_write，_read 是 私有函數(shù)的名字。 這些流具體怎么讀 怎么寫，就是通過修改各自類原型的私有方法來實現(xiàn)的

比如：

ReadStream.prototype._read = function(){

? ? this.push(' ...')

???? ...

}

// abstract method. ?to be overridden in specific implementation classes. ? ?

// call cb(er, data) where data is <= n in length. ? ?

// for virtual (non-string, non-buffer) streams, "length" is somewhat ? ?

// arbitrary, and perhaps not very meaningful. ? ?

Readable.prototype._read = function(n) { ? ?

this.emit('error', new Error('not implemented')); ? ?

}; ? ?
https://github.com/nodejs/node/blob/master/lib/_stream_readable.js

這是他的解釋，它是readable里的一個抽象方法，這里是對這個抽象方法進(jìn)行了實現(xiàn)，而fs.js里面已經(jīng)對這個方法實現(xiàn)過了

fs.createReadStream?=?function(path,?options)?{????
return?new?ReadStream(path,?options);????
};????
-----------------
ReadStream.prototype._read?=?function(n)?{????
if?(typeof?this.fd?!==?'number')????
return?this.once('open',?function()?{????
this._read(n);????
});????
if?(this.destroyed)????
return;????
if?(!pool?||?pool.length?-?pool.used?<?kMinPoolSpace)?{????
//?discard?the?old?pool.????
pool?=?null;????
allocNewPool(this._readableState.highWaterMark);????
}

代碼是部分的，大概是告訴你這是真的

https://github.com/nodejs/node/blob/master/lib/fs.js

有興趣自己去看看吧。