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

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

Try this out:

$ gcc -c test.c      $ objcopy -O binary -j .text 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 it with 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 

Answer 2

You can pass options to the linker directly with -Wl,<linker option>

The relevant documentation is copied below from the man gcc

-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map.

So when compiling with gcc if you pass -Wl,--oformat=binary you will generate a binary file instead of the elf format. Where --oformat=binary tells ld to generate a binary file.

This removes the need to objcopy separately.

Note that --oformat=binary can be expressed as OUTPUT_FORMAT("binary") from within a linker script. If you want to deal with flat binaries, there's a big chance that you would benefit from high level of control that linker scripts provide.