3 Rust’s standard library provides a lot of useful functionality, but assumes
4 support for various features of its host system: threads, networking, heap
5 allocation, and others. There are systems that do not have these features,
6 however, and Rust can work with those too! To do so, we tell Rust that we
7 don’t want to use the standard library via an attribute: `#![no_std]`.
9 > Note: This feature is technically stable, but there are some caveats. For
10 > one, you can build a `#![no_std]` _library_ on stable, but not a _binary_.
11 > For details on libraries without the standard library, see [the chapter on
12 > `#![no_std]`](using-rust-without-the-standard-library.html)
14 Obviously there's more to life than just libraries: one can use
15 `#[no_std]` with an executable, controlling the entry point is
16 possible in two ways: the `#[start]` attribute, or overriding the
17 default shim for the C `main` function with your own.
19 The function marked `#[start]` is passed the command line parameters
20 in the same format as C:
24 #![feature(lang_items)]
28 // Pull in the system libc library for what crt0.o likely requires
31 // Entry point for this program
33 fn start(_argc: isize, _argv: *const *const u8) -> isize {
37 // These functions and traits are used by the compiler, but not
38 // for a bare-bones hello world. These are normally
39 // provided by libstd.
40 #[lang = "eh_personality"] extern fn eh_personality() {}
41 #[lang = "panic_fmt"] extern fn panic_fmt() -> ! { loop {} }
42 # #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
43 # #[no_mangle] pub extern fn rust_eh_register_frames () {}
44 # #[no_mangle] pub extern fn rust_eh_unregister_frames () {}
45 # // fn main() {} tricked you, rustdoc!
48 To override the compiler-inserted `main` shim, one has to disable it
49 with `#![no_main]` and then create the appropriate symbol with the
50 correct ABI and the correct name, which requires overriding the
51 compiler's name mangling too:
55 #![feature(lang_items)]
62 #[no_mangle] // ensure that this symbol is called `main` in the output
63 pub extern fn main(argc: i32, argv: *const *const u8) -> i32 {
67 #[lang = "eh_personality"] extern fn eh_personality() {}
68 #[lang = "panic_fmt"] extern fn panic_fmt() -> ! { loop {} }
69 # #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
70 # #[no_mangle] pub extern fn rust_eh_register_frames () {}
71 # #[no_mangle] pub extern fn rust_eh_unregister_frames () {}
72 # // fn main() {} tricked you, rustdoc!
76 The compiler currently makes a few assumptions about symbols which are available
77 in the executable to call. Normally these functions are provided by the standard
78 library, but without it you must define your own.
80 The first of these two functions, `eh_personality`, is used by the failure
81 mechanisms of the compiler. This is often mapped to GCC's personality function
82 (see the [libstd implementation][unwind] for more information), but crates
83 which do not trigger a panic can be assured that this function is never
84 called. The second function, `panic_fmt`, is also used by the failure
85 mechanisms of the compiler.
87 [unwind]: https://github.com/rust-lang/rust/blob/master/src/libstd/sys/common/unwind/gcc.rs