3 The tracking issue for this feature is: None.
5 ------------------------
7 The `rustc` compiler has certain pluggable operations, that is,
8 functionality that isn't hard-coded into the language, but is
9 implemented in libraries, with a special marker to tell the compiler
10 it exists. The marker is the attribute `#[lang = "..."]` and there are
11 various different values of `...`, i.e. various different 'lang
14 For example, `Box` pointers require two lang items, one for allocation
15 and one for deallocation. A freestanding program that uses the `Box`
16 sugar for dynamic allocations via `malloc` and `free`:
19 #![feature(lang_items, box_syntax, start, libc, core_intrinsics)]
22 use core::panic::PanicInfo;
27 pub struct Box<T>(*mut T);
29 #[lang = "exchange_malloc"]
30 unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
31 let p = libc::malloc(size as libc::size_t) as *mut u8;
33 // Check if `malloc` failed:
42 unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
43 libc::free(ptr as *mut libc::c_void)
47 fn main(_argc: isize, _argv: *const *const u8) -> isize {
53 #[lang = "eh_personality"] extern fn rust_eh_personality() {}
54 #[lang = "panic_impl"] extern fn rust_begin_panic(info: &PanicInfo) -> ! { unsafe { intrinsics::abort() } }
55 #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
56 #[no_mangle] pub extern fn rust_eh_register_frames () {}
57 #[no_mangle] pub extern fn rust_eh_unregister_frames () {}
60 Note the use of `abort`: the `exchange_malloc` lang item is assumed to
61 return a valid pointer, and so needs to do the check internally.
63 Other features provided by lang items include:
65 - overloadable operators via traits: the traits corresponding to the
66 `==`, `<`, dereferencing (`*`) and `+` (etc.) operators are all
67 marked with lang items; those specific four are `eq`, `ord`,
68 `deref`, and `add` respectively.
69 - stack unwinding and general failure; the `eh_personality`,
70 `eh_unwind_resume`, `fail` and `fail_bounds_checks` lang items.
71 - the traits in `std::marker` used to indicate types of
72 various kinds; lang items `send`, `sync` and `copy`.
73 - the marker types and variance indicators found in
74 `std::marker`; lang items `covariant_type`,
75 `contravariant_lifetime`, etc.
77 Lang items are loaded lazily by the compiler; e.g. if one never uses
78 `Box` then there is no need to define functions for `exchange_malloc`
79 and `box_free`. `rustc` will emit an error when an item is needed
80 but not found in the current crate or any that it depends on.
82 Most lang items are defined by `libcore`, but if you're trying to build
83 an executable without the standard library, you'll run into the need
84 for lang items. The rest of this page focuses on this use-case, even though
85 lang items are a bit broader than that.
89 In order to build a `#[no_std]` executable we will need libc as a dependency.
90 We can specify this using our `Cargo.toml` file:
94 libc = { version = "0.2.14", default-features = false }
97 Note that the default features have been disabled. This is a critical step -
98 **the default features of libc include the standard library and so must be
101 ### Writing an executable without stdlib
103 Controlling the entry point is possible in two ways: the `#[start]` attribute,
104 or overriding the default shim for the C `main` function with your own.
106 The function marked `#[start]` is passed the command line parameters
107 in the same format as C:
110 #![feature(lang_items, core_intrinsics)]
113 use core::intrinsics;
114 use core::panic::PanicInfo;
116 // Pull in the system libc library for what crt0.o likely requires.
119 // Entry point for this program.
121 fn start(_argc: isize, _argv: *const *const u8) -> isize {
125 // These functions are used by the compiler, but not
126 // for a bare-bones hello world. These are normally
127 // provided by libstd.
128 #[lang = "eh_personality"]
130 pub extern fn rust_eh_personality() {
133 // This function may be needed based on the compilation target.
134 #[lang = "eh_unwind_resume"]
136 pub extern fn rust_eh_unwind_resume() {
139 #[lang = "panic_impl"]
141 pub extern fn rust_begin_panic(info: &PanicInfo) -> ! {
142 unsafe { intrinsics::abort() }
146 To override the compiler-inserted `main` shim, one has to disable it
147 with `#![no_main]` and then create the appropriate symbol with the
148 correct ABI and the correct name, which requires overriding the
149 compiler's name mangling too:
152 #![feature(lang_items, core_intrinsics)]
156 use core::intrinsics;
157 use core::panic::PanicInfo;
159 // Pull in the system libc library for what crt0.o likely requires.
162 // Entry point for this program.
163 #[no_mangle] // ensure that this symbol is called `main` in the output
164 pub extern fn main(_argc: i32, _argv: *const *const u8) -> i32 {
168 // These functions are used by the compiler, but not
169 // for a bare-bones hello world. These are normally
170 // provided by libstd.
171 #[lang = "eh_personality"]
173 pub extern fn rust_eh_personality() {
176 // This function may be needed based on the compilation target.
177 #[lang = "eh_unwind_resume"]
179 pub extern fn rust_eh_unwind_resume() {
182 #[lang = "panic_impl"]
184 pub extern fn rust_begin_panic(info: &PanicInfo) -> ! {
185 unsafe { intrinsics::abort() }
189 In many cases, you may need to manually link to the `compiler_builtins` crate
190 when building a `no_std` binary. You may observe this via linker error messages
191 such as "```undefined reference to `__rust_probestack'```". Using this crate
192 also requires enabling the library feature `compiler_builtins_lib`. You can read
193 more about this [here][compiler-builtins-lib].
195 [compiler-builtins-lib]: ../library-features/compiler-builtins-lib.md
197 ## More about the language items
199 The compiler currently makes a few assumptions about symbols which are
200 available in the executable to call. Normally these functions are provided by
201 the standard library, but without it you must define your own. These symbols
202 are called "language items", and they each have an internal name, and then a
203 signature that an implementation must conform to.
205 The first of these functions, `rust_eh_personality`, is used by the failure
206 mechanisms of the compiler. This is often mapped to GCC's personality function
207 (see the [libstd implementation][unwind] for more information), but crates
208 which do not trigger a panic can be assured that this function is never
209 called. The language item's name is `eh_personality`.
211 [unwind]: https://github.com/rust-lang/rust/blob/master/src/libpanic_unwind/gcc.rs
213 The second function, `rust_begin_panic`, is also used by the failure mechanisms of the
214 compiler. When a panic happens, this controls the message that's displayed on
215 the screen. While the language item's name is `panic_impl`, the symbol name is
218 A third function, `rust_eh_unwind_resume`, is also needed if the `custom_unwind_resume`
219 flag is set in the options of the compilation target. It allows customizing the
220 process of resuming unwind at the end of the landing pads. The language item's name
221 is `eh_unwind_resume`.
223 ## List of all language items
225 This is a list of all language items in Rust along with where they are located in
229 - `i8`: `libcore/num/mod.rs`
230 - `i16`: `libcore/num/mod.rs`
231 - `i32`: `libcore/num/mod.rs`
232 - `i64`: `libcore/num/mod.rs`
233 - `i128`: `libcore/num/mod.rs`
234 - `isize`: `libcore/num/mod.rs`
235 - `u8`: `libcore/num/mod.rs`
236 - `u16`: `libcore/num/mod.rs`
237 - `u32`: `libcore/num/mod.rs`
238 - `u64`: `libcore/num/mod.rs`
239 - `u128`: `libcore/num/mod.rs`
240 - `usize`: `libcore/num/mod.rs`
241 - `f32`: `libstd/f32.rs`
242 - `f64`: `libstd/f64.rs`
243 - `char`: `libcore/char.rs`
244 - `slice`: `liballoc/slice.rs`
245 - `str`: `liballoc/str.rs`
246 - `const_ptr`: `libcore/ptr.rs`
247 - `mut_ptr`: `libcore/ptr.rs`
248 - `unsafe_cell`: `libcore/cell.rs`
250 - `start`: `libstd/rt.rs`
251 - `eh_personality`: `libpanic_unwind/emcc.rs` (EMCC)
252 - `eh_personality`: `libpanic_unwind/seh64_gnu.rs` (SEH64 GNU)
253 - `eh_personality`: `libpanic_unwind/seh.rs` (SEH)
254 - `eh_unwind_resume`: `libpanic_unwind/seh64_gnu.rs` (SEH64 GNU)
255 - `eh_unwind_resume`: `libpanic_unwind/gcc.rs` (GCC)
256 - `msvc_try_filter`: `libpanic_unwind/seh.rs` (SEH)
257 - `panic`: `libcore/panicking.rs`
258 - `panic_bounds_check`: `libcore/panicking.rs`
259 - `panic_impl`: `libcore/panicking.rs`
260 - `panic_impl`: `libstd/panicking.rs`
262 - `owned_box`: `liballoc/boxed.rs`
263 - `exchange_malloc`: `liballoc/heap.rs`
264 - `box_free`: `liballoc/heap.rs`
266 - `not`: `libcore/ops/bit.rs`
267 - `bitand`: `libcore/ops/bit.rs`
268 - `bitor`: `libcore/ops/bit.rs`
269 - `bitxor`: `libcore/ops/bit.rs`
270 - `shl`: `libcore/ops/bit.rs`
271 - `shr`: `libcore/ops/bit.rs`
272 - `bitand_assign`: `libcore/ops/bit.rs`
273 - `bitor_assign`: `libcore/ops/bit.rs`
274 - `bitxor_assign`: `libcore/ops/bit.rs`
275 - `shl_assign`: `libcore/ops/bit.rs`
276 - `shr_assign`: `libcore/ops/bit.rs`
277 - `deref`: `libcore/ops/deref.rs`
278 - `deref_mut`: `libcore/ops/deref.rs`
279 - `index`: `libcore/ops/index.rs`
280 - `index_mut`: `libcore/ops/index.rs`
281 - `add`: `libcore/ops/arith.rs`
282 - `sub`: `libcore/ops/arith.rs`
283 - `mul`: `libcore/ops/arith.rs`
284 - `div`: `libcore/ops/arith.rs`
285 - `rem`: `libcore/ops/arith.rs`
286 - `neg`: `libcore/ops/arith.rs`
287 - `add_assign`: `libcore/ops/arith.rs`
288 - `sub_assign`: `libcore/ops/arith.rs`
289 - `mul_assign`: `libcore/ops/arith.rs`
290 - `div_assign`: `libcore/ops/arith.rs`
291 - `rem_assign`: `libcore/ops/arith.rs`
292 - `eq`: `libcore/cmp.rs`
293 - `ord`: `libcore/cmp.rs`
295 - `fn`: `libcore/ops/function.rs`
296 - `fn_mut`: `libcore/ops/function.rs`
297 - `fn_once`: `libcore/ops/function.rs`
298 - `generator_state`: `libcore/ops/generator.rs`
299 - `generator`: `libcore/ops/generator.rs`
301 - `coerce_unsized`: `libcore/ops/unsize.rs`
302 - `drop`: `libcore/ops/drop.rs`
303 - `drop_in_place`: `libcore/ptr.rs`
304 - `clone`: `libcore/clone.rs`
305 - `copy`: `libcore/marker.rs`
306 - `send`: `libcore/marker.rs`
307 - `sized`: `libcore/marker.rs`
308 - `unsize`: `libcore/marker.rs`
309 - `sync`: `libcore/marker.rs`
310 - `phantom_data`: `libcore/marker.rs`
311 - `freeze`: `libcore/marker.rs`
312 - `debug_trait`: `libcore/fmt/mod.rs`
313 - `non_zero`: `libcore/nonzero.rs`
314 - `arc`: `liballoc/sync.rs`
315 - `rc`: `liballoc/rc.rs`