posix_fadvise - 預先聲明對文件數據的訪問模式

內容簡介

#define _XOPEN_SOURCE 600
#include <fcntl.h>
int posix_fadvise(int fd**, off_t** offset**, off_t** len**, int** advice**);**

描述

程序可以使用  posix_fadvise () 公佈的意圖來訪問文件中的數據在未來的特定圖案，從而允許內核來執行適當的優化。

The advice applies to a (not necessarily existent) region starting at offset and extending for len bytes (or until the end of the file if len is 0) within the file referred to by fd. The advice is not binding; it merely constitutes an expectation on behalf of the application.

Permissible values for advice include:

標籤

描述

POSIX_FADV_NORMAL

 

表示該應用程序沒有建議提供有關其指定的數據訪問模式。如果沒有意見，給出了一個打開的文件，這是默認的假設。

POSIX_FADV_SEQUENTIAL

 

該應用程序需要訪問指定的數據順序（與以前高的人讀低偏移）。

POSIX_FADV_RANDOM

 

將指定的數據將會以隨機順序進行訪問。

POSIX_FADV_NOREUSE

 

將指定的數據將只訪問一次。

POSIX_FADV_WILLNEED

 

將指定的數據將在不久的將來訪問。

POSIX_FADV_DONTNEED

 

指定的數據不會在短期內被訪問。

返回值

On success, zero is returned. On error, -1 is returned, and  errno  is set appropriately.

錯誤

標籤

描述

EBADF

The fd argument was not a valid file descriptor.

EINVAL

An invalid value was specified for advice.

ESPIPE

The specified file descriptor refers to a pipe or FIFO. (Linux actually returns EINVAL in this case.)

注意

posix_fadvise () appeared in kernel 2.5.60.

Under Linux, POSIX_FADV_NORMAL sets the readahead window to the default size for the backing device; POSIX_FADV_SEQUENTIAL doubles this size, andPOSIX_FADV_RANDOM disables file readahead entirely. These changes affect the entire file, not just the specified region (but other open file handles to the same file are unaffected).

POSIX_FADV_WILLNEED and POSIX_FADV_NOREUSE both initiate a non-blocking read of the specified region into the page cache. The amount of data read may be decreased by the kernel depending on VM load. (A few megabytes will usually be fully satisfied, and more is rarely useful.)

POSIX_FADV_DONTNEED attempts to free cached pages associated with the specified region. This is useful, for example, while streaming large files. A program may periodically request the kernel to free cached data that has already been used, so that more useful cached pages are not discarded instead.

Pages that have not yet been written out will be unaffected, so if the application wishes to guarantee that pages will be released, it should call fsync() or fdatasync() first.

遵循於

POSIX.1-2001. Note that the type of the  len  parameter was changed from  size_t  to  off_t in POSIX.1-2003 TC5.

BUGS

In kernels before 2.6.6, if  len  was specified as 0, then this was interpreted literally as "zero bytes", rather than as meaning "all bytes through to the end of the file".

另請參閱

• posix_madvise (2)

• readahead (2)