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According to the C Standard, subclause 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.)

Compliance with this This rule is complicated by the fact that applicable only to code that runs on platforms where the char data type can, in any implementation, be signed or unsigned.is defined to have the same range, representation, and behavior as signed char.

Following are the character classification functions that this rule addressesThe following character classification functions are affected:

NoteNOTE: ^XSI denotes an X/Open System Interfaces Extension to ISO/IEC 9945—POSIX. The 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 is a generalization of this rule.

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 may might not be representable as an unsigned char:

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

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Related Vulnerabilities

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

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Bibliography

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