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flat assembler > Linux > Generating object file to link it with gcc

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vanquish



Joined: 25 Feb 2018
Posts: 3
I am new to assembly language, I would like to pass values from C program to Assembly using the linked assembly method instead of inline assembly method in C. Below is the assembly program which I am working on
Code:
format ELF64 executable 3

segment readable executable

entry bitgamesasm

bitgamesasm:
    push rbp 
    mov rbp, rsp 
    
    mov rbx, rdi        ;load rbx with a
    mov rcx, rsi        ;load rcx with b
    xor rdx, rdx        ;initialize rdx(result) 0

countBits:
    ror rbx, 1            ;rotate right
    jnc bitIsZero       ;if C flag not set do not increment 
    inc rdx               ;if C flag set increment rdx

bitIsZero:
    loop countBits              

exit:
    mov rax, rdx        ; return value store in rax
    mov rsp, rbp 
    pop rbp 
    ret     


And this is the C program from with I am trying to pass the values to assembly program

Code:
#include<stdio.h>

extern unsigned short bitgamesasm(int a, int b); 

int main(void){
    
    unsigned int a = 0;
    unsigned short b = 0;
    unsigned short result = 0;

    printf("Enter the number to check number of set bits: ");
    scanf("%d",&a);

    printf("Enter number of bits to check 32 or 64 : ");
    scanf("%hu", &b);

    result = bitgamesasm(a, b); 

    printf("For the number %d, number of set bits are %hu\n", a, result);
    return 0;
}
    


Below is the way I assemble and compile the program
Code:
$ fasm bitgamesasm.fasm bitgamesasm.o
flat assembler  version 1.71.59  (16384 kilobytes memory)
2 passes, 151 bytes.

$ gcc bitgames.c bitgamesasm.o -o bitgames
/tmp/cckEcarp.o: In function `main':
bitgames.cSad.text+0x7c): undefined reference to `bitgamesasm'
collect2: error: ld returned 1 exit status    


I am using fasm 1.71.59 in MX17 Linux OS.
I am unable to figure out the error. Kindly help me out. Thank you.
Post 25 Feb 2018, 08:21
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15973
Location: Qo'noS
To create a linkable object files the format needs to be non-executable (i.e. just ordinary ELF). And you have to declare external symbols (extrn) for your asm code to call, and declare exported symbols (public) for your C code to call.
Post 25 Feb 2018, 08:30
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vanquish



Joined: 25 Feb 2018
Posts: 3
@revolution Thank You very much. I did delete executable 3 and segment readable executable And add public keyword instead of entry I was able to assemble and run the program.

May I know why we should delete segment readable as well. I understood that `executable`needs to be removed since its going to be an object file, not an executable file.
Post 25 Feb 2018, 08:59
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 15973
Location: Qo'noS
I'm not sure about the segment, but I suspect that segments are for executable files only. For linkable object you would use section.


Last edited by revolution on 25 Feb 2018, 23:06; edited 1 time in total
Post 25 Feb 2018, 09:34
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redsock



Joined: 09 Oct 2009
Posts: 285
Location: Australia
I did a fairly extensive writeup on C/C++ linking with my library that may be of help: https://2ton.com.au/rants_and_musings/gcc_integration.html

_________________
2 Ton Digital - https://2ton.com.au/
Post 25 Feb 2018, 20:26
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vanquish



Joined: 25 Feb 2018
Posts: 3
@redsock. Thank you for the link will check it out.
Post 26 Feb 2018, 02:20
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