Is there a way to get gcc to output raw binary?

Question

Forgive me—newbie gcc question. Is there a set of command-line options that will convince gcc to produce a flat binary file from a self-contained source file? For example, suppose the contents of foo.c are

static int f(int x)
{
  int y = x*x;
  return y+2;
}

No external references, nothing to export to the linker. I'd like to get a small file with just the machine instructions for this function, without any other decoration. Sort of like a (DOS) .COM file except 32-bit protected mode.

Answer 1

You can use objcopy to pull the text segment out of the .o file or the a.out file.

$ cat q.c
f() {}
$ cc -S -O q.c
$ cat q.s
        .file   "q.c"
        .text
.globl f
        .type   f, @function
f:
        pushl   %ebp
        movl    %esp, %ebp
        popl    %ebp
        ret
        .size   f, .-f
        .ident  "GCC: (Ubuntu 4.3.3-5ubuntu4) 4.3.3"
        .section        .note.GNU-stack,"",@progbits
$ cc -c -O q.c
$ objcopy -O binary q.o q.bin
$ od -X q.bin
0000000 5de58955 000000c3
0000005
$ objdump -d q.o
q.o:     file format elf32-i386
Disassembly of section .text:
00000000 <f>:
   0:   55                    push   %ebp
   1:   89 e5                 mov    %esp,%ebp
   3:   5d                    pop    %ebp
   4:   c3                    ret

Answer 2

Try this out:

$ cc -c test.c
$ objcopy -O binary test.o binfile

You can make sure it's correct with objdump:

$ objdump -d test.o 
test.o:     file format pe-i386


Disassembly of section .text:

00000000 <_f>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 ec 04                sub    $0x4,%esp
   6:   8b 45 08                mov    0x8(%ebp),%eax
   9:   0f af 45 08             imul   0x8(%ebp),%eax
   d:   89 45 fc                mov    %eax,-0x4(%ebp)
  10:   8b 45 fc                mov    -0x4(%ebp),%eax
  13:   83 c0 02                add    $0x2,%eax
  16:   c9                      leave  
  17:   c3                      ret

And compare to the binary file:

$ hexdump -C binfile 
00000000  55 89 e5 83 ec 04 8b 45  08 0f af 45 08 89 45 fc  |U......E...E..E.|
00000010  8b 45 fc 83 c0 02 c9 c3                           |.E......|
00000018