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According to the C Standard, subclause 7.4 [ISO/IEC 9899:2011],

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.

(See also undefined behavior 113  of Appendix J.)

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

The following character classification functions are affected:

Note: XSI denotes an X/Open System Interfaces Extension to ISO/IEC 9945—POSIX. The functions are not defined by the C Standard.

STR34-C. Cast characters to unsigned char before converting to larger integer sizes is a generalization of this rule.

Noncompliant Code Example

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

#include <ctype.h>
#include <stddef.h>
 
ptrdiff_t count_preceding_whitespace(const char *s) {
  const char *t = s;

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

The argument to isspace() must be EOF or representable as an unsigned char; otherwise, the result is undefined.

Compliant Solution

This compliant solution casts the character to unsigned char before passing it as an argument to the isspace() function:

#include <ctype.h>
#include <stddef.h>
 
ptrdiff_t count_preceding_whitespace(const char *s) {
  const char *t = s;

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

Risk Assessment

Passing values to character handling functions that cannot be represented as an unsigned char results in undefined program behavior.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography