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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 is practice is recommended because the C99 standard C Standard requires only the digit characters ('0' - '9'0–9) to have consecutive numerical values C99 N1401. ThusConsequently, operations that rely on expected values for plain-character- or plain-wide-character-typed expressions can lead to unexpected behavior.

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• The binary + operator may be used to add integer values from 0 to 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 guideline STR04-C. Use plain char for characters in the basic character set.

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This noncompliant code example attempts to determine if the value of a character variable is between 'a' and 'c' inclusive. However, since because the C99 standard C Standard does not require the letter characters to be in consecutive or in alphabetical or alphabetic order, the check might not work as expected.

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

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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 guideline recommendation is primarily concerned with platform portability, for example, if code is migrated from ASCII systems to non-ASCII systems.

Risk

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Assessment

Automated Detection

Related Guidelines

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Bibliography

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[Jones 2009]

Section 5.2.1

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, "Character

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Sets"

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Image Removed      07. Characters and Strings (STR)      Image Added Image Added Image Modified