diff --git a/include/cc20.h b/include/cc20.h
index ec38702..a462d24 100644
--- a/include/cc20.h
+++ b/include/cc20.h
@@ -1,5 +1,11 @@
 #ifndef _CC20_H
 #define _CC20_H
+
+#ifdef __cplusplus
+extern "C" {
+#define restrict __restrict__
+#endif
+
 #include <stdint.h>
 #include <stddef.h>
 
@@ -7,56 +13,99 @@
 #define IV_SIZE 12
 
 enum cc20_mode {
+	/// Encrypt the data written to this stream
 	CC20_ENCRYPT,
+	/// Decrypt the data written to this stream
 	CC20_DECRYPT,
 };
 
+/// Result from a `cc20_` internal operation.
 typedef enum cc20_error {
+	/// An internal fatal error
 	CC20_ERR_PANIC = -1,
+	/// Operation succeeded
 	CC20_ERR_NONE = 0,
 
+	/// If a `FILE*` stream is used that is invalid or `NULL`.
 	CC20_ERR_INVALID_FILE,
+	/// If a pointer is `NULL` that should not be
 	CC20_ERR_NULL_PTR,
+	/// An underlying `libssl` call fails
 	CC20_ERR_SSL,
+	/// There is a generic I/O failure
 	CC20_ERR_IO
 } cc20_result_t;
 
 #define CC20_OK(v) ((v)==CC20_ERR_NONE)
 #define CC20_ERR(v) ((v)!=CC20_ERR_NONE)
 
+/// A chacha20_poly1305 key
 typedef uint8_t cc20_key_t[KEY_SIZE];
+/// A chacha20_poly1305 IV
 typedef uint8_t cc20_iv_t[IV_SIZE];
 
+/// Metadata used to construct an instance of `cc20_sink_t`.
 typedef struct cc20_metadata {
+	/// A valid, non-NULL, stream that the sink will write the transformed data to.
 	FILE* backing;
+	/// The key used for the cipher
 	cc20_key_t key;
+	/// The iv used to initialise the cipher.
 	cc20_iv_t iv;
+	/// The transformation mode
+	///
+	/// *MUST* be a valid `enum cc20_mode` disctiminant or UB.
 	enum cc20_mode mode;
 } cc20_meta_t;
 
+/// Configuration for a wrapper `FILE*` stream over a `cc20_sink_t`.
 struct cc20_wrap_cfg {
-	// default: false (0)
+	/// Keep the backing (`cc20_meta_t.backing`) stream alive (do not `fclose()` it) when the wrapper is closed.
+	/// # Default
+	///  * false (0)
 	int keep_alive;
 };
 
+/// An opaque type containing the cipher transform and the backing stream.
 typedef struct cc20_sink cc20_sink_t;
 
 // Functions //
+
+/// Parameters tagged with this must be valid, non-NULL and non-aliased; but can point to uninitialised memory for this type.
+/// They are guaranteed to be written to with a valid value if the function succeeds. If it fails, it is unspecified whether it will be written to.
+//TODO: Attribute non-NULL how?
 #define _cc20_OUT *restrict
 
+/// Generate a new securely random key and/or iv.
+///
+/// # Possible errors
+/// * `CC20_ERR_PANIC` - If the RNG fails.
 cc20_result_t cc20_keygen(cc20_key_t* restrict key,
 		cc20_iv_t* restrict iv);
 
+/// Write these parameters to `output` metadata.
+///  * `key` and `iv` can be NULL, if one or both are, the field(s) will be initialised to secure random data in the `output` metadata (same as `cc20_keygen`.
+///  * `file` must be non-NULL and valid.
+/// # Possible errors
+///  * `CC20_ERR_INVALID_FILE` - if `file` is NULL.
+///  * `CC20_ERR_NULL_PTR` - if `output` is NULL. (see `_cc20_OUT`.)
+/// ## Undefined behaviour
+/// If `mode` is not a valid distriminant of `enum cc20_mdoe`.
 cc20_result_t cc20_gen_meta(FILE* file,
 		const cc20_key_t* key,
 		const cc20_iv_t* iv,
 		enum cc20_mode mode,
 		struct cc20_metadata _cc20_OUT output);
 
+/// Create a sink from the specified metadata and write it to `output`.
+///
+/// # Possible errors
+///  * `CC20_ERR_NULL_PTR` - If `meta` or `output` is NULL.
 cc20_result_t cc20_gen_sink(const struct cc20_metadata* meta,
 			cc20_sink_t* _cc20_OUT output);
 
-
+//TODO: Rework this entire API interface. Its clunky.
+//TODO: Document the rest
 cc20_result_t cc20_gen_sink_full(FILE* file, 
 		const cc20_key_t* key,
 		const cc20_iv_t* iv,
@@ -79,4 +128,9 @@ cc20_result_t cc20_write(const void* ptr, size_t * restrict bytes, cc20_sink_t*
 
 FILE* cc20_wrap_sink(cc20_sink_t* sink, const struct cc20_wrap_cfg* cfg);
 
+#ifdef __cplusplus
+}
+#undef restrict
+#endif
+
 #endif /* _CC20_H */