posix_fadvise()函数 Unix/Linux
内容简介
#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() 公布的意图来访问文件中的数据在未来的特定图案，从而允许内核来执行适当的优化。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:
| 标签 | 描述 |
|---|---|
| POSIX_FADV_NORMAL | |
| 表示该应用程序没有建议提供有关其指定的数据访问模式。如果没有意见，给出了一个打开的文件，这是默认的假设。 | |
| POSIX_FADV_SEQUENTIAL | |
| 该应用程序需要访问指定的数据顺序（与以前高的人读低偏移）。 | |
| POSIX_FADV_RANDOM | |
| 将指定的数据将会以随机顺序进行访问。 | |
| POSIX_FADV_NOREUSE | |
| 将指定的数据将只访问一次。 | |
| POSIX_FADV_WILLNEED | |
| 将指定的数据将在不久的将来访问。 | |
| POSIX_FADV_DONTNEED | |
| 指定的数据不会在短期内被访问。 | |
返回值
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.错误
| 标签 | 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.
