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For portable applications, use only the assignment = operator, the equality operators == and !=, and the unary & operator on plain-character-typed or plain-wide-character-typed expressions.

This practice is recommended because the C Standard requires only the digit characters (0–9) to have consecutive numerical values. Consequently, operations that rely on expected values for plain-character- or plain-wide-character-typed expressions can lead to unexpected behavior.

However, because of the requirement for digit characters, other operators can be used for them according to the following restrictions:

  • The binary + operator may be used to add integer values 0 through 9 to '0'.
  • The binary - operator may be used to subtract character 0.
  • Relational operators <, <=, >, and >= can be used to check whether a character or wide character is a digit.

Character types should be chosen and used in accordance with STR04-C. Use plain char for characters in the basic character set.

Noncompliant Code Example

This noncompliant code example attempts to determine if the value of a character variable is between 'a' and 'c' inclusive. However, because the C Standard does not require the letter characters to be in consecutive or alphabetic order, the check might not work as expected.

char ch = 'b';
if ((ch >= 'a') && (ch <= 'c')) {
  /* ... */
}

Compliant Solution

In this example, the specific check is enforced using compliant operations on character expressions:

char ch = 't';
if ((ch == 'a') || (ch == 'b') || (ch == 'c')) {
  /* ... */
}

Exceptions

STR09-C-EX1: Consecutive values for characters like a~z can be assumed on platforms where ASCII or Unicode is used. This recommendation is primarily concerned with platform portability, for example, if code is migrated from ASCII systems to non-ASCII systems.

Risk Assessment

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

STR09-C

Low

Unlikely

Low

P3

L3

Automated Detection

Tool

Version

Checker

Description

Astrée
19.04

Supported indirectly via MISRA C:2012 rule 10.1.
Axivion Bauhaus Suite

6.9.0

CertC-STR09
LDRA tool suite
9.7.1
329 SFully implemented
Parasoft C/C++test
10.4.2
CERT_C-STR09-a
Expressions with type (plain) char and wchar_t shall not be used as operands to built-in operators other than  =,  ==, != and the unary & operator
PRQA QA-C

9.5

2106

2107


RuleChecker

19.04


Supported indirectly via MISRA C:2012 rule 10.1.

Related Guidelines

Bibliography

[Jones 2009]Section 5.2.1, "Character Sets"