The C Standard defines octal constants as a 0 followed by octal digits (0 1 2 3 4 5 6 7). Programming errors can occur when decimal values are mistakenly specified as octal constants.

Noncompliant Code Example

In this noncompliant code example, a decimal constant is mistakenly prefaced with zeros so that all the constants are a fixed length:

i_array[0] = 2719;
i_array[1] = 4435;
i_array[2] = 0042;

Although it may appear that i_array[2] is assigned the decimal value 42, it is actually assigned the decimal value 34.

Compliant Solution

To avoid using wrong values and to make the code more readable, do not preface constants with zeroes if the value is meant to be decimal:

i_array[0] = 2719;
i_array[1] = 4435;
i_array[2] =   42;

Risk Assessment

Misrepresenting decimal values as octal can lead to incorrect comparisons and assignments.

Automated Detection

Tool	Version	Checker	Description
Astrée		octal-constant	Fully checked
Axivion Bauhaus Suite		CertC-DCL18
CodeSonar		LANG.TYPE.OC	Octal constant
Helix QAC		C0339, C1272
Klocwork		MISRA.TOKEN.OCTAL.ESCAPE MISRA.TOKEN.OCTAL.INT
LDRA tool suite		83 S	Fully Implemented
Parasoft C/C++test		CERT_C-DCL18-a CERT_C-DCL18-b	Octal and hexadecimal escape sequences shall be terminated Octal constants (other than zero) shall not be used
PC-lint Plus		9001	Fully supported
Polyspace Bug Finder		CERT C: Rec. DCL18-C	Checks for use of octal constants (rec. fully covered)
PVS-Studio		V536
RuleChecker		octal-constant	Fully checked
SonarQube C/C++ Plugin		OctalConstantAndSequence

Related Guidelines