: mainAssignment0.c libs
    gcc -g -m32 -c -o mainAssignment0.o mainAssignment0.c
    gcc -g -m32 -o Assignment0.out mainAssignment0.o asmAssignment0.o 
.PHONY: clean
clean:
    rm -rf ./*.o Assignment0.out

If that's it, these will be duplicate: [language history - Why do C compilers prepend underscores to external names? - Stack Overflow](https://stackoverflow.com/questions/2627511/why-do-c-compilers-prepend-underscores-to-external-names) [c - What is the reason function names are prefixed with an underscore by the compiler? - Stack Overflow](https://stackoverflow.com/questions/5908568/what-is-the-reason-function-names-are-prefixed-with-an-underscore-by-the-compile) — MikeCAT, Mar 21 '21 at 14:18

score 1 · Accepted Answer · answered Mar 21 '21 at 14:19

You don't need to declare the function external.

int do_Str(char*);

Also, a function in C is prefixed with an underscore, so you must name it accordingly in your asm file.

    global _do_Str               
_do_Str:

The underscore is automatically added by the C compiler, so you don't have to use it in the C module.

score 0 · Answer 2 · answered Mar 21 '21 at 14:41

The reason for your error you quote here is that your compile line is wrong. You can tell because you're trying to create an object file, but getting errors from the linker, so something is clearly not right:

gcc -g -m32 -Wall -o mainAssignment0.o mainAssignment0.c
  ...
collect2: error: ld returned 1 exit status

The problem is you forgot to add the -c flag to this compile line, so that the compiler generates an object file.

However, in your makefile the -c is present, so clearly this error you quote is not generated from the makefile you show us.

exec: mainAssignment0.c libs
        gcc -g -m32 -c -o mainAssignment0.o mainAssignment0.c

C makefile issue, undefined reference to function

2 Answers2