Should fflush() not be used to flush a buffer even if it is an output stream? What is it useful for? How do we flush a buffer in general? More on stackoverflow.com

Imagine you're job is to change the marque just under the sign of a fast food restaurant. The message you want to display is "Open 24 hours". Here's one way to do it. Get a ladder Get a box of letters Find the capital O Climb the ladder Place the O Climb down the ladder Get the next letter Repeat until done No intelligent human would take these steps. It's too much work. But a computer is a slave to the program and will follow instructions with no complaints. So how would an intelligent human solve this problem? Here's how: Get a ladder Get a box of letters Get a bag Find the capital O Put it in the bag Find the next letter and repeat until all the letters are in the bag Climb the ladder Find the capital O Place the O Repeat placing each letter until finished Climb down the letter Notice how much less effort you had to exert by first putting all of the letters into the bag. That's because the work necessary to move the letters from the ground to the sign was only done once. Displaying a message in a computer is similar. While you may write a simple line of code to display a message, 1000s or 100,000s of lines of code are executed just to display your message. This is a lot of work. So for efficiency, your program's output is placed in a buffer. It's like our bag example, except it's more efficient since it's ordered. The first character that you want to display is the first thing in the buffer and so on. In the example with the bag, you had to find the letters all over again. Flushing a buffer is like emptying the bag. If you climb the ladder and don't empty the bag, your sign will say nothing. If your program exits WITHOUT flushing the buffer, then the data is never displayed. BTW, buffers are used everywhere. Network communications is one notable place. Imagine how much overhead it would be to write your friend a letter if you wrote one word on a piece of paper and then mailed that letter. Then wrote the next word on the next sheet and mailed that letter. We don't do that because it's inefficient. Instead we write all the words onto one or more sheets of paper and then mail them once. In network communications, we buffer the data we want to send until the buffer is full at which point it's automatically flushed or we have no more to send and we manually flush the buffer. Hope this helps. More on reddit.com

understand that this command flushes out the input stream No. Calling fflush() on stdin is undefined in C - it could do anything. "flushing" means removing everything from a buffer. In the case of the stdout output stream (for which calling fflush() is defined), then whatever is in the streams internal buffers (private memory) will be sent to the actual output device - if this is the screen, then whatever was in the buffer will be displayed. For alternative ways of clearing the input stream buffer, see http://c-faq.com/stdio/stdinflush2.html . More on reddit.com

A lot of I/O is buffered. This means that data is not immediately sent to the target; instead it is stored in a buffer and sent only when explicitly requested or when the buffer is full. Sending data in large chunks slightly delays it but is very optimal in terms of time spend for system/driver calls. More on reddit.com