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 c - What is the reason function names are prefixed with an underscore by the compiler? - Stack Overflow — MikeCAT
– MikeCAT, Commented Mar 21, 2021 at 14:18

Devolus · Accepted Answer · 2021-03-21 14:19:41Z

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.

MadScientist · Accepted Answer · 2021-03-21 14:41:58Z

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

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