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The three types char, signed char, and unsigned char are collectively called the character types. Compilers have the latitude to define char to have the same range, representation, and behavior as either signed char or unsigned char. Irrespective of the choice made, char is a separate type from the other two and is not compatible with either.

Only use Use only signed char and unsigned char types for the storage and use of numeric values, as this is the only way to (portably) guarantee the signedness of the character types.

Non-Compliant Code Example

In this non-compliant code example, the char-type variable c may be signed or unsigned. Assuming 8-bit, two's complement character types, this code may either print out i/c = 5 (unsigned) or i/c = -17 (signed). As a result, it is much more difficult to reason about the correctness of a program without knowing if these integers are signed or unsigned.

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

Compliant Solution

In this compliant solution, the variable c is declared as unsigned char. The subsequent division operation is now independent of the signedness of char and consequently has a predictable result.

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

Risk Assessment

This is a subtle error that results in a disturbingly broad range of potentially severe vulnerabilities.

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 6.2, "Signed and unsigned char type shall be used only for the storage and use of numeric values"

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