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This is complicated by the fact that 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 1

Pass character strings around explicitly using unsigned characters.

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This approach is inconvenient when you need to interwork with other functions that haven't been designed with this approach in mind, for example, the string handling functions found in the standard library.

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;
}

Priority: P3 Level: L3

References

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