Data.ByteString.Unsafe

A module containing unsafe ByteString operations.

While these functions have a stable API and you may use these functions in applications, do carefully consider the documented pre-conditions; incorrect use can break referential transparency or worse.

Unchecked access

unsafeHead :: ByteString -> Word8

A variety of head for non-empty ByteStrings. unsafeHead omits the check for the empty case, so there is an obligation on the programmer to provide a proof that the ByteString is non-empty.

unsafeTail :: ByteString -> ByteString

A variety of tail for non-empty ByteStrings. unsafeTail omits the check for the empty case. As with unsafeHead, the programmer must provide a separate proof that the ByteString is non-empty.

unsafeIndex :: ByteString -> Int -> Word8

Unsafe ByteString index (subscript) operator, starting from 0, returning a Word8 This omits the bounds check, which means there is an accompanying obligation on the programmer to ensure the bounds are checked in some other way.

unsafeTake :: Int -> ByteString -> ByteString

A variety of take which omits the checks on n so there is an obligation on the programmer to provide a proof that 0 <= n <= length xs.

unsafeDrop :: Int -> ByteString -> ByteString

A variety of drop which omits the checks on n so there is an obligation on the programmer to provide a proof that 0 <= n <= length xs.

Low level interaction with CStrings

Using ByteStrings with functions for CStrings

unsafeUseAsCString :: ByteString -> (CString -> IO a) -> IO a

O(1) construction Use a ByteString with a function requiring a CString.

This function does zero copying, and merely unwraps a ByteString to appear as a CString. It is unsafe in two ways:

• After calling this function the CString shares the underlying byte buffer with the original ByteString. Thus modifying the CString, either in C, or using poke, will cause the contents of the ByteString to change, breaking referential transparency. Other ByteStrings created by sharing (such as those produced via take or drop) will also reflect these changes. Modifying the CString will break referential transparency. To avoid this, use useAsCString, which makes a copy of the original ByteString.
• CStrings are often passed to functions that require them to be null-terminated. If the original ByteString wasn't null terminated, neither will the CString be. It is the programmers responsibility to guarantee that the ByteString is indeed null terminated. If in doubt, use useAsCString.

unsafeUseAsCStringLen :: ByteString -> (CStringLen -> IO a) -> IO a

O(1) construction Use a ByteString with a function requiring a CStringLen.

This function does zero copying, and merely unwraps a ByteString to appear as a CStringLen. It is unsafe:

• After calling this function the CStringLen shares the underlying byte buffer with the original ByteString. Thus modifying the CStringLen, either in C, or using poke, will cause the contents of the ByteString to change, breaking referential transparency. Other ByteStrings created by sharing (such as those produced via take or drop) will also reflect these changes. Modifying the CStringLen will break referential transparency. To avoid this, use useAsCStringLen, which makes a copy of the original ByteString.

Converting CStrings to ByteStrings

unsafePackCString :: CString -> IO ByteString

O(n) Build a ByteString from a CString. This value will have no finalizer associated to it, and will not be garbage collected by Haskell. The ByteString length is calculated using strlen(3), and thus the complexity is a O(n).

This function is unsafe. If the CString is later modified, this change will be reflected in the resulting ByteString, breaking referential transparency.

unsafePackCStringLen :: CStringLen -> IO ByteString

O(1) Build a ByteString from a CStringLen. This value will have no finalizer associated with it, and will not be garbage collected by Haskell. This operation has O(1) complexity as we already know the final size, so no strlen(3) is required.

This funtion is unsafe. If the original CStringLen is later modified, this change will be reflected in the resulting ByteString, breaking referential transparency.

unsafePackMallocCString :: CString -> IO ByteString

O(n) Build a ByteString from a malloced CString. This value will have a free(3) finalizer associated to it.

This funtion is unsafe. If the original CString is later modified, this change will be reflected in the resulting ByteString, breaking referential transparency.

This function is also unsafe if you call its finalizer twice, which will result in a double free error, or if you pass it a CString not allocated with malloc.

unsafePackAddress :: Addr# -> IO ByteString

O(n) Pack a null-terminated sequence of bytes, pointed to by an Addr# (an arbitrary machine address assumed to point outside the garbage-collected heap) into a ByteString. A much faster way to create an Addr# is with an unboxed string literal, than to pack a boxed string. A unboxed string literal is compiled to a static char [] by GHC. Establishing the length of the string requires a call to strlen(3), so the Addr# must point to a null-terminated buffer (as is the case with string# literals in GHC). Use unsafePackAddressLen if you know the length of the string statically.

An example:

 literalFS = unsafePackAddress "literal"#

This function is unsafe. If you modify the buffer pointed to by the original Addr# this modification will be reflected in the resulting ByteString, breaking referential transparency.

Note this also won't work if you Add# has embedded '\0' characters in the string (strlen will fail).

unsafePackAddressLen :: Int -> Addr# -> IO ByteString

O(1) unsafePackAddressLen provides constant-time construction of ByteStrings which is ideal for string literals. It packs a sequence of bytes into a ByteString, given a raw Addr# to the string, and the length of the string.

This function is unsafe in two ways:

• the length argument is assumed to be correct. If the length argument is incorrect, it is possible to overstep the end of the byte array.
• if the underying Addr# is later modified, this change will be reflected in resulting ByteString, breaking referential transparency.

If in doubt, don't use these functions.

unsafePackCStringFinalizer :: Ptr Word8 -> Int -> IO () -> IO ByteString

O(1) Construct a ByteString given a Ptr Word8 to a buffer, a length, and an IO action representing a finalizer. This function is not available on Hugs.

This function is unsafe, it is possible to break referential transparency by modifying the underlying buffer pointed to by the first argument. Any changes to the original buffer will be reflected in the resulting ByteString.

unsafeFinalize :: ByteString -> IO ()

Explicitly run the finaliser associated with a ByteString. References to this value after finalisation may generate invalid memory references.

This function is unsafe, as there may be other ByteStrings referring to the same underlying pages. If you use this, you need to have a proof of some kind that all ByteStrings ever generated from the underlying byte array are no longer live.