scanf not terminating

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.

Answer 2

scanf() will take input from the stdin file descriptor. If redirected, then will take from the appropriate source. getc() will read from any file descriptor. You're mixing inputs from a file and stdin.

You might be more interested in fscanf() to read from your file.

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