Add strncpy.

Author: Wassasin

README.md

@@ -12,6 +12,7 @@ This crate basically came about so that the [nrfxlib](https://github.com/NordicP
 * atoi
 * strcmp
 * strncmp
+* strncpy
 * strlen
 * strtol
 * strstr

src/lib.rs

@@ -19,6 +19,9 @@ pub use self::strcmp::strcmp;
 mod strncmp;
 pub use self::strncmp::strncmp;
 
+mod strncpy;
+pub use self::strncpy::strncpy;
+
 mod strlen;
 pub use self::strlen::strlen;
 

src/strncpy.rs

@@ -0,0 +1,53 @@
+//! Rust implementation of C library function `strncpy`
+//!
+//! Copyright (c) Wouter 'Wassasin' Geraedts 2021
+//! Licensed under the Blue Oak Model Licence 1.0.0
+
+use crate::CChar;
+
+/// Rust implementation of C library function `strncmp`. Passing NULL
+/// (core::ptr::null()) gives undefined behaviour.
+#[no_mangle]
+pub unsafe extern "C" fn strncpy(
+	dest: *mut CChar,
+	src: *const CChar,
+	count: usize,
+) -> *const CChar {
+	let mut i = 0isize;
+	while i < count as isize {
+		*dest.offset(i) = *src.offset(i);
+		let c = *dest.offset(i);
+		i += 1;
+		if c == 0 {
+			break;
+		}
+	}
+	for j in i..count as isize {
+		*dest.offset(j) = 0;
+	}
+	dest
+}
+
+#[cfg(test)]
+mod test {
+	use super::*;
+
+	#[test]
+	fn short() {
+		let src = b"hi\0";
+		let mut dest = *b"abcdef"; // no null terminator
+		let result = unsafe { strncpy(dest.as_mut_ptr(), src.as_ptr(), 5) };
+		assert_eq!(
+			unsafe { core::slice::from_raw_parts(result, 5) },
+			*b"hi\0\0\0"
+		);
+	}
+
+	#[test]
+	fn two() {
+		let src = b"hi\0";
+		let mut dest = [0u8; 2]; // no space for null terminator
+		let result = unsafe { strncpy(dest.as_mut_ptr(), src.as_ptr(), 2) };
+		assert_eq!(unsafe { core::slice::from_raw_parts(result, 2) }, b"hi");
+	}
+}