STR37-C. Arguments to character-handling functions must be representable as an unsigned char

According to the C Standard, 7.4.1 paragraph 1 [ISO/IEC 9899:2024],

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.

This rule is applicable only to code that runs on platforms where the char data type is defined to have the same range, representation, and behavior as signed char.

Following are the character classification functions that this rule addresses:

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

This rule is a specific instance of STR34-C. Cast characters to unsigned char before converting to larger integer sizes.

Noncompliant Code Example

On implementations where plain char is signed, this code example is noncompliant because the parameter to isspace(), *t, is defined as a const char *, and this value might not be representable as an unsigned char:

#include <ctype.h>
#include <string.h>
 
size_t count_preceding_whitespace(const char *s) {
  const char *t = s;
  size_t length = strlen(s) + 1;
  while (isspace(*t) && (t - s < length)) { 
    ++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 <string.h>
 
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;
} 

Risk Assessment

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

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-686 and STR37-C

Intersection( CWE-686, STR37-C) = Ø

STR37-C is not about the type of the argument passed (which is signed int), but about the restrictions placed on the value in this type (must be 0-UCHAR_MAX or EOF). I interpret ‘argument type’ to be specific to the C language, so CWE-686 does not apply to incorrect argument values, just incorrect types (which is relatively rare in C, but still possible).

CWE-704 and STR37-C

STR37-C = Subset( STR34-C)

CWE-683 and STR37-C

Intersection( CWE-683, STR37-C) = Ø

STR37-C excludes mis-ordered function arguments (assuming they pass type-checking), because there is no easy way to reliably detect violations of CWE-683.

Bibliography