zephyr: error: helper to convert return code and errno to Result

Various Zephyr functions have a return code where '-1' indicates an
error, but the exact error code is stored in the errno variable. Add a
helper function to convert this kind of return value into a Result.

Signed-off-by: Matt Rodgers <[email protected]>

Author: mrodgers-witekio

main.c

@@ -52,4 +52,12 @@ void rust_panic_wrap(void)
 	k_panic();
 }
 
+/* `errno` is usually defined as a macro rather than a variable, which makes it difficult to
+ * access in Rust. So create a simple getter function in C.
+ */
+int get_errno(void)
+{
+	return errno;
+}
+
 #endif

zephyr/Cargo.toml

@@ -22,6 +22,9 @@ cfg-if = "1.0"
 log = "0.4.22"
 arrayvec = { version = "0.7.6", default-features = false }
 
+# The num crate is used primarily for the Zero trait bound
+num = { version = "0.4", default-features = false }
+
 [dependencies.fugit]
 version = "0.3.7"
 

zephyr/src/error.rs

@@ -11,8 +11,14 @@
 //! this an enum itself, however, it would probably be better to auto-generate this enum instead of
 //! trying to maintain the list manually.
 
+use core::cmp::PartialOrd;
 use core::ffi::c_int;
 use core::fmt;
+use num::Zero;
+
+extern "C" {
+    fn get_errno() -> c_int;
+}
 
 // This is a little messy because the constants end up as u32 from bindgen, although the values are
 // negative.
@@ -51,6 +57,20 @@ pub fn to_result(code: c_int) -> Result<c_int> {
     }
 }
 
+/// Map a return result and errno from Zephyr into a Result.
+///
+/// Some Zephyr functions (particularly those intended for compatibility with POSIX function
+/// signatures) return -1 on error, with the specific error code stored in the `errno` variable.
+#[inline(always)]
+pub fn to_result_errno<T: PartialOrd + Zero>(code: T) -> Result<T> {
+    if code < T::zero() {
+        let e = unsafe { get_errno() };
+        Err(Error(e as u32))
+    } else {
+        Ok(code)
+    }
+}
+
 /// Map a return result, with a void result.
 #[inline(always)]
 pub fn to_result_void(code: c_int) -> Result<()> {