10 KiB
Buffers
Pure Javascript is Unicode friendly but not nice to binary data. When dealing with TCP streams or the file system, it's necessary to handle octet streams. Node has several strategies for manipulating, creating, and consuming octet streams.
Raw data is stored in instances of the Buffer class. A Buffer is similar
to an array of integers but corresponds to a raw memory allocation outside
the V8 heap. A Buffer cannot be resized.
The Buffer object is global.
Converting between Buffers and JavaScript string objects requires an explicit encoding method. Here are the different string encodings;
-
'ascii'- for 7 bit ASCII data only. This encoding method is very fast, and will strip the high bit if set. -
'utf8'- Multi byte encoded Unicode characters. Many web pages and other document formats use UTF-8. -
'ucs2'- 2-bytes, little endian encoded Unicode characters. It can encode only BMP(Basic Multilingual Plane, U+0000 - U+FFFF). -
'base64'- Base64 string encoding. -
'binary'- A way of encoding raw binary data into strings by using only the first 8 bits of each character. This encoding method is deprecated and should be avoided in favor ofBufferobjects where possible. This encoding will be removed in future versions of Node. -
'hex'- Encode each byte as two hexidecimal characters.
new Buffer(size)
Allocates a new buffer of size octets.
new Buffer(array)
Allocates a new buffer using an array of octets.
new Buffer(str, encoding='utf8')
Allocates a new buffer containing the given str.
buffer.write(string, offset=0, encoding='utf8')
Writes string to the buffer at offset using the given encoding. Returns
number of octets written. If buffer did not contain enough space to fit
the entire string, it will write a partial amount of the string.
The method will not write partial characters.
Example: write a utf8 string into a buffer, then print it
buf = new Buffer(256);
len = buf.write('\u00bd + \u00bc = \u00be', 0);
console.log(len + " bytes: " + buf.toString('utf8', 0, len));
The number of characters written (which may be different than the number of
bytes written) is set in Buffer._charsWritten and will be overwritten the
next time buf.write() is called.
buffer.toString(encoding, start=0, end=buffer.length)
Decodes and returns a string from buffer data encoded with encoding
beginning at start and ending at end.
See buffer.write() example, above.
buffer[index]
Get and set the octet at index. The values refer to individual bytes,
so the legal range is between 0x00 and 0xFF hex or 0 and 255.
Example: copy an ASCII string into a buffer, one byte at a time:
str = "node.js";
buf = new Buffer(str.length);
for (var i = 0; i < str.length ; i++) {
buf[i] = str.charCodeAt(i);
}
console.log(buf);
// node.js
Buffer.isBuffer(obj)
Tests if obj is a Buffer.
Buffer.byteLength(string, encoding='utf8')
Gives the actual byte length of a string. This is not the same as
String.prototype.length since that returns the number of characters in a
string.
Example:
str = '\u00bd + \u00bc = \u00be';
console.log(str + ": " + str.length + " characters, " +
Buffer.byteLength(str, 'utf8') + " bytes");
// ½ + ¼ = ¾: 9 characters, 12 bytes
buffer.length
The size of the buffer in bytes. Note that this is not necessarily the size
of the contents. length refers to the amount of memory allocated for the
buffer object. It does not change when the contents of the buffer are changed.
buf = new Buffer(1234);
console.log(buf.length);
buf.write("some string", "ascii", 0);
console.log(buf.length);
// 1234
// 1234
buffer.copy(targetBuffer, targetStart=0, sourceStart=0, sourceEnd=buffer.length)
Does a memcpy() between buffers.
Example: build two Buffers, then copy buf1 from byte 16 through byte 19
into buf2, starting at the 8th byte in buf2.
buf1 = new Buffer(26);
buf2 = new Buffer(26);
for (var i = 0 ; i < 26 ; i++) {
buf1[i] = i + 97; // 97 is ASCII a
buf2[i] = 33; // ASCII !
}
buf1.copy(buf2, 8, 16, 20);
console.log(buf2.toString('ascii', 0, 25));
// !!!!!!!!qrst!!!!!!!!!!!!!
buffer.slice(start, end=buffer.length)
Returns a new buffer which references the
same memory as the old, but offset and cropped by the start and end
indexes.
Modifying the new buffer slice will modify memory in the original buffer!
Example: build a Buffer with the ASCII alphabet, take a slice, then modify one byte from the original Buffer.
var buf1 = new Buffer(26);
for (var i = 0 ; i < 26 ; i++) {
buf1[i] = i + 97; // 97 is ASCII a
}
var buf2 = buf1.slice(0, 3);
console.log(buf2.toString('ascii', 0, buf2.length));
buf1[0] = 33;
console.log(buf2.toString('ascii', 0, buf2.length));
// abc
// !bc
buffer.readUInt8(offset, endian)
Reads an unsigned 8 bit integer from the buffer at the specified offset. Endian must be either 'big' or 'little' and specifies what endian ordering to read the bytes from the buffer in.
Example:
var buf = new Buffer(4);
buf[0] = 0x3;
buf[1] = 0x4;
buf[2] = 0x23;
buf[3] = 0x42;
for (ii = 0; ii < buf.length; ii++) {
console.log(buf.readUInt8(ii, 'big');
console.log(buf.readUInt8(ii, 'little');
}
// 0x3
// 0x3
// 0x4
// 0x4
// 0x23
// 0x23
// 0x42
// 0x42
buffer.readUInt16(offset, endian)
Reads an unsigned 16 bit integer from the buffer at the specified offset. Endian must be either 'big' or 'little' and specifies what endian ordering to read the bytes from the buffer in.
Example:
var buf = new Buffer(4);
buf[0] = 0x3;
buf[1] = 0x4;
buf[2] = 0x23;
buf[3] = 0x42;
console.log(buf.readUInt16(0, 'big');
console.log(buf.readUInt16(0, 'little');
console.log(buf.readUInt16(1, 'big');
console.log(buf.readUInt16(1, 'little');
console.log(buf.readUInt16(2, 'big');
console.log(buf.readUInt16(2, 'little');
// 0x0304
// 0x0403
// 0x0423
// 0x2304
// 0x2342
// 0x4223
buffer.readUInt32(offset, endian)
Reads an unsigned 32 bit integer from the buffer at the specified offset. Endian must be either 'big' or 'little' and specifies what endian ordering to read the bytes from the buffer in.
Example:
var buf = new Buffer(4);
buf[0] = 0x3;
buf[1] = 0x4;
buf[2] = 0x23;
buf[3] = 0x42;
console.log(buf.readUInt32(0, 'big');
console.log(buf.readUInt32(0, 'little');
// 0x03042342
// 0x42230403
buffer.readInt8(offset, endian)
Reads a signed 8 bit integer from the buffer at the specified offset. Endian must be either 'big' or 'little' and specifies what endian ordering to read the bytes from the buffer in.
Works as buffer.readUInt8, except buffer contents are treated as twos
complement signed values.
buffer.readInt16(offset, endian)
Reads a signed 16 bit integer from the buffer at the specified offset. Endian must be either 'big' or 'little' and specifies what endian ordering to read the bytes from the buffer in.
Works as buffer.readUInt16, except buffer contents are treated as twos
complement signed values.
buffer.readInt32(offset, endian)
Reads a signed 32 bit integer from the buffer at the specified offset. Endian must be either 'big' or 'little' and specifies what endian ordering to read the bytes from the buffer in.
Works as buffer.readUInt32, except buffer contents are treated as twos
complement signed values.
buffer.writeUInt8(value, offset, endian)
Writes value to the buffer at the specified offset with specified endian
format. Note, value must be a valid 8 bit unsigned integer.
Example:
var buf = new Buffer(4);
buf.writeUInt8(0x3, 0, 'big');
buf.writeUInt8(0x4, 1, 'big');
buf.writeUInt8(0x23, 2, 'big');
buf.writeUInt8(0x42, 3, 'big');
console.log(buf);
buf.writeUInt8(0x3, 0, 'little');
buf.writeUInt8(0x4, 1, 'little');
buf.writeUInt8(0x23, 2, 'little');
buf.writeUInt8(0x42, 3, 'little');
console.log(buf);
// <Buffer 03 04 23 42>
// <Buffer 03 04 23 42>
buffer.writeUInt16(value, offset, endian)
Writes value to the buffer at the specified offset with specified endian
format. Note, value must be a valid 16 bit unsigned integer.
Example:
var buf = new Buffer(4);
buf.writeUInt16(0xdead, 0, 'big');
buf.writeUInt16(0xbeef, 2, 'big');
console.log(buf);
buf.writeUInt16(0xdead, 0, 'little');
buf.writeUInt16(0xbeef, 2, 'little');
console.log(buf);
// <Buffer de ad be ef>
// <Buffer ad de ef be>
buffer.writeUInt32(value, offset, endian)
Writes value to the buffer at the specified offset with specified endian
format. Note, value must be a valid 32 bit unsigned integer.
Example:
var buf = new Buffer(4);
buf.writeUInt32(0xfeedface, 0, 'big');
console.log(buf);
buf.writeUInt32(0xfeedface, 0, 'little');
console.log(buf);
// <Buffer fe ed fa ce>
// <Buffer ce fa ed fe>
buffer.writeInt8(value, offset, endian)
Writes value to the buffer at the specified offset with specified endian
format. Note, value must be a valid 16 bit signed integer.
Works as buffer.writeUInt8, except value is written out as a two's complement
signed integer into buffer.
buffer.writeInt16(value, offset, endian)
Writes value to the buffer at the specified offset with specified endian
format. Note, value must be a valid 16 bit unsigned integer.
Works as buffer.writeUInt16, except value is written out as a two's complement
signed integer into buffer.
buffer.writeInt32(value, offset, endian)
Writes value to the buffer at the specified offset with specified endian
format. Note, value must be a valid 16 bit signed integer.
Works as buffer.writeUInt832, except value is written out as a two's complement signed integer into buffer`.
buffer.fill(value, offset=0, length=-1)
Fills the buffer with the specified value. If the offset and length are not given it will fill the entire buffer.
var b = new Buffer(50);
b.fill("h");