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In most cases, the only portable operators on plain char types are assignment and equality operators (=, ==, != ).  An exception is the translation to and from digits.  For example, if the char c is a digit, c - '0' is a value between 0 and 9.

Non-Compliant Code Example

The following non-compliant code example simply illustrates calling the standard string handling function strlen() with a plain character string, a signed character string, and an unsigned character string:

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If this C code were compiled using a C+\+ compiler, conversions from {{unsigned char\[\]}} to {{const char *}} and from {{signed char\[\]}} to {{const char *}} would be be flagged as errors requiring casts.

Compliant Solution

The compliant solution uses plain char for character data.

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Conversions are not required and the code compiles cleanly at high warning levels without casts.

Risk Assessment

Automated Detection

Fortify SCA Version 5.0 with CERT C Rule Pack can detect violations of this recommendation, except cases involving signed char.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.2.5, "Types"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 6.1, "The plain char type shall be used only for the storage and use of character values"

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STR06STR03-A. Do not assume that strtok() leaves the parse string unchangedinadvertently truncate a null-terminated byte string      07. Characters and Strings (STR)       Image Removed STR05-A. Prefer making string literals const-qualified