INT07-C. Use only explicitly signed or unsigned char type for numeric values

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.

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 STR00-C. Represent characters using an appropriate type for more information on representing characters.

Noncompliant Code Example

In this noncompliant 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). 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);

Exceptions

INT07-EX1: 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.

Risk Assessment

This is a subtle error that results in a disturbingly broad range of potentially severe vulnerabilities. At the very least, this error can lead to unexpected numerical results on different platforms. Unexpected arithmetic values when applied to arrays or pointers can yield buffer overflows or other invalid memory access.

Automated Detection

The LDRA tool suite V 7.6.0 can detect violations of this recommendation.

Fortify SCA Version 5.0 with the CERT C Rule Pack can detect violations of this recommendation.

Splint Version 3.1.1 can detect violations of this recommendation.

Compass/ROSE 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.

Related Vulnerabilities

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

Other Languages

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

References

[[ISO/IEC 9899:1999]] Section 6.2.5, "Types"
[[ISO/IEC PDTR 24772]] "STR Bit Representations"
[[MISRA 04]] Rule 6.2, "Signed and unsigned char type shall be used only for the storage and use of numeric values"
[[MITRE 07]] CWE ID 682, "Incorrect Calculation"

INT06-C. Use strtol() or a related function to convert a string token to an integer      04. Integers (INT)