How can I prevent scanf() to wait forever for an input character?

Question

I want to fulfill the following things in a console application:

1. If user inputs a character, the application will do the corresponding task. For example, if us

Answer 1

As others have said, the best way to do truly async IO is with select(...).

But a quick and dirty way to do what you want is with getline(...) which will return the number of bytes read every time (not hanging on IO) and returns -1 on no bytes read.

The following is from the getline(3) man page:

// The following code fragment reads lines from a file and writes them to standard output.  
// The fwrite() function is used in case the line contains embedded NUL characters.

char *line = NULL;
size_t linecap = 0;
ssize_t linelen;
while ((linelen = getline(&line, &linecap, fp)) > 0)
    fwrite(line, linelen, 1, stdout);