Question

So, I’ve got the following code (C):

char c = getc(in);
if (c == '(')
   ...
if (c == '%')
   ...
if (isdigit(c))
{
   int n;
   ungetc(c, in);
   scanf("%i", &n);
   ...
}

Everything is all fine and dandy when I’m reading in input from stdin but, when reading in input from from a file, the call to scanf does not terminate.

I added some code around the call to see what’s going on before the call to scanf. One such instance is

1. c = '0'
2. the character right after c is )

Is the buffer not flushing after ungetc or something? What might be happening, that it works fine when the input is stdin but not when its a file? (I’m not that familiar with IO in C).

edit: Should have used fscanf… boy is my face red.

Answer 1

1. getc() will return EOF when you
   reach the end or an error occurs so
   make sure you check for that. Your c have to be an int, not a char to be able to distinguish EOF.

2. The same goes for scanf check its
   return value for EOF. For scanf the
   conversion might also fail,
   scanf("%i", &n) should return 1 if
   it successfully parsed something
   into n so make sure you check for
   that too.1.

3. You are also operating on in, as
   in getc(in), suggesting you’re
   reading from a particular FILE* ,
   however your scanf call still reads
   from stdin.

   Use fscanf instead of scanf there.