3 By default, `std` is linked to every Rust crate. In some contexts,
4 this is undesirable, and can be avoided with the `#![no_std]`
5 attribute attached to the crate.
7 Obviously there's more to life than just libraries: one can use
8 `#[no_std]` with an executable, controlling the entry point is
9 possible in two ways: the `#[start]` attribute, or overriding the
10 default shim for the C `main` function with your own.
12 The function marked `#[start]` is passed the command line parameters
13 in the same format as C:
17 #![feature(lang_items)]
21 // Pull in the system libc library for what crt0.o likely requires
24 // Entry point for this program
26 fn start(_argc: isize, _argv: *const *const u8) -> isize {
30 // These functions and traits are used by the compiler, but not
31 // for a bare-bones hello world. These are normally
32 // provided by libstd.
33 #[lang = "eh_personality"] extern fn eh_personality() {}
34 #[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }
35 # #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
36 # #[no_mangle] pub extern fn rust_eh_register_frames () {}
37 # #[no_mangle] pub extern fn rust_eh_unregister_frames () {}
38 # // fn main() {} tricked you, rustdoc!
41 To override the compiler-inserted `main` shim, one has to disable it
42 with `#![no_main]` and then create the appropriate symbol with the
43 correct ABI and the correct name, which requires overriding the
44 compiler's name mangling too:
48 #![feature(lang_items)]
55 #[no_mangle] // ensure that this symbol is called `main` in the output
56 pub extern fn main(argc: i32, argv: *const *const u8) -> i32 {
60 #[lang = "eh_personality"] extern fn eh_personality() {}
61 #[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }
62 # #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
63 # #[no_mangle] pub extern fn rust_eh_register_frames () {}
64 # #[no_mangle] pub extern fn rust_eh_unregister_frames () {}
65 # // fn main() {} tricked you, rustdoc!
69 The compiler currently makes a few assumptions about symbols which are available
70 in the executable to call. Normally these functions are provided by the standard
71 library, but without it you must define your own.
73 The first of these two functions, `eh_personality`, is used by the
74 failure mechanisms of the compiler. This is often mapped to GCC's
75 personality function (see the
76 [libstd implementation](../std/rt/unwind/index.html) for more
77 information), but crates which do not trigger a panic can be assured
78 that this function is never called. The second function, `panic_fmt`, is
79 also used by the failure mechanisms of the compiler.
83 > **Note**: the core library's structure is unstable, and it is recommended to
84 > use the standard library instead wherever possible.
86 With the above techniques, we've got a bare-metal executable running some Rust
87 code. There is a good deal of functionality provided by the standard library,
88 however, that is necessary to be productive in Rust. If the standard library is
89 not sufficient, then [libcore](../core/index.html) is designed to be used
92 The core library has very few dependencies and is much more portable than the
93 standard library itself. Additionally, the core library has most of the
94 necessary functionality for writing idiomatic and effective Rust code. When
95 using `#![no_std]`, Rust will automatically inject the `core` crate, just like
96 we do for `std` when we’re using it.
98 As an example, here is a program that will calculate the dot product of two
99 vectors provided from C, using idiomatic Rust practices.
103 #![feature(lang_items)]
113 pub extern fn dot_product(a: *const u32, a_len: u32,
114 b: *const u32, b_len: u32) -> u32 {
115 use core::raw::Slice;
117 // Convert the provided arrays into Rust slices.
118 // The core::raw module guarantees that the Slice
119 // structure has the same memory layout as a &[T]
122 // This is an unsafe operation because the compiler
123 // cannot tell the pointers are valid.
124 let (a_slice, b_slice): (&[u32], &[u32]) = unsafe {
126 Slice { data: a, len: a_len as usize },
127 Slice { data: b, len: b_len as usize },
131 // Iterate over the slices, collecting the result
133 for (i, j) in a_slice.iter().zip(b_slice.iter()) {
139 #[lang = "panic_fmt"]
140 extern fn panic_fmt(args: &core::fmt::Arguments,
146 #[lang = "eh_personality"] extern fn eh_personality() {}
147 # #[start] fn start(argc: isize, argv: *const *const u8) -> isize { 0 }
148 # #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
149 # #[no_mangle] pub extern fn rust_eh_register_frames () {}
150 # #[no_mangle] pub extern fn rust_eh_unregister_frames () {}
154 Note that there is one lang item here whose signature differs from the examples
155 above, `panic_fmt`. This must be defined by consumers of libcore because the
156 core library declares panics, but it does not define it. The `panic_fmt`
157 lang item is this crate's definition of panic, and it must be guaranteed to
160 As can be seen in this example, the core library is intended to provide the
161 power of Rust in all circumstances, regardless of platform requirements. Further
162 libraries, such as liballoc, add functionality to libcore which make other
163 platform-specific assumptions, but continue to be more portable than the
164 standard library itself.