#define _XOPEN_SOURCE 600 #include <fcntl.h> int posix_fadvise(int fd, off_t offset, off_t len, int advice); |
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 | |
| 表示該應用程式沒有建議提供有關其指定的資料存取模式。如果沒有意見，給出了一個開啟的檔案，這是預設的假設。 | |
| POSIX_FADV_SEQUENTIAL | |
| 該應用程式需要存取指定的資料順序（與以前高的人讀低偏移）。 | |
| POSIX_FADV_RANDOM | |
| 將指定的資料將會以隨機順序進行存取。 | |
| POSIX_FADV_NOREUSE | |
| 將指定的資料將只存取一次。 | |
| POSIX_FADV_WILLNEED | |
| 將指定的資料將在不久的將來存取。 | |
| POSIX_FADV_DONTNEED | |
| 指定的資料不會在短期內被存取。 | |
| 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.) |
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.