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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.

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

Automated Detection

Tool

Version

Checker

Description

Astrée
19.04

Supported indirectly via MISRA C:2012 rules 10.1, 10.3 and 10.4.
Axivion Bauhaus Suite

6.9.0

CertC-INT07
CodeSonar
5.1p0
LANG.TYPE.IOTInappropriate operand type
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

ECLAIR

1.2

CC2.INT07

Fully implemented

Klocwork
2018
PORTING.SIGNED.CHAR
LDRA tool suite
9.7.1

93 S, 96 S, 101 S, 329 S, 432 S, 458 S

Fully implemented

Parasoft C/C++test
10.4.2

CERT_C-INT07-a
CERT_C-INT07-b

The plain char type shall be used only for the storage and use of character values
signed and unsigned char type shall be used only for the storage and use of numeric values

Polyspace Bug Finder

R2019b

CERT C: Rec. INT07-C


Checks for use of plain char type for numeric value (rec. fully covered)

Splint
3.1.1



PRQA QA-C
9.5

1292, 1293, 4401, 4421, 4431, 4441, 4451


Partially implemented
RuleChecker

19.04


Supported indirectly via MISRA C:2012 rules 10.1, 10.3 and 10.4.
SonarQube C/C++ Plugin
3.11
S820

Related Vulnerabilities

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

Related Guidelines

SEI CERT C++ Coding StandardVOID INT07-CPP. Use only explicitly signed or unsigned char type for numeric values
ISO/IEC TR 24772:2013Bit Representations [STR]
MISRA C:2012Rule 10.1 (required)
Rule 10.3 (required)
Rule 10.4 (required)
MITRE CWECWE-682, Incorrect calculation



4 Comments

  1. What about numeric values too large to fit into a char?

    1. This rule is only to be used when you intend to use a character type to hold a numeric value (in constrast to an actual character).

  2. I think the Exception needs to be removed from this guideline because it is based on void FIO34-C, which isn't replaced/superseded by any other guideline.