Undefined reference error in C external asm is included in C

Question

I am including external asm into c, when I try to compile I am getting error.

I am compiling c file like this - g++ testing.c

Error:

cc0FHCkn.o:testing.c:(.text+0xe): undefined reference to helloWorld collect2.exe: error: ld returned 1 exit status

C code:

#include<stdio.h>
extern "C" int helloWorld();
int main() {
    printf("Its - ",helloWorld());
}

ASM code:

.code
helloWorld proc
    mov rax, 123
    ret
helloWorld endp
end

Answer 1

Note : I use that answer to be able to say more than it is possible through a remark, and using gcc.

First, just doing g++ testing.c g++ is not able to link with the assembler file which is not specified, so of course helloWorld is missing.

---

If I have the file hw.c :

int helloWorld()
{
  return 123;
}

I ask to produce the source assembler through the option -S (I also use -O to reduce the assembler source size), so I do not have to write the assembler file by hand and I am sure it is compatible with gcc :

/tmp % gcc -O -S hw.c

That produced the file hw.s :

/tmp % cat hw.s
    .file   "hw.c"
    .text
.globl helloWorld
    .type   helloWorld, @function
helloWorld:
.LFB0:
    .cfi_startproc
    movl    $123, %eax
    ret
    .cfi_endproc
.LFE0:
    .size   helloWorld, .-helloWorld
    .ident  "GCC: (GNU) 4.4.7 20120313 (Red Hat 4.4.7-16)"
    .section    .note.GNU-stack,"",@progbits
/tmp % 

Also having the file m.c :

#include <stdio.h>

extern int helloWorld();

int main()
{
  printf("%d\n", helloWorld());
  return 0;
}

I can do :

/tmp % gcc m.c hw.s
/tmp % ./a.out
123

---

I propose you to do the same as, write helloWorld in C then generate the assembler with option -S, doing that you are sure to follow the gcc requirements in the function definition

Answer 2

1.) Create an ELF object file from the assembly file

nasm -f elf64 -o assembly.o assembly.asm

2.) Create an ELF object file of testing.c file

gcc -c testing.c -o testing.o

3.) Link ELF object file together to create final executable file.

gcc -o testing assembly.o testing.o

4.) Run final executable file

./testing

use extern int hellowrold();