What does fflushstdin do

Using fflush stdin is an unspecified behavior pattern, as per C standard. However, some compilers like the visual studio in Microsoft permit this.

How are those compilers used in these? A buffer does not get cleared over the next input even when taking an integer number with spaces and discusses the previous input for the same.

Learn more. What does fflush stdin do in C programing? Asked 7 years, 7 months ago. Active 3 years, 11 months ago. Viewed 87k times. Improve this question. C graphics C graphics 6, 18 18 gold badges 76 76 silver badges bronze badges. Don't do it, don't copy it from those that do. Nothing in OP's post is Linux specific.

Tags are for the Post's subject, not for the type of people seeking. Add a comment. Active Oldest Votes. Adding an fflush stdout after the printf solves the problem: printf "Enter a number: " ; fflush stdout ; Now, working by analogy, people often think that fflush stdin should discard any unused input, but if you think about it a little bit that doesn't make much sense.

The C standard says: If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.

POSIX says also explicitly defers to C standard : If stream points to an output stream or an update stream in which the most recent operation was not input, fflush shall cause any unwritten data for that stream to be written to the file, A tool to try open-source projects quickly.

Software Engineering Bootcamps worth consider? Get Ahead by Going Headless. Software Development. Follow us! Get the Latest Bytes Updates. By using this site, you agree to our Privacy Policy and Terms of Use. What does fflush stdin do? Jun 7 '10 reply Banfa 9, Expert Mod 8TB As mayankarpit says fflush is defined by the standard to work on output streams. Jun 7 '10 reply Message. Cancel Changes. Only when the output stream encounters a newline or end of file , will the contents be written to the stdout file.

Notice the lack of a newline in our program! But, for stderr , the stderr stream is not buffered, so the contents are immediately written to it! Only after the end of file output is reached, will the contents of stdout be written! So if you want to have stdout output first, without using a newline, you must flush the output buffer using fflush.

Makes sense? But for input streams , there is no way of knowing where the input will end up eventually. For all you know, there may be some external program which feeds input randomly to this program, and you have no way of knowing what will happen to it!