Data.ByteString.Unsafe

bytestring-0.9.1.10: Fast, packed, strict and lazy byte arrays with a list interface

Portability | portable
Stability | experimental
Maintainer | dons@cse.unsw.edu.au, duncan@haskell.org

Data.ByteString.Unsafe

Contents

- Unchecked access
- Low level interaction with CStrings
  - Using ByteStrings with functions for CStrings
  - Converting CStrings to ByteStrings

Description

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.

Synopsis

- unsafeHead :: ByteString -> Word8
- unsafeTail :: ByteString -> ByteString
- unsafeIndex :: ByteString -> Int -> Word8
- unsafeTake :: Int -> ByteString -> ByteString
- unsafeDrop :: Int -> ByteString -> ByteString
- unsafeUseAsCString :: ByteString -> (CString -> IO a) -> IO a
- unsafeUseAsCStringLen :: ByteString -> (CStringLen -> IO a) -> IO a
- unsafePackCString :: CString -> IO ByteString
- unsafePackCStringLen :: CStringLen -> IO ByteString
- unsafePackMallocCString :: CString -> IO ByteString
- unsafePackAddress :: Addr# -> IO ByteString
- unsafePackAddressLen :: Int -> Addr# -> IO ByteString
- unsafePackCStringFinalizer :: Ptr Word8 -> Int -> IO () -> IO ByteString
- unsafeFinalize :: ByteString -> IO ()

# 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 `ByteString`s ever generated from the underlying byte array are no longer live.