rust/src/doc/trpl/no-stdlib.md

% No stdlib

By default, std is linked to every Rust crate. In some contexts, this is undesirable, and can be avoided with the #![no_std] attribute attached to the crate.

Obviously there's more to life than just libraries: one can use #[no_std] with an executable, controlling the entry point is possible in two ways: the #[start] attribute, or overriding the default shim for the C main function with your own.

The function marked #[start] is passed the command line parameters in the same format as C:

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

// Pull in the system libc library for what crt0.o likely requires
extern crate libc;

// Entry point for this program
#[start]
fn start(_argc: isize, _argv: *const *const u8) -> isize {
    0
}

// These functions and traits are used by the compiler, but not
// for a bare-bones hello world. These are normally
// provided by libstd.
#[lang = "eh_personality"] extern fn eh_personality() {}
#[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }
# #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
# // fn main() {} tricked you, rustdoc!

To override the compiler-inserted main shim, one has to disable it with #![no_main] and then create the appropriate symbol with the correct ABI and the correct name, which requires overriding the compiler's name mangling too:

# #![feature(libc)]
#![feature(no_std)]
#![feature(lang_items)]
#![feature(start)]
#![no_std]
#![no_main]

extern crate libc;

#[no_mangle] // ensure that this symbol is called `main` in the output
pub extern fn main(argc: i32, argv: *const *const u8) -> i32 {
    0
}

#[lang = "eh_personality"] extern fn eh_personality() {}
#[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }
# #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
# // fn main() {} tricked you, rustdoc!

The compiler currently makes a few assumptions about symbols which are available in the executable to call. Normally these functions are provided by the standard library, but without it you must define your own.

The first of these two functions, eh_personality, is used by the failure mechanisms of the compiler. This is often mapped to GCC's personality function (see the libstd implementation for more information), but crates which do not trigger a panic can be assured that this function is never called. The second function, panic_fmt, is also used by the failure mechanisms of the compiler.

Using libcore

Note

: the core library's structure is unstable, and it is recommended to use the standard library instead wherever possible.

With the above techniques, we've got a bare-metal executable running some Rust code. There is a good deal of functionality provided by the standard library, however, that is necessary to be productive in Rust. If the standard library is not sufficient, then libcore is designed to be used instead.

The core library has very few dependencies and is much more portable than the standard library itself. Additionally, the core library has most of the necessary functionality for writing idiomatic and effective Rust code. When using #![no_std], Rust will automatically inject the core crate, just like we do for std when we’re using it.

As an example, here is a program that will calculate the dot product of two vectors provided from C, using idiomatic Rust practices.

# #![feature(libc)]
#![feature(lang_items)]
#![feature(start)]
#![feature(no_std)]
#![feature(core)]
#![feature(core_slice_ext)]
#![feature(raw)]
#![no_std]

extern crate libc;

use core::mem;

#[no_mangle]
pub extern fn dot_product(a: *const u32, a_len: u32,
                          b: *const u32, b_len: u32) -> u32 {
    use core::raw::Slice;

    // Convert the provided arrays into Rust slices.
    // The core::raw module guarantees that the Slice
    // structure has the same memory layout as a &[T]
    // slice.
    //
    // This is an unsafe operation because the compiler
    // cannot tell the pointers are valid.
    let (a_slice, b_slice): (&[u32], &[u32]) = unsafe {
        mem::transmute((
            Slice { data: a, len: a_len as usize },
            Slice { data: b, len: b_len as usize },
        ))
    };

    // Iterate over the slices, collecting the result
    let mut ret = 0;
    for (i, j) in a_slice.iter().zip(b_slice.iter()) {
        ret += (*i) * (*j);
    }
    return ret;
}

#[lang = "panic_fmt"]
extern fn panic_fmt(args: &core::fmt::Arguments,
                    file: &str,
                    line: u32) -> ! {
    loop {}
}

#[lang = "eh_personality"] extern fn eh_personality() {}
# #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
# #[start] fn start(argc: isize, argv: *const *const u8) -> isize { 0 }
# fn main() {}

Note that there is one extra lang item here which differs from the examples above, panic_fmt. This must be defined by consumers of libcore because the core library declares panics, but it does not define it. The panic_fmt lang item is this crate's definition of panic, and it must be guaranteed to never return.

As can be seen in this example, the core library is intended to provide the power of Rust in all circumstances, regardless of platform requirements. Further libraries, such as liballoc, add functionality to libcore which make other platform-specific assumptions, but continue to be more portable than the standard library itself.