Document no_std gotchas, provide c-stubs

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "littlefs2"
 description = "Idiomatic Rust API for littlefs"
-version = "0.3.1"
+version = "0.3.2"
 authors = ["Nicolas Stalder <[email protected]>", "Brandon Edens <[email protected]>"]
 edition = "2018"
 license = "Apache-2.0 OR MIT"
@@ -43,6 +43,7 @@ dir-entry-path = []
 ll-assertions = ["littlefs2-sys/assertions"]
 # enable trace in backend C code
 ll-trace = ["littlefs2-sys/trace"]
+c-stubs = []
 
 log-all = []
 log-none = []

README.md

@@ -44,6 +44,16 @@ Upstream release: [v2.1.4][upstream-release]
 [littlefs2-sys]: https://lib.rs/littlefs2-sys
 [upstream-release]: https://github.com/ARMmbed/littlefs/releases/tag/v2.1.4
 
+## `no_std`
+
+This library is `no_std` compatible, but there are two gotchas.
+
+- The dev-dependency `memchr` of `littlefs2-sys` has its `std` features activated. To prevent this, upgrade to at least Rust 1.51
+  and add `resolver = "2"` in the consuming code's `[package]` section. This will be the default in Rust 2021 edition.
+
+- At link time, `lfs.c` has a dependency on `strcpy`. When not linking to a `libc` with this symbol, activate the `c-stubs` feature
+  to provide an implementation.
+
 #### License
 
 <sup>littlefs is licensed under [BSD-3-Clause](https://github.com/ARMmbed/littlefs/blob/master/LICENSE.md).</sup>

src/c_stubs.rs

@@ -0,0 +1,29 @@
+//! C functions not provided by compiler-builtins
+//!
+//! Use this instead of linking to libc if you only need a handful of free functions
+
+use cty::{c_char, c_void, size_t};
+
+extern "C" {
+    // provided by `compiler-builtins`
+    fn memcpy(dst: *mut c_void, src: *const c_void, n: size_t) -> *mut c_void;
+}
+
+/// # Safety
+/// - `src` must be a valid C string (null terminated)
+/// - `dst` must be large enough to hold `src`
+#[no_mangle]
+unsafe fn strcpy(dst: *mut c_char, src: *const c_char) -> *mut c_char {
+    memcpy(dst as *mut c_void, src as *const c_void, strlen(src)) as *mut c_char
+}
+
+/// # Safety
+/// `s` must point to valid memory; `s` will be treated as a null terminated string
+pub unsafe fn strlen(mut s: *const c_char) -> size_t {
+    let mut n = 0;
+    while *s != 0 {
+        s = s.add(1);
+        n += 1;
+    }
+    n
+}

src/lib.rs

@@ -54,6 +54,16 @@ terms of actual and `typenum` constants, and an implementation supplies methods
 The filesystem and each open file need memory for state and caching, this has to be allocated
 beforehand and passed to constructors.
 
+### `no_std`
+
+This library is `no_std` compatible, but there are two gotchas.
+
+- The dev-dependency `memchr` of `littlefs2-sys` has its `std` features activated. To prevent this, upgrade to at least Rust 1.51
+  and add `resolver = "2"` in the consuming code's `[package]` section. This will be the default in Rust 2021 edition.
+
+- At link time, `lfs.c` has a dependency on `strcpy`. When not linking to a `libc` with this symbol, activate the `c-stubs` feature
+  to provide an implementation.
+
 ### Design notes
 
 All operations on the filesystem require passing a `&mut Storage`, which guarantees by Rust's
@@ -124,6 +134,9 @@ generate_macros!();
 #[macro_use]
 pub mod macros;
 
+#[cfg(feature = "c-stubs")]
+mod c_stubs;
+
 pub mod consts;
 pub mod driver;