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Identifiers in mutually visible scopes must be deemed unique by the compiler, 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 instance, two functions may each have a local variable with the same name, as their scopes 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 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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On implementations that support only the minimum requirements for significant characters required by the standard, the following this code example is noncompliant because the first 31 characters of the external identifiers are identical:

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Noncompliant Code Example (Universal Characters)

In the following this noncompliant code example, both external identifiers consist of four universal characters. Because the first three universal characters of each identifier are identical, both identify the same integer array.

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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.

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

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