Assembling and Linking (gcc)

Compile C to assembly, then compile and link assembly in a single step:

$ cat hello_world.c
#include <stdio.h>

int main(void) {
    printf("Hello, world!\n");
    return 0;
}

# Compile C code to assembly (.s) file:
$ gcc -S -m64 -masm=intel hello_world.c

$ file hello_world.s
hello_world.s: assembler source, ASCII text

$ cat hello_world.s

	.file	"hello_world.c"
	.intel_syntax noprefix
	.text
	.section	.rodata
.LC0:
	.string	"Hello, world!"
	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	push	rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	mov	rbp, rsp
	.cfi_def_cfa_register 6
	lea	rdi, .LC0[rip]
	call	puts@PLT
	mov	eax, 0
	pop	rbp
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.ident	"GCC: (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0"
	.section	.note.GNU-stack,"",@progbits

# Compile assembly into object (.o) file as machine code, and link into binary executable.
# This one gcc command calls `as` to compile and then `ld` to link in one command:
$ gcc -o hello_world -m64 hello_world.s

$ file hello_world
hello_world: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=7b99531e675d43c0d8c674076b910c2c60dcd8a8, not stripped

$ ./hello_world 
Hello, world!
$ echo $?
0

Compile C to assembly, then compile and link assembly in a separate steps:

# Alternatively, from the generated (.s) assembly code file,
# perform the steps seperately:
$ gcc -S -m64 -masm=intel hello_world.c # Produces .s assembly code

$ file hello_world.s
hello_world.s: assembler source, ASCII text

# Compile the linked assembly into an object (.o) file with machine code:
gcc -g -c hello_world.s

$ file hello_world.o
hello_world.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), with debug_info, not stripped

# Optionlly dump the machine code:
$ objdump -d -M intel -S hello_world.o

hello_world.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <main>:
    .globl  main
    .type   main, @function
main:
.LFB0:
    .cfi_startproc
    push    rbp
   0:   55                      push   rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    mov rbp, rsp
   1:   48 89 e5                mov    rbp,rsp
    .cfi_def_cfa_register 6
    lea rdi, .LC0[rip]
   4:   48 8d 3d 00 00 00 00    lea    rdi,[rip+0x0]        # b <main+0xb>
    call    puts@PLT
   b:   e8 00 00 00 00          call   10 <main+0x10>
    mov eax, 0
  10:   b8 00 00 00 00          mov    eax,0x0
    pop rbp
  15:   5d                      pop    rbp
    .cfi_def_cfa 7, 8
    ret
  16:   c3                      ret    


# Then link into an executable:
$ gcc -g hello_world.o -o hello_world

$ file hello_world
hello_world: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=4f046a79f6957a2c47ac7ac5ff7b156147c2e4b4, with debug_info, not stripped

$ ./hello_world 
Hello, world!
$ echo $?
0

 

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