How to generate assembly code with gcc that can be compiled with nasm
I am trying to learn assembly language as a hobby and I frequently use gcc -S to produce assembly output. This is pretty much straightforward, but I fail to compile the assembly output. I was just curious whether this can be done at all. I tried using both standard assembly output and intel syntax using the -masm=intel. Both can't be compiled with nasm and linked with ld.
Therefore I would like to ask whether it is possible to generate assembly code, that can be then compiled.
To be more precise I used the following C code.
>> cat csimp.c
int main (void){
int i,j;
for(i=1;i<21;i++)
j= i + 100;
return 0;
}
Generated assembly with gcc -S -O0 -masm=intel csimp.c and tried to compile with nasm -f elf64 csimp.s and link with ld -m elf_x86_64 -s -o test csimp.o. The output I got from nasm reads:
csimp.s:1: error: attempt to define a local label before any non-local labels
csimp.s:1: error: parser: instruction expected
csimp.s:2: error: attempt to define a local label before any non-local labels
csimp.s:2: error: parser: instruction expected
This is most probably due to broken assembly syntax. My hope is that I would be able to fix this without having to manually correct the output of gcc -S
Edit:
I was given a hint that my problem is solved in another question; unfortunately, after testing the method described there, I was not able to produce nasm assembly format. You can see the output of objconv below.
Therefore I still need your help.
>>cat csimp.asm
; Disassembly of file: csimp.o
; Sat Jan 30 20:17:39 2016
; Mode: 64 bits
; Syntax: YASM/NASM
; Instruction set: 8086, x64
global main: ; **the ':' should be removed !!!**
SECTION .text ; section number 1, code
main: ; Function begin
push rbp ; 0000 _ 55
mov rbp, rsp ; 0001 _ 48: 89. E5
mov dword [rbp-4H], 1 ; 0004 _ C7. 45, FC, 00000001
jmp ?_002 ; 000B _ EB, 0D
?_001: mov eax, dword [rbp-4H] ; 000D _ 8B. 45, FC
add eax, 100 ; 0010 _ 83. C0, 64
mov dword [rbp-8H], eax ; 0013 _ 89. 45, F8
add dword [rbp-4H], 1 ; 0016 _ 83. 45, FC, 01
?_002: cmp dword [rbp-4H], 20 ; 001A _ 83. 7D, FC, 14
jle ?_001 ; 001E _ 7E, ED
pop rbp ; 0020 _ 5D
ret ; 0021 _ C3
; main End of function
SECTION .data ; section number 2, data
SECTION .bss ; section number 3, bss
Apparent solution:
I made a mistake when cleaning up the output of objconv. I should have run:
sed -i "s/align=1//g ; s/[a-z]*execute//g ; s/: *function//g; /default *rel/d" csimp.asm
All steps can be condensed in a bash script
#! /bin/bash
a=$( echo $1 | sed "s/\.c//" ) # strip the file extension .c
# compile binary with minimal information
gcc -fno-asynchronous-unwind-tables -s -c ${a}.c
# convert the executable to nasm format
./objconv/objconv -fnasm ${a}.o
# remove unnecesairy objconv information
sed -i "s/align=1//g ; s/[a-z]*execute//g ; s/: *function//g; /default *rel/d" ${a}.asm
# run nasm for 64-bit binary
nasm -f elf64 ${a}.asm
# link --> see comment of MichaelPetch below
ld -m elf_x86_64 -s ${a}.o
Running this code I get the ld warning:
ld: warning: cannot find entry symbol _start; defaulting to 0000000000400080
The executable produced in this manner crashes with segmentation fault message. I would appreciate your help.
Answer
You basically can't, at least directly. GCC does output assembly in Intel syntax; but NASM/MASM/TASM have their own Intel syntax. They are largely based on it, but there are as well some differences the assembler may not be able to understand and thus fail to compile.
The closest thing is probably having objdump show the assembly in Intel format:
objdump -d $file -M intel
Peter Cordes suggests in the comments that assembler directives will still target GAS, so they won't be recognized by NASM for example. They typically have the same name, but GAS-like directives start with a . as in .section text (vs section text).