diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5e95810..bef093f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@
 * Add `wordPaddedDec4`.
 * Add `reversedOnto` and `commitsOntoChunks`.
 * Add `ascii(2|3|4|5|6)`.
+* Add `cstringLen` unbounded builder.
 
 ## 0.3.3.0 -- 2020-02-10
 
diff --git a/src/Data/ByteArray/Builder.hs b/src/Data/ByteArray/Builder.hs
index d1f8264..2b96d33 100644
--- a/src/Data/ByteArray/Builder.hs
+++ b/src/Data/ByteArray/Builder.hs
@@ -26,6 +26,7 @@ module Data.ByteArray.Builder
   , shortTextUtf8
   , shortTextJsonString
   , cstring
+  , cstringLen
   , stringUtf8
     -- * Encode Integral Types
     -- ** Human-Readable
@@ -132,6 +133,7 @@ import Data.Primitive (ByteArray(..),MutableByteArray(..),PrimArray(..))
 import Data.Text.Short (ShortText)
 import Data.WideWord (Word128,Word256)
 import Data.Word (Word64,Word32,Word16,Word8)
+import Foreign.C.String (CStringLen)
 import GHC.ByteOrder (ByteOrder(BigEndian,LittleEndian),targetByteOrder)
 import GHC.Exts (Int(I#),Char(C#),Int#,State#,ByteArray#,(>=#))
 import GHC.Exts (RealWorld,MutableByteArray#,(+#),(-#),(<#))
@@ -324,11 +326,11 @@ fromBoundedOne (UnsafeBounded.Builder f) = Builder $ \buf0 off0 len0 cs0 s0 ->
    in case f buf1 off1 s1 of
         (# s2, off2 #) -> (# s2, buf1, off2, len1 -# (off2 -# off1), cs1 #)
 
--- | Create a builder from an unsliced byte sequence.
+-- | Create a builder from an unsliced byte sequence. Implemented with 'bytes'.
 byteArray :: ByteArray -> Builder
 byteArray a = bytes (Bytes a 0 (PM.sizeofByteArray a))
 
--- | Create a builder from a short bytestring.
+-- | Create a builder from a short bytestring. Implemented with 'bytes'.
 shortByteString :: ShortByteString -> Builder
 shortByteString (SBS x) = bytes (Bytes a 0 (PM.sizeofByteArray a))
   where a = ByteArray x
@@ -368,6 +370,20 @@ copy (Bytes (ByteArray src# ) (I# soff# ) (I# slen# )) = Builder
   where
   !(I# newSz) = max (I# slen#) 4080
 
+-- | Create a builder from a C string with explicit length. The builder
+-- must be executed before the C string is freed.
+cstringLen :: CStringLen -> Builder
+cstringLen (Exts.Ptr src#, I# slen# ) = Builder
+  (\buf0 off0 len0 cs0 s0 -> case len0 <# slen# of
+    1# -> case Exts.newByteArray# newSz s0 of
+        (# s1, buf1 #) -> case Exts.copyAddrToByteArray# src# buf1 0# slen# s1 of
+          s2 -> (# s2, buf1, slen#, newSz -# slen#, Mutable buf0 off0 cs0 #)
+    _ -> let !s1 = Exts.copyAddrToByteArray# src# buf0 off0 slen# s0 in
+      (# s1, buf0, off0 +# slen#, len0 -# slen#, cs0 #)
+  )
+  where
+  !(I# newSz) = max (I# slen#) 4080
+
 -- | Create a builder from a byte sequence. This never calls @memcpy@.
 -- Instead, it pushes a chunk that references the argument byte sequence.
 -- This wastes the remaining space in the active chunk, so it may adversely