#include <ctype.h>
#include <stdio.h>

typedef struct {
  size_t space;
  size_t alnum;
  size_t punct;
} chartypes;

chartypes classify(FILE *f) {
  chartypes types = { 0, 0, 0 };
  int ch;

  while ((ch = fgetc(f)) != EOF) {
    types.space += !!isspace(ch);
    types.alnum += !!isalnum(ch);
    types.punct += !!ispunct(ch);
  }

  return types;
}

The classify function reads characters from a stream and counts the number of spaces, alphanumeric and punctuation characters. It avoids several pitfalls.

• When reading a character from a stream, the result is saved as an int, since otherwise there would be an ambiguity between reading EOF (the end-of-file marker) and a character that has the same bit pattern.
• The character classification functions (e.g. isspace) expect their argument to be either representable as an unsigned char, or the value of the EOF macro. Since this is exactly what the fgetc returns, there is no need for conversion here.
• The return value of the character classification functions only distinguishes between zero (meaning false) and nonzero (meaning true). For counting the number of occurrences, this value needs to be converted to a 1 or 0, which is done by the double negation, !!.