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Use only signed char and unsigned char types for the storage and use of numeric values, as this is the only portable way to guarantee the signedness of the character types. See guideline STR00-C. Represent characters using an appropriate type for more information on representing characters.

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unsigned char c = 200;
int i = 1000;
printf("i/c = %d\n", i/c);

Exceptions

INT07-EX1: Guideline FIO34-C. Use int to capture the return value of character IO functions mentions that certain character IO functions return a value of type int. Despite being returned in an arithmetic type, the value is not actually numeric in nature so it is acceptable to later store the result into a variable of type char.

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Automated Detection

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Tool	Version	Checker	Description
Section LDRA tool suite

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	Include Page c:LDRA_V c:LDRA_V
Section Fortify SCA	Section V. 5.0		Section can detect violations of this recommendation

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with the CERT C Rule Pack

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Section Splint	Include Page c:Splint_V c:Splint_V
Section Compass/ROSE			Section can detect violations of this recommendation. In particular, it flags any instance of a variable of type char (without a signed or unsigned qualifier) that appears in an arithmetic expression

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Related Vulnerabilities

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

Other Languages

Related Guidelines

This rule appears in the C++ Secure Coding Standard as : INT07-CPP. Use only explicitly signed or unsigned char type for numeric values.

Bibliography

\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 6.2.5, "Types"
\[[ISO/IEC PDTR 24772|AA. Bibliography#ISO/IEC PDTR 24772]\] "STR Bit Representations"
\[[MISRA 042004|AA. Bibliography#MISRA 04]\] Rule 6.2, "Signed and unsigned char type shall be used only for the storage and use of numeric values"
\[[MITRE 072007|AA. Bibliography#MITRE 07]\] [CWE ID 682|http://cwe.mitre.org/data/definitions/682.html], "Incorrect Calculation"

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