fflush(stdin) and fflush(stdout)

fflush(stdin) is relatively easy to understand: it clears the standard input stream, discarding any excess data that has not yet been processed.

For example, consider this small program:

void main()
{
    int a;
    char str[10];
    cin>>a;
    cout<<a<<endl;
    cin>>str;
    cout<<str<<endl;
}

The goal is simple: get an integer from stdin into a and print it immediately; then get a string from stdin into str and print it immediately. However, the following scenario might require special consideration: if two integers are entered on the first line, after cin>>a, there's still an integer left in the stdin buffer that hasn't been read. Subsequently, without waiting for string input, this extra number might be directly stored into str and printed.

To some extent, this is caused by non-standard operation, but a program should be robust, and programmers should anticipate such non-standard operations. One could prompt the user with "Please enter 1 integer" on the program interface, and sometimes even persistent emphasis and warnings are necessary. Of course, for simplicity, this example doesn't focus on UI friendliness. In such cases, fflush(stdin) can be inserted before the cin>>str statement to clear any excess data in the standard input buffer.

fflush(stdout) is similar to fflush(stdin) in that it clears the standard output stream, but it doesn't discard data; instead, it prints the data to the screen immediately. To better understand this, one needs to know a fact: standard output is typically performed "line by line," meaning data is printed to the screen only when a \n is encountered. This can cause delays, as shown in these lines of code:

int a;
printf_s("input one number:");
fflush(stdout);\\\\#1
scanf_s("%d",&a);

Without line #1 (the fflush(stdout) call), on some platforms, "input one number:" might not appear on the screen for a while because there's no carriage return. In this situation, fflush(stdout) plays the role of forcing immediate output.

However, on Windows platforms, there seems to be no noticeable difference. This suggests that MSFT has already made stdout output effective immediately.

The fflush function is widely used in message processing for multithreading and network programming.

fflush(stdout): Clears the output buffer and outputs its contents.