What happen when the stream is in an error state?

In C++, when the stream is in an error state (e.g. eofbit being set), we cannot perform either read or write operation on that stream. But if it's in C, what happen when the stream is in an error state? It seems that we can still read or write on that stream. Consider the following code:

Code:

#include <stdio.h>

int main( void )
{
    int ch;

    while ( getchar() != EOF ) ;
    if ( feof(stdin) )
    {
        printf("Oops...EOF!\n");
    }
    /* Surprisingly! Can still read from stdin */
    putchar( getchar() );
    if ( !feof(stdin) )
    {
        printf("Why EOF is unset?");
    }

    return 0;
}

runing this program, I got

Code:

745a2344faz
^Z
Oops...EOF!
1212
1Why EOF is unset?

From the output, we know that stdin is in an error state before the second getchar. But as we can see, getchar was still called successfully. Why? And after that getchar was called, the EOF state for stdin was also unset, why?

ps, I complied this code with Dev-C++

>Why?
Because the C standard imposes no requirements on how an implementation should react to an "error" state such as those returned by feof or ferror. Ignoring it in favor of letting the calling code deal with it is a reasonable, if confusing, interpretation. I chose to go with the disallowing of future operations (like C++) until the indicators are cleared. I think that makes more sense and promotes more robust code, but my personal implementation isn't widely used.