A simple freestanding program using libc

unsafe_freestanding.rs

#![feature(start)]
#![no_std]

extern crate libc;
use core::panic::PanicInfo;
use libc::{c_char, c_int, c_void, size_t};

extern "C" {
    fn malloc(size: size_t) -> *mut c_void;
    fn free(p: *mut c_void);
    fn exit(status: c_int) -> !;
    fn printf(fmt: *const c_char, ...) -> c_int;
}

#[start]
fn main(argc: isize, _argv: *const *const u8) -> isize {
    unsafe {
        let ptr1 = malloc(32 as size_t) as *mut i32;
        if ptr1 as usize == 0 {
            printf("Memory not allocated.\0".as_ptr() as *const i8);
            exit(0);
        }
        *ptr1 = 2;

        let ptr2 = malloc(32 as size_t) as *mut i32;
        if ptr2 as usize == 0 {
            printf("Memory not allocated.\0".as_ptr() as *const i8);
            exit(0);
        }
        *ptr2 = 5;

        printf(
            "%d + %d = %d\0".as_ptr() as *const i8,
            *ptr1,
            *ptr2,
            *ptr1 + *ptr2,
        );

        free(ptr1 as *mut c_void);
        free(ptr2 as *mut c_void);
    }

    0
}

#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
    loop {}
}

cargo.toml

[package]
name = "libc"
version = "0.1.0"
authors = ["nyinyithann nyinyithann@gmail.com"]
edition = "2018"

[profile.dev]
panic = "abort"

[profile.release]
panic = "abort"

[dependencies]
libc = { version = "0.2.48", default-features = false }