Perform file readahead into page cache

# eio_readahead

(PECL eio >= 0.0.1dev)

eio_readahead — Perform file readahead into page cache

### 说明

function eio_readahead(
mixed `$fd`,
int `$offset`,
int `$length`,
int `$pri` = EIO_PRI_DEFAULT,
callable `$callback` = NULL,
mixed `$data` = NULL
): resource

eio_readahead() populates the page cache with data from a file so that subsequent reads from that file will not block on disk I/O. See `READAHEAD(2)` man page for details.

### 参数

`fd`
  Stream, Socket resource, or numeric file descriptor
`offset`
  Starting point from which data is to be read.
`length`
  Number of bytes to be read.
`pri`
  请求的优先级：`EIO_PRI_DEFAULT`，`EIO_PRI_MIN`，`EIO_PRI_MAX` 或 `null`。如果是 `null`，`pri` 将设为 `EIO_PRI_DEFAULT`。
`callback`

  `callback` 函数在请求完成时被调用。其应匹配一下原型：

      void callback(mixed $data, int $result[, resource $req]);

  `data`
    传递给请求的用户数据。
  `result`
    针对请求的结果的值。通常是相应的系统调用返回的值。
  `req`
    可选的请求资源，可被 eio_get_last_error() 之类的函数使用。
`data`
  Arbitrary variable passed to `callback`.

### 返回值

eio_readahead() returns request resource on success, 或者在失败时返回 `false`.

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