Linking an assembler program: error "undefined reference to `printf'"
I'm trying to compile this x86 assembly code on x64 Debian :
BITS 32
%include 'training.s'
global main
extern exit
; ===============================================
section .text
main:
; The program begins here:
call read_hex
mov edx,eax
call read_hex
add eax,edx
add eax,eax
inc eax
call print_eax
; Exit the process:
push 0
call exit
I'm getting these errors:
~$nasm -f elf -g 0_strange_calc.asm && ld -o 0_strange_calc 0_strange_calc.o
ld: i386 architecture of input file `0_strange_calc.o' is incompatible with i386:x86-64 output
ld: warning: cannot find entry symbol _start; defaulting to 00000000004000b0
0_strange_calc.o:training.s:25: undefined reference to `printf'
0_strange_calc.o:training.s:35: undefined reference to `printf'
0_strange_calc.o:training.s:45: undefined reference to `printf'
0_strange_calc.o:training.s:56: undefined reference to `read'
0_strange_calc.o:training.s:77: undefined reference to `scanf'
0_strange_calc.o:training.s:97: undefined reference to `scanf'
0_strange_calc.o:training.s:108: undefined reference to `printf'
0_strange_calc.o:training.s:129: undefined reference to `printf'
0_strange_calc.o:training.s:137: undefined reference to `printf'
0_strange_calc.o:0_strange_calc.asm:50: undefined reference to `exit'
~$ yasm -f elf64 0_strange_calc.asm
~$ gcc -m32 -nostdlib -nostdinc 0_strange_calc.o -o 0_strange_calc
/usr/bin/ld: i386:x86-64 architecture of input file `0_strange_calc.o' is incompatible with i386 output
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 00000000080480c0
0_strange_calc.o: In function `no symbol':
0_strange_calc.asm:(.text+0x8): undefined reference to `printf'
0_strange_calc.asm:(.text+0x19): undefined reference to `printf'
0_strange_calc.asm:(.text+0x2a): undefined reference to `printf'
0_strange_calc.asm:(.text+0x39): undefined reference to `read'
0_strange_calc.asm:(.text+0x5b): undefined reference to `scanf'
0_strange_calc.asm:(.text+0x7a): undefined reference to `scanf'
0_strange_calc.asm:(.text+0x89): undefined reference to `printf'
0_strange_calc.asm:(.text+0xa8): undefined reference to `printf'
0_strange_calc.asm:(.text+0xb9): undefined reference to `printf'
0_strange_calc.o: In function `main':
0_strange_calc.asm:(.text+0xdb): undefined reference to `exit'
collect2: error: ld returned 1 exit status
This is my dump of 0_strange_calc.o:
~$ objdump -M intel -d 0_strange_calc.o
0_strange_calc.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <main-0xc2>:
0: 60 (bad)
1: 50 push rax
2: 68 00 00 00 00 push 0x0
7: e8 00 00 00 00 call c <main-0xb6>
c: 83 c4 08 add esp,0x8
f: 61 (bad)
10: c3 ret
11: 60 (bad)
12: 50 push rax
13: 68 00 00 00 00 push 0x0
18: e8 00 00 00 00 call 1d <main-0xa5>
1d: 83 c4 08 add esp,0x8
20: 61 (bad)
21: c3 ret
22: 60 (bad)
23: b8 00 00 00 00 mov eax,0x0
28: 50 push rax
29: e8 00 00 00 00 call 2e <main-0x94>
2e: 83 c4 04 add esp,0x4
31: 61 (bad)
32: c3 ret
33: 60 (bad)
34: 51 push rcx
35: 57 push rdi
36: 6a 00 push 0x0
38: e8 00 00 00 00 call 3d <main-0x85>
3d: 83 c4 0c add esp,0xc
40: 31 d2 xor edx,edx
42: c6 04 07 00 mov BYTE PTR [rdi+rax*1],0x0
46: 61 (bad)
47: c3 ret
48: 55 push rbp
49: 89 e5 mov ebp,esp
4b: 83 ec 04 sub esp,0x4
4e: 53 push rbx
4f: 51 push rcx
50: 52 push rdx
51: 8d 5d fc lea ebx,[rbp-0x4]
54: 53 push rbx
55: 68 00 00 00 00 push 0x0
5a: e8 00 00 00 00 call 5f <main-0x63>
5f: 83 c4 08 add esp,0x8
62: 8b 03 mov eax,DWORD PTR [rbx]
64: 5a pop rdx
65: 59 pop rcx
66: 5b pop rbx
67: c9 leave
68: c3 ret
69: 55 push rbp
6a: 89 e5 mov ebp,esp
6c: 83 ec 04 sub esp,0x4
6f: 8d 5d fc lea ebx,[rbp-0x4]
72: 60 (bad)
73: 53 push rbx
74: 68 00 00 00 00 push 0x0
79: e8 00 00 00 00 call 7e <main-0x44>
7e: 83 c4 08 add esp,0x8
81: 61 (bad)
82: 8b 03 mov eax,DWORD PTR [rbx]
84: c9 leave
85: c3 ret
86: 60 (bad)
87: 56 push rsi
88: e8 00 00 00 00 call 8d <main-0x35>
8d: 83 c4 04 add esp,0x4
90: 61 (bad)
91: c3 ret
92: 60 (bad)
93: b9 20 00 00 00 mov ecx,0x20
98: d1 c0 rol eax,1
9a: 89 c2 mov edx,eax
9c: 83 e2 01 and edx,0x1
9f: 51 push rcx
a0: 50 push rax
a1: 52 push rdx
a2: 68 00 00 00 00 push 0x0
a7: e8 00 00 00 00 call ac <main-0x16>
ac: 83 c4 08 add esp,0x8
af: 58 pop rax
b0: 59 pop rcx
b1: e2 e5 loop 98 <main-0x2a>
b3: 68 00 00 00 00 push 0x0
b8: e8 00 00 00 00 call bd <main-0x5>
bd: 83 c4 04 add esp,0x4
c0: 61 (bad)
c1: c3 ret
00000000000000c2 <main>:
c2: e8 81 ff ff ff call 48 <main-0x7a>
c7: 89 c2 mov edx,eax
c9: e8 7a ff ff ff call 48 <main-0x7a>
ce: 01 d0 add eax,edx
d0: 01 c0 add eax,eax
d2: 40 e8 28 ff ff ff rex call 0 <main-0xc2>
d8: 6a 00 push 0x0
da: e8 00 00 00 00 call df <main+0x1d>
It seems to have been converted successfully in x64 asm, other simple code I had compiled and linked without problems. What am I doing wrong? And how can I fix it?
Answer
You have a couple of options
- Use LD to link to a final executable
- Use GCC to link to a final executable
Using LD Method
Your command lines use LD, unfortunately that presents a number of problems. The first:
ld: i386 architecture of input file `0_strange_calc.o' is incompatible with i386:x86-6
You are on 64-bit Debian, trying to produce a 32-bit executable. -f elf on the NASM command line generates 32-bit ELF (-f elf64 generate 64 bit objects). Your LD command line is by default trying to generate a 64-bit executable thus the error above is given. You can force LD to generate a 32-bit executable by adding the -m elf_i386 option to LD's command line.
ld: warning: cannot find entry symbol _start; defaulting to 00000000004000b0
You should tell LD that your entry point is main . LD by default looks for an entry point of _start. You can add -e main to the LD command line to resolve that.
Errors like this suggest you need the C library (where printf exists):
0_strange_calc.asm:(.text+0x8): undefined reference to `printf'
Since your code doesn't use printf directly I can only assume that is required by the functions in training.s . In order to link in the C library you will need to add it after the .o files in your command line. You can do this with -lc on your LD command line. You'll also need to tell LD specifically what dynamic linker library you will need to use (In this case a 32-bit one). In a Debian environment that will usually look like: -dynamic-linker /lib/ld-linux.so.2
So your NASM and LD lines should look like this:
nasm -f elf -g 0_strange_calc.asm
ld -melf_i386 -e main -dynamic-linker /lib/ld-linux.so.2 -o 0_strange_calc 0_strange_calc.o -lc
Using GCC Method
You can simplify linking to the C library by using GCC to link you your object file to an executable. To build a 32-bit executable you could use:
nasm -f elf -g 0_strange_calc.asm
gcc -m32 0_strange_calc.o -o 0_strange_calc
The C library and runtime has a _start method that does C startup initialization and in turn calls a function called main which happens to be the function in your assembly file. -m32 tells GCC you are also linking to a 32-bit executable.
Special Considerations
You may also need to to install the Multlilib versions of gcc (and g++ if you want to) so that you can properly build and run 32-bit applications on 64-bit Debian using the appropriate C libraries. That can be done with this command line:
apt-get install gcc-multilib g++-multilib
On Ubuntu based systems you'd need to use:
sudo apt-get install gcc-multilib g++-multilib