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Foreign.C.String

Portability | portable
Stability | provisional
Maintainer | ffi@haskell.org

Contents

C strings
  Using a locale-dependent encoding
  Using 8-bit characters
C wide strings

Description

Utilities for primitive marshalling of C strings.

The marshalling converts each Haskell character, representing a Unicode code point, to one or more bytes in a manner that, by default, is determined by the current locale. As a consequence, no guarantees can be made about the relative length of a Haskell string and its corresponding C string, and therefore all the marshalling routines include memory allocation. The translation between Unicode and the encoding of the current locale may be lossy.

Synopsis

type CString = Ptr CChar
type CStringLen = (Ptr CChar, Int)
peekCString :: CString -> IO String
peekCStringLen :: CStringLen -> IO String
newCString :: String -> IO CString
newCStringLen :: String -> IO CStringLen
withCString :: String -> (CString -> IO a) -> IO a
withCStringLen :: String -> (CStringLen -> IO a) -> IO a
charIsRepresentable :: Char -> IO Bool
castCharToCChar :: Char -> CChar
castCCharToChar :: CChar -> Char
peekCAString :: CString -> IO String
peekCAStringLen :: CStringLen -> IO String
newCAString :: String -> IO CString
newCAStringLen :: String -> IO CStringLen
withCAString :: String -> (CString -> IO a) -> IO a
withCAStringLen :: String -> (CStringLen -> IO a) -> IO a
type CWString = Ptr CWchar
type CWStringLen = (Ptr CWchar, Int)
peekCWString :: CWString -> IO String
peekCWStringLen :: CWStringLen -> IO String
newCWString :: String -> IO CWString
newCWStringLen :: String -> IO CWStringLen
withCWString :: String -> (CWString -> IO a) -> IO a
withCWStringLen :: String -> (CWStringLen -> IO a) -> IO a

C strings
type CString = Ptr CChar
A C string is a reference to an array of C characters terminated by NUL.
type CStringLen = (Ptr CChar, Int)
A string with explicit length information in bytes instead of a terminating NUL (allowing NUL characters in the middle of the string).
Using a locale-dependent encoding
Currently these functions are identical to their `CAString` counterparts; eventually they will use an encoding determined by the current locale.
peekCString :: CString -> IO String
Marshal a NUL terminated C string into a Haskell string.
peekCStringLen :: CStringLen -> IO String
Marshal a C string with explicit length into a Haskell string.
newCString :: String -> IO CString

Marshal a Haskell string into a NUL terminated C string.

- the Haskell string may not contain any NUL characters
- new storage is allocated for the C string and must be explicitly freed using `free` or `finalizerFree`.

newCStringLen :: String -> IO CStringLen

Marshal a Haskell string into a C string (ie, character array) with explicit length information.

- new storage is allocated for the C string and must be explicitly freed using `free` or `finalizerFree`.

withCString :: String -> (CString -> IO a) -> IO a

Marshal a Haskell string into a NUL terminated C string using temporary storage.

- the Haskell string may not contain any NUL characters
- the memory is freed when the subcomputation terminates (either normally or via an exception), so the pointer to the temporary storage must not be used after this.

withCStringLen :: String -> (CStringLen -> IO a) -> IO a

Marshal a Haskell string into a C string (ie, character array) in temporary storage, with explicit length information.

- the memory is freed when the subcomputation terminates (either normally or via an exception), so the pointer to the temporary storage must not be used after this.

charIsRepresentable :: Char -> IO Bool
Determines whether a character can be accurately encoded in a `CString`. Unrepresentable characters are converted to `'?'`. Currently only Latin-1 characters are representable.
Using 8-bit characters
These variants of the above functions are for use with C libraries that are ignorant of Unicode. These functions should be used with care, as a loss of information can occur.
castCharToCChar :: Char -> CChar
Convert a Haskell character to a C character. This function is only safe on the first 256 characters.
castCCharToChar :: CChar -> Char
Convert a C byte, representing a Latin-1 character, to the corresponding Haskell character.
peekCAString :: CString -> IO String
Marshal a NUL terminated C string into a Haskell string.
peekCAStringLen :: CStringLen -> IO String
Marshal a C string with explicit length into a Haskell string.
newCAString :: String -> IO CString

Marshal a Haskell string into a NUL terminated C string.

- the Haskell string may not contain any NUL characters
- new storage is allocated for the C string and must be explicitly freed using `free` or `finalizerFree`.

newCAStringLen :: String -> IO CStringLen

Marshal a Haskell string into a C string (ie, character array) with explicit length information.

- new storage is allocated for the C string and must be explicitly freed using `free` or `finalizerFree`.

withCAString :: String -> (CString -> IO a) -> IO a

Marshal a Haskell string into a NUL terminated C string using temporary storage.

- the Haskell string may not contain any NUL characters
- the memory is freed when the subcomputation terminates (either normally or via an exception), so the pointer to the temporary storage must not be used after this.

withCAStringLen :: String -> (CStringLen -> IO a) -> IO a

Marshal a Haskell string into a C string (ie, character array) in temporary storage, with explicit length information.

- the memory is freed when the subcomputation terminates (either normally or via an exception), so the pointer to the temporary storage must not be used after this.

C wide strings

These variants of the above functions are for use with C libraries that encode Unicode using the C `wchar_t` type in a system-dependent way. The only encodings supported are

- UTF-32 (the C compiler defines `__STDC_ISO_10646__`), or
- UTF-16 (as used on Windows systems).

type CWString = Ptr CWchar
A C wide string is a reference to an array of C wide characters terminated by NUL.
type CWStringLen = (Ptr CWchar, Int)
A wide character string with explicit length information in bytes instead of a terminating NUL (allowing NUL characters in the middle of the string).
peekCWString :: CWString -> IO String
Marshal a NUL terminated C wide string into a Haskell string.
peekCWStringLen :: CWStringLen -> IO String
Marshal a C wide string with explicit length into a Haskell string.
newCWString :: String -> IO CWString

Marshal a Haskell string into a NUL terminated C wide string.

- the Haskell string may not contain any NUL characters
- new storage is allocated for the C wide string and must be explicitly freed using `free` or `finalizerFree`.

newCWStringLen :: String -> IO CWStringLen

Marshal a Haskell string into a C wide string (ie, wide character array) with explicit length information.

- new storage is allocated for the C wide string and must be explicitly freed using `free` or `finalizerFree`.

withCWString :: String -> (CWString -> IO a) -> IO a

Marshal a Haskell string into a NUL terminated C wide string using temporary storage.

- the Haskell string may not contain any NUL characters
- the memory is freed when the subcomputation terminates (either normally or via an exception), so the pointer to the temporary storage must not be used after this.

withCWStringLen :: String -> (CWStringLen -> IO a) -> IO a

Marshal a Haskell string into a NUL terminated C wide string using temporary storage.

- the Haskell string may not contain any NUL characters
- the memory is freed when the subcomputation terminates (either normally or via an exception), so the pointer to the temporary storage must not be used after this.

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