Haskell Libraries
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# 7.5. Posix Input and Output Primitives

    createPipe :: IO (Fd, Fd)

`createPipe` calls `pipe` to create a pipe and returns a pair of `Fd`s, the first for reading and the second for writing.

    dup :: Fd -> IO Fd

`dup fd` calls `dup` to duplicate `Fd` `fd` to another `Fd`.

    dupTo :: Fd -> Fd -> IO ()

`dupTo src dst` calls `dup2` to duplicate `Fd` `src` to `Fd` `dst`.

    fdClose :: Fd -> IO ()

`fdClose fd` calls `close` to close `Fd` `fd`.

    fdRead :: Fd -> ByteCount -> IO (String, ByteCount)

`fdRead fd nbytes` calls `read` to read at most `nbytes` bytes from `Fd` `fd`, and returns the result as a string paired with the number of bytes actually read.

The operation may fail with:

`EOF`

  End of file has been reached.

`SystemError`

  Various other causes.

    fdWrite :: Fd -> String -> IO ByteCount

`fdWrite fd s` calls `write` to write the string `s` to `Fd` `fd` as a contiguous sequence of bytes. It returns the number of bytes successfully written.

    queryFdOption :: FdOption -> Fd -> IO Bool

`getFdOption opt fd` calls `fcntl` to determine whether or not the flag associated with `FdOption` `opt` is set for `Fd` `fd`.

    setFdOption :: Fd -> FdOption -> Bool -> IO ()

`setFdOption fd opt val` calls `fcntl` to set the flag associated with `FdOption` `opt` on `Fd` `fd` to `val`.

    getLock :: Fd -> FileLock -> IO (Maybe (ProcessID, FileLock))

`getLock fd lock` calls `fcntl` to get the first `FileLock` for `Fd` `fd` which blocks the `FileLock` `lock`. If no such `FileLock` exists, `getLock` returns `Nothing`. Otherwise, it returns `Just (pid, block)`, where `block` is the blocking `FileLock` and `pid` is the `ProcessID` of the process holding the blocking `FileLock`.

    setLock :: Fd -> FileLock -> IO ()

`setLock fd lock` calls `fcntl` with `F_SETLK` to set or clear a lock segment for `Fd` `fd` as indicated by the `FileLock` `lock`. `setLock` does not block, but fails with `SystemError` if the request cannot be satisfied immediately.

    waitToSetLock :: Fd -> FileLock -> IO ()

`waitToSetLock fd lock` calls `fcntl` with `F_SETLKW` to set or clear a lock segment for `Fd` `fd` as indicated by the `FileLock` `lock`. If the request cannot be satisfied immediately, `waitToSetLock` blocks until the request can be satisfied.

    fdSeek :: Fd -> SeekMode -> FileOffset -> IO FileOffset

`fdSeek fd whence offset` calls `lseek` to position the `Fd` `fd` at the given `offset` from the starting location indicated by `whence`. It returns the resulting offset from the start of the file in bytes.

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