====== Buffered (fread,fwrite) Vs non buffered (read, write) i/o ======

Buffered I/O operations use an intermediary buffer managed by the C library. The intermediary buffer stores data to be read or written to a stream (file or device). This mode provides better I/O performance when compared to the non buffered I/O mode because it tends to reduce the number of actual reads and writes.

Since buffered I/O is not synchronous in real time, data may not be actually written in the stream until the intermediary buffer is flushed.

For this reason, when using fread/fwrite in real time systems, one must make sure to flush the I/O buffers or to force the C library to write all the provided data.


===== How to ensure data is written to the stream when using fwrite =====

There are two ways for doing this 

==== Using the fflush function ====
The function fflush() forces a write of all user-space buffered data for the given output or update stream via the stream's underlying write function. The open status of the stream is unaffected. See man pages for more details.


==== Manually flushing the output buffer ====
If the fflush function is not available. The following construct ensures the full buffer provided to fwrite is output.

<code C>

	for (;;) {

		j=fwrite(buffer,1,data_length,output_stream);

		if (j <= 0) { 
                       perror("fwrite error"); 
                       goto err; 
                }
		
                if (i == j) break;
		buffer+=j; i-=j;
		
       }

</code>


{{tag>coding embedded howto}}


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