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 because it is the only portable way to guarantee the signedness of the character types . See (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 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.
| Code Block | ||||
|---|---|---|---|---|
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char c = 200;
int i = 1000;
printf("i/c = %d\n", i/c);
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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.
| Code Block | ||||
|---|---|---|---|---|
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unsigned char c = 200;
int i = 1000;
printf("i/c = %d\n", i/c);
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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.
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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
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.
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Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Astrée |
| Supported indirectly via MISRA C:2012 rules 10.1, 10.3 and 10.4. | |||||||
| Axivion Bauhaus Suite |
| CertC-INT07 | |||||||
| CodeSonar |
| LANG.TYPE.IOT | Inappropriate operand type | ||||||
| Compass/ROSE | Can detect violations of this recommendation. In particular, it flags any instance of a variable of type | ||||||||
| CC2.INT07 | Fully implemented | |||||||
| Helix QAC |
| C1292, C1293, C4401, C4421, C4431, C4441, C4451 | |||||||
| Klocwork |
| PORTING.SIGNED.CHAR | |||||||
| LDRA tool suite |
| 93 S, 96 S, 101 S, 329 S, 432 S, 458 S | Fully implemented | ||||||
| Parasoft C/C++test |
| CERT_C-INT07-a | The plain char type shall be used only for the storage and use of character values | ||||||
| PC-lint Plus |
| 9112 | Fully supported | ||||||
| Checks for use of plain char type for numeric value (rec. fully covered) | ||||||||
| Splint |
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| RuleChecker |
| Supported indirectly via MISRA C:2012 rules 10.1, 10.3 and 10.4. | |||||||
| SonarQube C/C++ Plugin |
| S820 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Other Languages
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Related Guidelines
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Bibliography
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| 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 |
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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"
\[[MISRA 04|AA. Bibliography#MISRA 04]\] Rule 6.2, "Signed and unsigned char type shall be used only for the storage and use of numeric values"
\[[MITRE 07|AA. Bibliography#MITRE 07]\] [CWE ID 682|http://cwe.mitre.org/data/definitions/682.html], "Incorrect Calculation"INT06-C. Use strtol() or a related function to convert a string token to an integer 04. Integers (INT)