Synchronous File Writing

The synchronous method of file writing involves writing data to a file before returning execution to the running thread. This provides the advantage of allowing you to write multiple times in the same section of code, but it can be a disadvantage if the file writes hold up other threads, as discussed earlier.

To write to a file synchronously, first open it using openSync() to get a file descriptor and then use fs.writeSync() to write data to the file. The following is the syntax for fs.writeSync():

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fs.writeSync(fd, data, offset, length, position)

The fd parameter is the file descriptor that openSync() returns. The data parameter specifies the String or Buffer object that will be written to the file. The offset parameter specifies the index in the data parameter to begin reading from. If you want to begin at the current index in the string or buffer, this value should be null. The length parameter specifies the number of bytes to write; you can specify null to write until the end of the data buffer. The position argument specifies the position in the file to begin writing; to use the current file position, specify null for this value.

The code in Listing 6.2 shows how to implement basic synchronous writing to store a series of string data in a file. Figure 6.2 shows the output of the code in Listing 6.2.

Listing 6.2 file_write_sync.js: Performing synchronous writes to a file

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1 var fs = require('fs'),
2 var veggieTray = ['carrots', 'celery', 'olives'];
3 fd = fs.openSync('../data/veggie.txt', 'w'),
4 while (veggieTray.length){
5   veggie = veggieTray.pop() + " ";
6   var bytes = fs.writeSync(fd, veggie, null, null);
7   console.log("Wrote %s %dbytes", veggie, bytes);
8 }
9 fs.closeSync(fd);

Figure 6.2 Output of file_write_sync.js, writing synchronously to a file.