Fill "Beneath std" (#413)

Author: kadiwa4

src/beneath-std.md

@@ -1,10 +1,108 @@
-# Beneath std
+# Beneath `std`
 
-This section documents (or will document) features that are provided by the standard library and
-that `#![no_std]` developers have to deal with (i.e. provide) to build `#![no_std]` binary crates. A
-(likely incomplete) list of such features is shown below:
+This section documents features that are normally provided by the `std` crate and
+that `#![no_std]` developers have to deal with (i.e. provide) to build
+`#![no_std]` binary crates.
 
-- `#[lang = "eh_personality"]`
-- `#[lang = "start"]`
-- `#[lang = "termination"]`
-- `#[panic_implementation]`
+## Using `libc`
+
+In order to build a `#[no_std]` executable we will need `libc` as a dependency.
+We can specify this using our `Cargo.toml` file:
+
+```toml
+[dependencies]
+libc = { version = "0.2.146", default-features = false }
+```
+
+Note that the default features have been disabled. This is a critical step -
+**the default features of `libc` include the `std` crate and so must be
+disabled.**
+
+Alternatively, we can use the unstable `rustc_private` private feature together
+with an `extern crate libc;` declaration as shown in the examples below.
+
+## Writing an executable without `std`
+
+We will probably need a nightly version of the compiler to produce
+a `#![no_std]` executable because on many platforms, we have to provide the
+`eh_personality` [lang item], which is unstable.
+
+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.
+Additionally, it's required to define a [panic handler function](panic-handler.html).
+
+The function marked `#[start]` is passed the command line parameters
+in the same format as C (aside from the exact integer types being used):
+
+```rust
+#![feature(start, lang_items, core_intrinsics, rustc_private)]
+#![allow(internal_features)]
+#![no_std]
+
+// Necessary for `panic = "unwind"` builds on some platforms.
+#![feature(panic_unwind)]
+extern crate unwind;
+
+// Pull in the system libc library for what crt0.o likely requires.
+extern crate libc;
+
+use core::panic::PanicInfo;
+
+// Entry point for this program.
+#[start]
+fn main(_argc: isize, _argv: *const *const u8) -> isize {
+    0
+}
+
+// These functions are used by the compiler, but not for an empty program like this.
+// They are normally provided by `std`.
+#[lang = "eh_personality"]
+fn rust_eh_personality() {}
+#[panic_handler]
+fn panic_handler(_info: &PanicInfo) -> ! { core::intrinsics::abort() }
+```
+
+To override the compiler-inserted `main` shim, we have 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:
+
+```rust
+#![feature(lang_items, core_intrinsics, rustc_private)]
+#![allow(internal_features)]
+#![no_std]
+#![no_main]
+
+// Necessary for `panic = "unwind"` builds on some platforms.
+#![feature(panic_unwind)]
+extern crate unwind;
+
+// Pull in the system libc library for what crt0.o likely requires.
+extern crate libc;
+
+use core::ffi::{c_char, c_int};
+use core::panic::PanicInfo;
+
+// Entry point for this program.
+#[no_mangle] // ensure that this symbol is included in the output as `main`
+extern "C" fn main(_argc: c_int, _argv: *const *const c_char) -> c_int {
+    0
+}
+
+// These functions are used by the compiler, but not for an empty program like this.
+// They are normally provided by `std`.
+#[lang = "eh_personality"]
+fn rust_eh_personality() {}
+#[panic_handler]
+fn panic_handler(_info: &PanicInfo) -> ! { core::intrinsics::abort() }
+```
+
+If you are working with a target that doesn't have binary releases of the
+standard library available via rustup (this probably means you are building the
+`core` crate yourself) and need compiler-rt intrinsics (i.e. you are probably
+getting linker errors when building an executable:
+``undefined reference to `__aeabi_memcpy'``), you need to manually link to the
+[`compiler_builtins` crate] to get those intrinsics and solve the linker errors.
+
+[`compiler_builtins` crate]: https://crates.io/crates/compiler_builtins
+[lang item]: https://doc.rust-lang.org/nightly/unstable-book/language-features/lang-items.html