According to the C Standard, 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.

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:

isalnum()

isalpha()

isascii()XSI

isblank()

iscntrl()

isdigit()

isgraph()

islower()

isprint()

ispunct()

isspace()

isupper()

isxdigit()

toascii()XSI

toupper()

tolower()

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.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

STR37-C

Low

Unlikely

Low

P3

L3

Automated Detection

Tool

Version

Checker

Description

Astrée
ctype-limitsPartially checked
Axivion Bauhaus Suite

CertC-STR37Fully implemented
CodeSonar
MISC.NEGCHARNegative character value
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

ECLAIR

CC2.STR37

Fully implemented

LDRA tool suite
663 SFully implemented
Parasoft C/C++test

CERT_C-STR37-aDo not pass incorrect values to ctype.h library functions
Polyspace Bug Finder

CERT C: Rule STR37-C

Checks for invalid use of standard library integer routine (rule fully covered)

PRQA QA-C
4413, 4414Fully implemented
PRQA QA-C++
3051 
RuleChecker

ctype-limitsPartially checked
TrustInSoft Analyzer

valid_charPartially verified.

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)

Taxonomy

Taxonomy item

Relationship

CERT C Secure Coding StandardSTR34-C. Cast characters to unsigned char before converting to larger integer sizesPrior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961Passing arguments to character-handling functions that are not representable as unsigned char [chrsgnext]Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-704, Incorrect Type Conversion or Cast2017-06-14: CERT: Rule subset of CWE

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

[ISO/IEC 9899:2011]7.4, "Character Handling <ctype.h>"
[Kettlewell 2002]Section 1.1, "<ctype.h> and Characters Types"