Asynchronous file reading not asynchronous or programmer poop?

[Maverick Peppers]

// Start of program4 process

var fs = require('fs');

// Asyncronously read and send the buffer object of a file whose path

// is set int 2nd argument of the process to the callback function.

var buffer = 0;

fs.readFile(process.argv[2], fileReadCallBack);

while(buffer == 0) {}

var numOfNewLines = buffer.toString().split('\n').length-1;

console.log(numOfNewLines);

// Our callback function for our asyncronous file reading

function fileReadCallBack(err, data) { if(!err){ buffer = data; } }

I'm using NodeSchool.io's workshop to learn Nodejs. I'm on problem 4, Async File I/O and the above code runs forever. If the file reading is asynchronous, I'm thinking var buffer should no longer be 0 and the while-loop will terminate and move on. 

I know the correct solution would place the logic inside of the callback function, but I'm curious as to why the code above doesn't work as expected.

Thanks!

[Adrien Risser]

On Sun, Jan 25, 2015 at 12:25 AM, Maverick Peppers <bmpe...@gmail.com> wrote:

// Start of program4 process

var fs = require('fs');

// Asyncronously read and send the buffer object of a file whose path

// is set int 2nd argument of the process to the callback function.

var buffer = 0;

fs.readFile(process.argv[2], fileReadCallBack);

while(buffer == 0) {}

Code is blocked here as instructions are executed sequentially.

fileReadCallback is never called as you never free the thread's execution.

 

var numOfNewLines = buffer.toString().split('\n').length-1;

console.log(numOfNewLines);

// Our callback function for our asyncronous file reading

function fileReadCallBack(err, data) { if(!err){ buffer = data; } }

I'm using NodeSchool.io's workshop to learn Nodejs. I'm on problem 4, Async File I/O and the above code runs forever. If the file reading is asynchronous, I'm thinking var buffer should no longer be 0 and the while-loop will terminate and move on. 

I know the correct solution would place the logic inside of the callback function, but I'm curious as to why the code above doesn't work as expected.

Thanks!

Try this:

var buffer = 0;

console.log('outside setTimeout');

setTimeout(function () {

  console.log('inside setTimeout');

  buffer = 1;

}, 10);

console.log('before while');

while(buffer == 0) {}

console.log('after while');

 Cheers,

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Adrien Risser,

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[Thomas Shinnick]

On Sat, Jan 24, 2015 at 5:25 PM, Maverick Peppers <bmpe...@gmail.com> wrote:

while(buffer == 0) {}

The above line does not mean wait around 'awhile', it means "run forever, not giving anyone else any CPU time to do anything or respond to anything".  You are not 'cooperating' with Node or anyone else. 

There are a number of ways/ideas to say "let me wait around until the system/Node can respond", but this would be one replacement for the above line:

while ( buffer == 0 ) 
    process.nextTick();

With that you are saying "let me check on something fairly often, but if it hasn't happened yet, give me control to the system/Node so it can do 'whatever' needs to be done".

[Ryan Schmidt]

Because the line

> while(buffer == 0) {}

blocks the event loop.

[Angel Java Lopez]

The file read could be async, but your code fileReadCallBack will never be called.

After fs.readFile(...) all the async magic take place, OK.

BUT YOUR JAVASCRIPT code will run the next command, after fs.readFile. It is an infinite loop, becase fileReadCallBack will be called when your JavaScript code free the JavaScript machine, so it can attend any callback

If you don't free the JavaScript machine, fileReadCallBack will never executed, and your buffer will be empty for ever

Maybe better is

fs.readFile(process.argv[2], fileReadCallBack);

// Our callback function for our asyncronous file reading

function fileReadCallBack(err, data) { 

  if(err)

      return; // or maybe better, inform the problem

var numOfNewLines = data.toString().split('\n').length-1;

console.log(numOfNewLines);

} }

Apologize my English ;-)

Angel "Java" Lopez

@ajlopez

--

[genericlady]

The while statement is an infinite loop because it doesn't have a break.  From top to bottom you have your fs object so that's good.  Since your setting up your objects maybe even put the path into it's own ;)

The readFile method is not being used correctly. The way a callback works in Node.js is you pass an anonymous function as the callback.  When reading documentation for fs.readFile this is what we see.

fs.readFile('/etc/passwd', function (err, data) {

  if (err) throw err;

  console.log(data);

});

Hope this helps!