Why doesn't tcflush work for scanf?

Question

I have this simple code to accept 3 characters,:

char a,b,c;
scanf("%c",&a);
scanf("%c",&b);
scanf("%c",&c);
printf("%c",a);
printf("%c",b);
printf("%c",c);

I understand why this will only accept 2 characters because the second scanf accepts the carriage return. However if if use __fpurge(stdin); between each scanf the code works as expected. But if I use read(STDIN_FILENO,&a,1); instead of scanf, it does not work. For read() , only tcflush(STDIN_FILENO,TCIOFLUSH); works but it fails with scanf. Can someone explain me why?

Answer 1

fpurge empties the buffer at C level, which is the level at which scanf works.

tcflush does it at a lower level (system level), which is the level at which read works.

scanf uses read to fill its own buffer.

So in the first case: emptying the C-buffer with scanf works well, but does nothing at the system-level.

In the second case, emptying the system-buffer of course works with read but not with scanf because when you use scanf , data up at least up to the carriage return are already present in the C-buffer. The first scanf , reads plenty of data, place them into a buffer and then use that buffer to return you only one char. Then you tcflush which flushes the system-level buffer but does nothing to the C-buffer, so the following scanf is able to find the carriage return in it.