filehandle.createReadStream([options])
options
<Object>- 返回: <fs.ReadStream>
与 <stream.Readable> 的 16 KiB 默认 highWaterMark
不同,此方法返回的流的默认 highWaterMark
为 64 KiB。
options
可以包括 start
和 end
值,以从文件中读取一定范围的字节,而不是整个文件。
start
和 end
都包含在内并且从 0 开始计数,允许的值在 [0, Number.MAX_SAFE_INTEGER
] 范围内。
如果省略 start
或为 undefined
,则 filehandle.createReadStream()
从当前的文件位置开始依次读取。
encoding
可以是 <Buffer> 接受的任何一种。
如果 FileHandle
指向只支持阻塞读取的字符设备(如键盘或声卡),则读取操作在数据可用之前不会完成。
这可以防止进程退出和流自然关闭。
默认情况下,流将在销毁后触发 'close'
事件。
将 emitClose
选项设置为 false
以更改此行为。
import { open } from 'node:fs/promises';
const fd = await open('/dev/input/event0');
// 从某个字符设备创建流。
const stream = fd.createReadStream();
setTimeout(() => {
stream.close(); // 这可能不会关闭流。
// 人为地标记流结束,就好像底层资源自己指示了文件结束一样,允许流关闭。
// 这不会取消挂起的读操作,如果有这样的操作,进程可能仍然无法成功退出,直到它完成。
stream.push(null);
stream.read(0);
}, 100);
如果 autoClose
为 false,则即使出现错误,文件描述符也不会关闭。
关闭它并确保没有文件描述符泄漏是应用程序的责任。
如果 autoClose
设置为 true(默认行为),则在 'error'
或 'end'
时,文件描述符将自动关闭。
读取 100 个字节长的文件的最后 10 个字节的示例:
import { open } from 'node:fs/promises';
const fd = await open('sample.txt');
fd.createReadStream({ start: 90, end: 99 });
options
<Object>- Returns: <fs.ReadStream>
Unlike the 16 KiB default highWaterMark
for a <stream.Readable>, the stream
returned by this method has a default highWaterMark
of 64 KiB.
options
can include start
and end
values to read a range of bytes from
the file instead of the entire file. Both start
and end
are inclusive and
start counting at 0, allowed values are in the
[0, Number.MAX_SAFE_INTEGER
] range. If start
is
omitted or undefined
, filehandle.createReadStream()
reads sequentially from
the current file position. The encoding
can be any one of those accepted by
<Buffer>.
If the FileHandle
points to a character device that only supports blocking
reads (such as keyboard or sound card), read operations do not finish until data
is available. This can prevent the process from exiting and the stream from
closing naturally.
By default, the stream will emit a 'close'
event after it has been
destroyed. Set the emitClose
option to false
to change this behavior.
import { open } from 'node:fs/promises';
const fd = await open('/dev/input/event0');
// Create a stream from some character device.
const stream = fd.createReadStream();
setTimeout(() => {
stream.close(); // This may not close the stream.
// Artificially marking end-of-stream, as if the underlying resource had
// indicated end-of-file by itself, allows the stream to close.
// This does not cancel pending read operations, and if there is such an
// operation, the process may still not be able to exit successfully
// until it finishes.
stream.push(null);
stream.read(0);
}, 100);
If autoClose
is false, then the file descriptor won't be closed, even if
there's an error. It is the application's responsibility to close it and make
sure there's no file descriptor leak. If autoClose
is set to true (default
behavior), on 'error'
or 'end'
the file descriptor will be closed
automatically.
An example to read the last 10 bytes of a file which is 100 bytes long:
import { open } from 'node:fs/promises';
const fd = await open('sample.txt');
fd.createReadStream({ start: 90, end: 99 });