How do I compile the asm generated by GCC?

Question

I'm playing around with some asm code, and something is bothering me.

I compile this:

#include <stdio.h>

int main(int argc, char** argv){
  printf("Hello World\n");
  return 0;
}

with gcc file.c -S -o file.S this generates a nice little piece of asm code:

    .cstring
LC0:
    .ascii "Hello World\0"
    .text
.globl _main
_main:
LFB3:
    pushq   %rbp
LCFI0:
    movq    %rsp, %rbp
LCFI1:
    subq    $16, %rsp
LCFI2:
    movl    %edi, -4(%rbp)
    movq    %rsi, -16(%rbp)
    leaq    LC0(%rip), %rdi
    call    _puts
    movl    $0, %eax
    leave
    ret
LFE3:
    .section __TEXT,__eh_frame,coalesced,no_toc+strip_static_syms+live_support
EH_frame1:
    .set L$set$0,LECIE1-LSCIE1
    .long L$set$0
LSCIE1:
    .long   0x0
    .byte   0x1
    .ascii "zR\0"
    .byte   0x1
    .byte   0x78
    .byte   0x10
    .byte   0x1
    .byte   0x10
    .byte   0xc
    .byte   0x7
    .byte   0x8
    .byte   0x90
    .byte   0x1
    .align 3
LECIE1:
.globl _main.eh
_main.eh:
LSFDE1:
    .set L$set$1,LEFDE1-LASFDE1
    .long L$set$1
LASFDE1:
    .long   LASFDE1-EH_frame1
    .quad   LFB3-.
    .set L$set$2,LFE3-LFB3
    .quad L$set$2
    .byte   0x0
    .byte   0x4
    .set L$set$3,LCFI0-LFB3
    .long L$set$3
    .byte   0xe
    .byte   0x10
    .byte   0x86
    .byte   0x2
    .byte   0x4
    .set L$set$4,LCFI1-LCFI0
    .long L$set$4
    .byte   0xd
    .byte   0x6
    .align 3
LEFDE1:
    .subsections_via_symbols

My next problem is really, how do I compile this output, and can I make GCC do it for me?

Answer 1

Yes, You can use gcc to compile your asm code. Use -c for compilation like this:

gcc -c file.S -o file.o

This will give object code file named file.o. To invoke linker perform following after above command:

gcc file.o -o file

Answer 2

gcc can use an assembly file as input, and invoke the assembler as needed. There is a subtlety, though:

• If the file name ends with ".s" (lowercase 's'), then gcc calls the assembler.
• If the file name ends with ".S" (uppercase 'S'), then gcc applies the C preprocessor on the source file (i.e. it recognizes directives such as #if and replaces macros), and then calls the assembler on the result.

So, on a general basis, you want to do things like this:

gcc -S file.c -o file.s
gcc -c file.s

Answer 3

Yes, gcc can also compile assembly source code. Alternatively, you can invoke as, which is the assembler. (gcc is just a "driver" program that uses heuristics to call C compiler, C++ compiler, assembler, linker, etc..)

Answer 4

You can embed the assembly code in a normal C program. Here's a good introduction. Using the appropriate syntax, you can also tell GCC you want to interact with variables declared in C. The program below instructs gcc that:

• eax shall be foo
• ebx shall be bar
• the value in eax shall be stored in foo after the assembly code executed

\n

int main(void)
{
        int foo = 10, bar = 15;
        __asm__ __volatile__("addl  %%ebx,%%eax"
                             :"=a"(foo)
                             :"a"(foo), "b"(bar)
                             );
        printf("foo+bar=%d\n", foo);
        return 0;
}