CertC-INT07ΒΆ

Use only explicitly signed or unsigned char type for numeric values

Required inputs: IR

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 because it 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 print out either 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-C-EX1: void FIO34-C. Use int to capture the return value of character IO functions that might be used to check for end of file 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.

Recommendation Severity Likelihood Remediation Cost Priority Level
INT07-C Medium Probable Medium P8 L2
Related Guidelines
SEI CERT C++ Coding Standard VOID INT07-CPP. Use only explicitly signed or unsigned char type for numeric values
ISO/IEC TR 24772:2013 Bit Representations [STR]
MISRA C:2012 Rule 10.1 (required)
Rule 10.3 (required)
Rule 10.4 (required)
MITRE CWE CWE-682, Incorrect calculation
Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/recommendations/integers-int/int07-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

char_used_numerically

Plain char datatype used for non-character data.

None

False

Options