INT36-C. Arguments to character handling functions must be representable as an unsigned char

According to Section 7.4 of C99:

The header <ctype.h> declares several functions useful for classifying and mapping characters. In all cases the argument is an int, the value of which shall be representable as an unsigned char or shall equal the value of the macro EOF. If the argument has any other value, the behavior is undefined.

The char data type might, in any implementation, be signed or unsigned.

Non-Compliant Code Example

This non-compliant code example may pass illegal values to the ctype functions.

size_t count_whitespace(const char *s) {
  const char *t = s;
  while(isspace(*t))  /* possibly *t < 0 */
    ++t;
  return t - s;
}

Compliant Solution 2

This compliant solution uses an explicit cast.

size_t count_whitespace(const char *s) {
  const char *t = s;
  while(isspace((unsigned char)*t))
    ++t;
  return t - s;
}

References

• ISO/IEC 9899-1999 Section 7.4 Character handling <ctype.h>
• Kettlewell 02 Section 1.1 <ctype.h> And Characters Types