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According to subclause 6.2.7 of the C Standard [ISO/IEC 9899:2011],

All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined.

(See also undefined behavior 15 of Annex J.)

Further, according to subclause 6.4.2.1,

Any identifiers that differ in a significant character are different identifiers. If two identifiers differ only in nonsignificant characters, the behavior is undefined.

(See also undefined behavior 31 of Annex J.)

Identifiers in mutually visible scopes must be deemed unique by the compiler , in order to prevent confusion about which variable or function is being referenced. Implementations can allow additional non-unique nonunique characters to be appended to the end of identifiers, making the identifiers appear unique while actually being indistinguishable.

It is perfectly fine reasonable for scopes that are not visible to each other to have duplicate identifiers. For instanceexample, two functions may can each have a local variable with the same name , as because their scopes can not cannot access each other. But a function's local variable names should be distinct from each other , as well as from all static variables declared within the function's file (as well as and from all included header files.)

To guarantee that identifiers are unique, first the number of significant characters recognized by ( the most restrictive ) compiler used must be determined. This assumption must be documented in the code.

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• 63 significant initial characters in an internal identifier or a macro name. (each Each universal character name or extended source character is considered a single character.)
• 31 significant initial characters in an external identifier. (each Each universal character name specifying a short identifier of 0000FFFF or less is considered 6 characters, ; each universal character name specifying a short identifier of 00010000 or more is considered 10 characters, ; and each extended source character, if any exist, is considered the same number of characters as the corresponding universal character name, if any.)

Restriction of the significance of an external name to fewer than 255 characters in the standard (considering each universal character name or extended source character as a single character) is an obsolescent feature that is a concession to existing implementations. As a result, it is not necessary to comply with this restriction , as long as the identifiers are unique and the assumptions concerning the number of significant characters are documented.

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Noncompliant Code Example (Source Character Set)

On implementations that support only the minimum requirements for significant characters required by the standard, the following this code example is non-compliant noncompliant because the first 31 characters of the external identifiers are identical:

extern int *global_symbol_definition_lookup_table_a;
extern int *global_symbol_definition_lookup_table_b;

Compliant Solution (Source Character Set)

In a compliant solution, the significant characters in each identifier must differ.:

extern int *a_global_symbol_definition_lookup_table;
extern int *b_global_symbol_definition_lookup_table;

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Noncompliant Code Example (Universal

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Character Names)

In the following non-compliant this noncompliant code example, both external identifiers consist of four universal characterscharacter names. Because the first three universal characters character names of each identifier are identical, both identify the same integer array .on implementations that support only the minimum requirements for significant characters required by the standard:

extern int *\U00010401\U00010401\U00010401\U00010401;
extern int *\U00010401\U00010401\U00010401\U00010402;

Compliant Solution (Universal

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Character Names)

For portability, the first three universal character combination name combinations used in an identifier must be unique.:

extern int *\U00010401\U00010401\U00010401\U00010401;
extern int *\U00010402\U00010401\U00010401\U00010401;

Risk Assessment

Non-unique Nonunique identifiers can lead to abnormal program termination, denial-of-service attacks, or unintended information disclosure.

Automated Detection

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The LDRA tool suite V 7.6.0 is able to detect violations of this rule.

Tool	Version	Checker	Description
Astrée	Include Page Astrée_V Astrée_V		Supported indirectly via MISRA C:2012 Rules 5.1, 5.2, 5.3, 5.4 and 5.5.
Axivion Bauhaus Suite	Include Page Axivion Bauhaus Suite_V Axivion Bauhaus Suite_V	CertC-DCL23
CodeSonar	Include Page CodeSonar_V CodeSonar_V	LANG.ID.ND.EXT LANG.ID.ND.MM LANG.ID.ND.MO LANG.ID.ND.NEST LANG.ID.ND.SS LANG.ID.NU.EXT LANG.ID.NU.INT LANG.ID.NU.LIBFN LANG.ID.NU.TAG LANG.ID.NU.TYPE LANG.STRUCT.DECL.MGT	Non-distinct identifiers: external names Non-distinct identifiers: macro/macro Non-distinct identifiers: macro/other Non-distinct identifiers: nested scope Non-distinct identifiers: same scope Non-unique identifiers: external name Non-unique identifiers: internal name Library Function Override Non-unique identifiers: tag Non-unique identifiers: typedef Global variable declared with different types
Compass/ROSE			Can detect some violations of this rule but cannot flag violations involving universal names
Helix QAC	Include Page Helix QAC_V Helix QAC_V	C0627, C0776, C0777, C0778, C0779, C0789, C0791, C0793
Klocwork	Include Page Klocwork_V Klocwork_V	MISRA.IDENT.DISTINCT.C99.2012
LDRA tool suite	Include Page LDRA_V LDRA_V	17 D 355 S 61 X	Fully implemented
PC-lint Plus	Include Page PC-lint Plus_V PC-lint Plus_V	621	Fully supported
Polyspace Bug Finder	Include Page Polyspace Bug Finder_V Polyspace Bug Finder_V	CERT C: Rec. DCL23-C	Checks for: External identifiers not distinct Identifiers in same scope and namespace not distinct Macro identifier not distinct Name for macros and identifiers not distinct Rec. fully covered.
RuleChecker	Include Page RuleChecker_V RuleChecker_V		Supported indirectly via MISRA C:2012 Rules 5.1, 5.2, 5.3, 5.4 and 5.5.
SonarQube C/C++ Plugin	Include Page SonarQube C/C++ Plugin_V SonarQube C/C++ Plugin_V	IdentifierLongerThan31

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 5.2.4.1, "Translation limits"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "AJN Choice of Filenames and other External Identifiers" and "YOW Identifier name reuse"
\[[MISRA 04|AA. C References#MISRA 04]\] Rules 5.1 and 8.9

Related Guidelines

Bibliography

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Image Added Image Added Image AddedImage Removed      02. Declarations and Initialization (DCL)       DCL33-C. Ensure that restrict-qualified source and destination pointers in function arguments do not reference overlapping objects