Making C Programs Small: The Beauty of C

Introduction

Let’s write a simple hello-world program in C language and compile a static binary with GCC:

1
2
3
4
5
6

#include <stdio.h>

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

1
2
3

$ gcc -static -O2 -o hello hello.c
$ du -h --apparent-size hello
843K	hello

Eight hundred forty-three kilobytes for such a simple program! What a waste of space in 2020, yeah? Nevertheless, in this post, we’ll try to make our programs smaller (maybe a lot).

Getting rid of libc

The fact is that by default, C compilers automatically link our code with GNU C Library and most of the binary size is taken by it, so our initial goal is going to be to make our program independent from that. To make it possible, we need to write an entry point to our program and wrappers for system calls in Assembler. Let’s do that (I’m using x86-64 instructions set):

Dumb calculator

For the purpose of showing that one can do something meaningful without exploiting C library, I shall implement a CLI program that takes +|-|*|/ as an operation from an argument and reduces the list from the stdin with it.

Write a Makefile:

Now let’s build and test it:

1
2
3
4
5
6
7

$ make
as -64  -o start.o start.s
as -64  -o sys.o sys.s
cc -Wall -Wextra -Werror -nostdlib -static -O2   -c -o io.o io.c
cc -Wall -Wextra -Werror -nostdlib -static -O2   -c -o ops.o ops.c
cc -Wall -Wextra -Werror -nostdlib -static -O2   -c -o main.o main.c
ld -m elf_x86_64 -nodefaultlibs start.o sys.o io.o ops.o main.o -o calc

1
2
3
4
5
6
7
8
9

$ ./calc some unappropriate arguments
Usage:
  ./calc [+|-|*|/]
$ echo | ./calc +
Error: empty list
$ echo "12 34 -56" | ./calc +
-10
$ echo "1 2 3 4 5" | ./calc '*'
120

How small is it?

1
2

$ du -h --apparent-size calc
3.5K	calc

The size equals 3.5KB, which is approximately $99.6\%$ smaller than the initial hello-world program. Sounds fascinating 🎉. Of course, we become seriously limited by the inability to use functions from the standard library. However, the critical point is that we could get independent from runtime libraries and thus have almost zero initialization runtime, making C the only good choice for implementing system kernels and developing for cheap microcontrollers.

Source code of the example is available here.