NUL terminator on String Reprs

cheatsheet.md

@@ -12,12 +12,12 @@ For a value `v`, we write `«v»` for the binary encoding of `v`.
 
                           «#f» = [0xA0]
                           «#t» = [0xA1]
-                           «F» = [0xA2] ++ binary32(F)  if F ∈ Float
-                           «D» = [0xA2] ++ binary64(D)  if D ∈ Double
-                           «x» = [0xA3] ++ intbytes(x)  if x ∈ SignedInteger
-                           «S» = [0xA4] ++ utf8(S)      if S ∈ String
-                                 [0xA5] ++ S            if S ∈ ByteString
-                                 [0xA6] ++ utf8(S)      if S ∈ Symbol
+                           «F» = [0xA2] ++ binary32(F)     if F ∈ Float
+                           «D» = [0xA2] ++ binary64(D)     if D ∈ Double
+                           «x» = [0xA3] ++ intbytes(x)     if x ∈ SignedInteger
+                           «S» = [0xA4] ++ utf8(S) ++ [0]  if S ∈ String
+                                 [0xA5] ++ S               if S ∈ ByteString
+                                 [0xA6] ++ utf8(S)         if S ∈ Symbol
 
                «<L F_1...F_m>» = [0xA7] ++ seq(«L», «F_1», ..., «F_m»)
                  «[X_1...X_m]» = [0xA8] ++ seq(«X_1», ..., «X_m»)

preserves-binary.md

@@ -153,14 +153,22 @@ For example,
 
 ### Strings, ByteStrings and Symbols.
 
-Syntax for these three types varies only in the tag used. For `String`
-and `Symbol`, the data following the tag is a UTF-8 encoding of the
-`Value`'s code points, while for `ByteString` it is the raw data
-contained within the `Value` unmodified.
+    «S» = [0xA4] ++ utf8(S) ++ [0]  if S ∈ String
+          [0xA5] ++ S               if S ∈ ByteString
+          [0xA6] ++ utf8(S)         if S ∈ Symbol
 
-    «S» = [0xA4] ++ utf8(S)  if S ∈ String
-          [0xA5] ++ S        if S ∈ ByteString
-          [0xA6] ++ utf8(S)  if S ∈ Symbol
+For `String` and `Symbol`, the data following the tag is a UTF-8
+encoding of the `Value`'s code points, while for `ByteString` it is the
+raw data contained within the `Value` unmodified.
+
+Each `String` has a trailing zero byte appended. This extra byte *MUST
+NOT* be treated as part of the `Value`: it exists to permit zero-copy C
+interoperability.[^zero-copy-c-string-interop]
+
+  [^zero-copy-c-string-interop]: Some care must still be taken when
+    passing `String` `Repr`s directly to a C-style ABI, since `String`s
+    may contain the zero Unicode code point, which C library routines
+    will usually misinterpret as an end-of-string marker.
 
 ### Booleans.
 
@@ -221,7 +229,8 @@ the same `Value` to yield different binary `Repr`s.
 ## Acknowledgements
 
 The exclusion of lengths from `Repr`s, placing lengths instead ahead of
-contained values in sequences, is inspired by [argdata][].
+contained values in sequences, is inspired by [argdata][], as is the
+inclusion of a `NUL` byte in `String` `Repr`s for C interoperability.
 
 ## Appendix. Autodetection of textual or binary syntax