STR37-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.

Compliance with this rule is complicated by the fact that the char data type might, in any implementation, be signed or unsigned.

The following character classification functions are affected:

Noncompliant Code Example

This noncompliant code example may pass invalid values to the isspace() function.

size_t count_preceding_whitespace(const char *s) {
  const char *t = s;
  size_t length = strlen(s) + 1;

  /* possibly *t < 0 */
  while (isspace(*t) && (t - s < length)) {
    ++t;
  }
  return t - s;
}

Compliant Solution (Unsigned Char)

Pass character strings around explicitly using unsigned characters.

size_t count_preceding_whitespace(const char *s) {
  const unsigned char *t = s;
  size_t length = strlen(s) + 1;

  while (isspace(*t) && (t - s < length)) {
    ++t;
  }
  return t - s;
}

This approach is inconvenient when you need to interwork with other functions that haven't been designed with this approach in mind, such as the string handling functions found in the standard library [[Kettlewell 02]].

Compliant Solution (Cast)

This compliant solution uses a cast.

size_t count_preceding_whitespace(const char *s) {
  const char *t = s;
  size_t length = strlen(s) + 1;

  while (isspace((unsigned char)*t) && (t - s < length)) {
    ++t;
  }
  return t - s;
}

Automated Detection

The tool Compass / ROSE could detect violations of this rule by seeing if the argument to a character-handling function (listed above) is not an unsigned char.

Risk Assessment

Passing values to character handling functions that cannot be represented as an unsigned char may result in unintended program behavior.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ISO/IEC 9899:1999]] Section 7.4, "Character handling <ctype.h>"
[[Kettlewell 02]] Section 1.1, "<ctype.h> And Characters Types"

STR36-C. Do not specify the bound of a character array initialized with a string literal      07. Characters and Strings (STR)       08. Memory Management (MEM)