Description of the fs. read method in node. js, node. jsfs. read

Description of the fs. read method in node. js, node. jsfs. read

Method description:

Reads file data based on the specified file descriptor fd and writes it to the buffer object pointed to by the buffer. Compared with readFile, it provides a more underlying interface.

Generally, this method is not recommended to read files because it requires you to manually manage the buffer and file pointer, especially when you do not know the file size, this will be a very troublesome task.

Syntax:

Copy codeThe Code is as follows:
Fs. read (fd, buffer, offset, length, position, [callback (err, bytesRead, buffer)])

Because this method belongs to the fs module, we need to introduce the fs module (var fs = require ("fs") before use "))

Receiving parameters:

Fs file descriptor

Buffer, data will be written.

Offset buffer write offset

Length (integer) specifies the length of the number of bytes that the file reads.

Position (integer) specifies the start position of File Reading. If this parameter is set to null, data is read from the current file pointer.

The callback passes three parameters: err, bytesRead, and buffer.

· Err exception

· BytesRead: number of bytes read

· Buffer: buffer object

Example:

Copy codeThe Code is as follows:
Var fs = require ('fs ');
Fs.open('123.txt ', 'R', function (err, fd ){
If (err ){
Console. error (err );
Return;
}
 
Var buf = new Buffer (8 );
Fs. read (fd, buf, 0, 8, null, function (err, bytesRead, buffer ){
If (err ){
Console. log (err );
Return;
}
Console. log ('bytesread' + bytesRead );
Console. log (buffer );
})
})

Source code:

Copy codeThe Code is as follows:
Fs. read = function (fd, buffer, offset, length, position, callback ){
If (! Util. isBuffer (buffer )){
// Legacy string interface (fd, length, position, encoding, callback)
Var cb = arguments [4],
Encoding = arguments [3];
AssertEncoding (encoding );
Position = arguments [2];
Length = arguments [1];
Buffer = new Buffer (length );
Offset = 0;
Callback = function (err, bytesRead ){
If (! Cb) return;
Var str = (bytesRead> 0 )? Buffer. toString (encoding, 0, bytesRead ):'';
(Cb) (err, str, bytesRead );
};
}
Function wrapper (err, bytesRead ){
// Retain a reference to buffer so that it can't be GC 'ed too soon.
Callback & callback (err, bytesRead | 0, buffer );
}
Binding. read (fd, buffer, offset, length, position, wrapper );
};