What is in an executable besides the raw machine instructions?

Question

I am on a quest to understand low-level computing. I have noticed my compiled binaries are a lot bigger then I think they should be. So I tried to build the smallest possible c program without any stdlib code as follows:

void _start()
{
    while(1) {};
}

gcc -nostdlib -o minimal minimal.c

When I disasseble the binary, it shows me exactly what I expect, namely this exact code in three lines of assembly.

$ objdump -d minimal

minimal:     file format elf64-x86-64


Disassembly of section .text:

0000000000001000 <_start>:
    1000:   55                      push   %rbp
    1001:   48 89 e5                mov    %rsp,%rbp
    1004:   eb fe                   jmp    1004 <_start+0x4>

But my actual executable is still 13856 Bytes in size. What is it, that makes this so large? What else is in that file? Does the OS need more than these 6 Bytes of machine code?

Edit #1: The output of size is:

$ size -A minimal
minimal  :
section              size    addr
.interp                28     680
.note.gnu.build-id     36     708
.gnu.hash              28     744
.dynsym                24     776
.dynstr                 1     800
.text                   6    4096
.eh_frame_hdr          20    8192
.eh_frame              52    8216
.dynamic              208   16176
.comment               18       0
Total                 421

Answer 1

Modern compilers and linkers aren't really optimized for producing ultra-small code on full-scale platforms. Not because the job is difficult, but because there's usually no need to. It isn't necessarily that the compiler or linker adds additional code (although it might), but rather that it won't try hard to pack your data and code into the smallest possible space.

In your case, I note that you're using dynamic linking, even though nothing is actually linked. Using "-static" will shave off about 8kB. "-s" (strip) will get rid of a bit more.

I don't know if it's even possible with gcc to make a truly minimal ELF executable. In your case, that ought to be about 400 bytes, nearly all of which will be the various ELF headers, section table, etc.

I don't know if I'm allowed to link my own website (I'm sure somebody will put me right if not), but I have an article on producing a tiny ELF executable by building it from scratch in binary:

http://kevinboone.me/elfdemo.html