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According to Section the C Standard, 7.4 of C99.1 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 is complicated by the fact that rule is applicable only to code that runs on platforms where the char data type might, in any implementation, be signed or unsigned.

Non-Compliant Code Example

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:This non-compliant code example may pass illegal values to the ctype functions.

#include <ctype.h>
#include <string.h>
 
size_t count_preceding_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.

size_t count_whitespace(const unsigned *s) {
  const unsigned char *t = s;
  while(isspace(*t))
    ++t;
  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 \[[Kettlewell 02|AA. C References#Kettlewell 02]\].

Compliant Solution 2

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:This compliant solution uses an explicit cast.

#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.

References

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

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Image Added Image Added Image Added Wiki Markup\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]] Section 7.4, "Character handling <ctype.h>" \[[Kettlewell 02|AA. C References#Kettle 02]] Section 1.1, "<ctype.h> And Characters Types"