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A brief analysis of Buffer in NodeJS

青灯夜游

Release： 2020-11-18 17:59:51

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A brief analysis of Buffer in NodeJS

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I read Pu Ling's Nine Lights and One Deep NodeJS to get started with Node. , Park Dada’s book talks very little about practice and devotes more space to explaining principles. After listening to so many principles, he later started to use NodeJS in the field of front-end engineering but was hampered everywhere. Summarizing the reasons, he found that the difficult part in NodeJS is nothing more than files and files. Networking, file operations and networking all rely on a very important object - Stream, which is exactly what Park did not mention in his book.

Buffer Park Dada mentioned it in the book, but because the stream is actually processing the Buffer, it still needs to be briefly summarized.

What is Buffer

As introduced in the official API, before ES6 introduced TypedArray, JavaScript had no mechanism to read or operate binary data streams. The Buffer class was introduced as part of the NodeJS API to be able to interact with network streams such as TCP and file streams.

Now that TypedArray has been added to ES6, the Buffer class implements the Unit8Array API in a way that is more optimized and suitable for NodeJS operations.

In short, the Buffer class is used to process binary data. Because it is so commonly used, it is placed directly in a global variable, and there is no need to require when using it.

Instances of the Buffer class are similar to integer arrays, but the size of the buffer is determined when it is created and cannot be adjusted. The difference with the Buffer object is that it does not go through V8's memory allocation mechanism. Buffer is a module that combines JavaScript and C. The memory is applied for by C and allocated by JavaScript.

We will not discuss the related knowledge of Buffer memory allocation. Interested students can read Park Laoshi's book.

Instancing Buffer

Before NodeJS v6, Buffer was instantiated by calling the constructor, returning different results according to the parameters. For security reasons, this method has been abolished in versions after v6, and

Buffer.from()
Buffer.alloc()

Three separate functions with clear responsibilities handle the work of instantiating Buffer.

Buffer.from(arrayBuffer): Returns a new Buffer that shares memory with the given ArrayBuffer
Buffer.from(buffer): Returns the given Buffer A copy of Buffer
Buffer.from(string [, encoding]): Returns a Buffer containing the given string
Buffer.alloc(size [, fill [, encoding]]) : Returns a Buffer of the specified size and "filled"
Buffer.allocUnsafe(size): Returns a Buffer of the specified size, the content must be filled with methods such as buf.fill(0)

// 0x 表示 16 进制

Buffer.from([1, 2, 3]) // [0x1, 0x2, 0x3]

Buffer.from('test', 'utf-8') // [0x74, 0x65, 0x73, 0x74]

Buffer.alloc(5, 1) // [0x1, 0x1, 0x1, 0x1, 0x1]

Buffer.allocUnsafe(5); // 值不确定，后面详谈

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Buffer.allocUnsafe() will execute faster than Buffer.alloc() It is unsafe to look at the name, and it is indeed unsafe.

When

Buffer.allocUnsafe() is called, the allocated memory segment has not been initialized (not returned to zero), so the memory allocation speed is very slow, but the allocated memory segment may contain old data. If you do not overwrite these old data when using it, it may cause memory leaks. Although it is fast, try to avoid using it.

Encoding

Buffer supports the following encoding formats

utf8
utf16le
base64
binary
hex

Buffer and String conversion

String to Buffer is relatively simple

Buffer.from(string [, encoding])

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At the same time, the Buffer instance also has a toString method to convert the Buffer into a string

buf.toString([encoding[, start[, end]]])

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Buffer splicing

Use the concat method to speak multiple Buffer instances are spliced into one Buffer instance

Buffer.concat(list[, totalLength])

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StringDecoder

In NodeJS, a Chinese character is represented by three bytes. If we use something other than 3 when processing Chinese characters Multiples of the number of bytes will cause garbled character splicing problems.

const buf = Buffer.from('中文字符串！');

for(let i = 0; i < buf.length; i+=5){
  var b = Buffer.allocUnsafe(5);
  buf.copy(b, 0, i);
  console.log(b.toString());
}

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You can see that there are garbled characters in the results

But if you use the string_decoder module, you can solve this problem

const StringDecoder = require('string_decoder').StringDecoder;
const decoder = new StringDecoder('utf8');

const buf = Buffer.from('中文字符串！');

for(let i = 0; i < buf.length; i+=5){
  var b = Buffer.allocUnsafe(5);
  buf.copy(b, 0, i);
  console.log(decoder.write(b));
}

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StringDecoder in After getting the encoding, we know that wide bytes occupy 3 bytes under UTF-8, so when processing the incomplete bytes at the end, they will be retained until the second write(). Currently only UTF-8, Base64 and UCS-2/UTF-16LE can be processed.

Buffer Other commonly used APIs

There are also some Buffer commonly used APIs

Buffer.isBuffer: Determine whether the object is a Buffer
Buffer.isEncoding: Determine the encoding of the Buffer object
buf.length: Return the bytes of memory requested for this Buffer instance The number is not the number of bytes of the Buffer instance content
buf.indexOf: Similar to the indexOf of the array, returns the position of a certain string, acsii code or buf in the modified buf
buf. copy: Copy (part of) the contents of one buf to another buf

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