diff --git a/Cargo.toml b/Cargo.toml
index 3a4e29b..f34333e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -39,6 +39,7 @@ all = [
     "strncmp",
     "strncpy",
     "strrchr",
+    "strspn",
     "strstr",
     "strtoimax",
     "strtol",
@@ -71,6 +72,7 @@ strncasecmp = []
 strncmp = []
 strncpy = []
 strrchr = []
+strspn = []
 strstr = []
 strtoimax = []
 strtol = []
diff --git a/src/lib.rs b/src/lib.rs
index 7ff3df7..687b0b4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -27,6 +27,7 @@ mod strncasecmp;
 mod strncmp;
 mod strncpy;
 mod strrchr;
+mod strspn;
 mod strstr;
 mod strtol;
 
@@ -79,6 +80,8 @@ pub use self::strncmp::strncmp;
 pub use self::strncpy::strncpy;
 #[cfg(feature = "strrchr")]
 pub use self::strrchr::strrchr;
+#[cfg(feature = "strspn")]
+pub use self::strspn::strspn;
 #[cfg(feature = "strstr")]
 pub use self::strstr::strstr;
 #[cfg(feature = "strtoimax")]
diff --git a/src/strspn.rs b/src/strspn.rs
new file mode 100644
index 0000000..937a1ed
--- /dev/null
+++ b/src/strspn.rs
@@ -0,0 +1,83 @@
+//! Rust implementation of C library function `strspn`
+//!
+//! Copyright (c) Ferrous Systems UK Ltd
+//! Licensed under the Blue Oak Model Licence 1.0.0
+
+use crate::{CChar, CInt};
+
+/// Rust implementation of C library function `strspn`
+#[cfg_attr(feature = "strspn", no_mangle)]
+pub unsafe extern "C" fn strspn(s: *const CChar, charset: *const CChar) -> usize {
+	if s.is_null() {
+		return 0;
+	}
+	if charset.is_null() {
+		return 0;
+	}
+
+	let s = unsafe { core::ffi::CStr::from_ptr(s.cast()) };
+
+	let charset = unsafe { core::ffi::CStr::from_ptr(charset.cast()) };
+
+	for (idx, b) in s.to_bytes().iter().enumerate() {
+		if !is_c_in_charset(*b, charset) {
+			return idx;
+		}
+	}
+
+	s.count_bytes()
+}
+
+fn is_c_in_charset(c: u8, charset: &core::ffi::CStr) -> bool {
+	for b in charset.to_bytes() {
+		if c == *b {
+			return true;
+		}
+	}
+	false
+}
+
+#[cfg(test)]
+mod test {
+	#[test]
+	fn complete() {
+		let charset = c"0123456789";
+		let s = c"987654321";
+		assert_eq!(
+			unsafe { super::strspn(s.as_ptr().cast(), charset.as_ptr().cast()) },
+			9
+		);
+	}
+
+	#[test]
+	fn subset() {
+		let charset = c"0123456789";
+		let s = c"98xx7654321";
+		assert_eq!(
+			unsafe { super::strspn(s.as_ptr().cast(), charset.as_ptr().cast()) },
+			2
+		);
+	}
+
+	#[test]
+	fn empty_charset() {
+		let charset = c"";
+		let s = c"AABBCCDD";
+		assert_eq!(
+			unsafe { super::strspn(s.as_ptr().cast(), charset.as_ptr().cast()) },
+			0
+		);
+	}
+
+	#[test]
+	fn empty_string() {
+		let charset = c"0123456789";
+		let s = c"";
+		assert_eq!(
+			unsafe { super::strspn(s.as_ptr().cast(), charset.as_ptr().cast()) },
+			0
+		);
+	}
+}
+
+// End of file