CODEWITHSUNDEEP
node.jsfilestreamfs
Kang Hyuk Suh
Kang Hyuk Suh

Reputation: 61

How can I use fs.read() in Node js

I try to use Nodejs fs.read Method in Mac OS. However it doesn't work.. I use below source code

    var fs = require('fs');
    fs.open('helloworld.txt', 'r', function(err, fd) {
        fs.fstat(fd, function(err, stats) {

            var bufferSize=stats.size  ,
                chunkSize=512,
                buffer=new Buffer(bufferSize),
                bytesRead = 0;

            while (bytesRead < bufferSize) {
                if ((bytesRead + chunkSize) > bufferSize) {
                    chunkSize = (bufferSize - bytesRead);
                }

                fs.read(fd, buffer, bytesRead, chunkSize, bytesRead, testCallback);
                bytesRead += chunkSize;
            }
            console.log(buffer.toString('utf8'));
        });
        fs.close(fd);
    });

    var testCallback = function(err, bytesRead, buffer){
        console.log('err : ' +  err);
    };

Actually, I use some example in stackoverflow.

When I execute the source,

err : Error: EBADF, read

this err is returned.

However if I use readFile method, it works well.

    fs.readFile('helloworld.txt', function (err, data) {
       if (err) throw err;    
       console.log(data.toString('utf8'));
    });

result is

Hello World!

Of course, it's same file.

Please, let me know what the problem is.

Thank you.

Upvotes: 6

Views: 2819

Answers (3)

Snehil Agrahari
Snehil Agrahari

Reputation: 288

The error in the code is that you are currently using asynchronus functions in your application. Which simply means that before performing the first operation the other one starts in parallel i.e. fs.close() closes your file before performing the operation. You can simple replace them with synchronus functions provided by the same fs module from node.js.

Upvotes: 0

Nuno Barreto
Nuno Barreto

Reputation: 121

The difference is not int the functions you are using, but in the way you are using them.

All those fs.* functions you are using are asynchronous, that means they run in parallel. So, when you run fs.close, the others have not finished yet.

You should close it inside the fs.stat block:

var fs = require('fs');
fs.open('helloworld.txt', 'r', function(err, fd) {
    fs.fstat(fd, function(err, stats) {

        var bufferSize=stats.size  ,
            chunkSize=512,
            buffer=new Buffer(bufferSize),
            bytesRead = 0;

        while (bytesRead < bufferSize) {
            if ((bytesRead + chunkSize) > bufferSize) {
                chunkSize = (bufferSize - bytesRead);
            }

            fs.read(fd, buffer, bytesRead, chunkSize, bytesRead, testCallback);
            bytesRead += chunkSize;
        }
        console.log(buffer.toString('utf8'));
        fs.close(fd);
    });
});

var testCallback = function(err, bytesRead, buffer){
    console.log('err : ' +  err);
};

Upvotes: 3

Jaffer
Jaffer

Reputation: 2968

fs.read and fs.fstat are async functions.

And just after calling, fstat, you are closing the file (fs.close(fd);).

That could be the reason for this error.

Upvotes: 0

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