CODEWITHSUNDEEP
assemblyx86printfnasm
David K.
David K.

Reputation: 477

Nasm - ld can't find printf

I'm on Ubuntu using nasm 2.14.02.

I'm trying to write a simple hello world program with nasm by pushing the variable onto the stack and then calling printf:

; Import c-lib printf
extern printf

; Define Message and Length
section .data:
  msg:      db "Hello People!", 0xA
  msg_len   equ  $-msg

section .text:
  global _start

_start:
  push msg
  call printf

  mov eax, 1
  mov ebx, 0
  int 80h

Assembling the program with nasm works fine, but when I try to link it with ld it throws this error:

ld: hello_world.o: in function `_start':
hello_world.asm:(.text:+0x6): undefined reference to `printf'

So it seems extern does not import printf. What am I doing wrong. This is literally my first assembler program, so please use simple words.

Edit:

The commands I was using are:

nasm hello_world.asm -f elf64
ld hello_world.o -o hello_world

Upvotes: 3

Views: 1924

Answers (1)

Stacy J
Stacy J

Reputation: 178

You need to link your code with a library that provides printf; that's provided by the C standard library. The easiest way to get that is to use gcc to call the linker for you.

nasm -felf32 hello_world.asm
gcc -static -o hello_world hello_world.o

The above will not work, however, because gcc will try to add some additional code found in crt1.o that contains _start to start the C runtime and then invoke main.

The easiest fix for your code is to write a main method instead.

  section .text:
  global main

main:
  push ebp     ; these two lines are the usual function entry
  mov ebp, esp

  push msg
  call printf

  xor eax, eax ; set eax to zero as the return value

  mov esp, ebp ; these two lines are the usual function exit
  pop ebp
  ret

I'm not on a 32-bit machine, so I can't easily test that, but I think that's what you need. Use nasm and then gcc, as above, to assemble and link it.

If you do want to use _start, that's fine. You need to link with the C standard library (statically... using GOT is a whole other thing), probably with -lc.

Upvotes: 1

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