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An identifier can be Linkage can make an identifier declared in different scopes or declared multiple times within the same scope refer to the same object or function. Identifiers are classified as externally linked, internally linked, or not -linked.

An identifier that is classified as externally linked includes identifiers:

• whose declaration contains the storage-class specifier extern, where no prior declaration of that identifier is visible.
• for a function whose declaration contains no storage-class specifier.
• for an object with file scope whose declaration contains no storage-class specifier.

An identifier that is classified as internally linked includes identifiers whose declaration contains the storage-class specifier static.

An identifier that is classified as not-linked include:

• An identifier declared to be anything other than an object or a function.
• An identifier declared to be a function parameter.
• A block scope identifier for an object declared without the storage-class specifier extern.

If a prior declaration is visible and has no linkage, the latter declaration is externally linked. If a prior declaration is visible and has either internal or external linkage, the latter declaration is classified with the same linkage as the prior declaration.

Use of an identifier (within one translational unit) classified as both internally and externally linked causes undefined behavior. A translational unit includes the source file together with its headers, and all source files included via the preprocessing directive #include.

According to C99:

linked. These three kinds of linkage have the following characteristics [Kirch-Prinz 2002]:

• External linkage: An identifier with external linkage represents the same object or function throughout the entire program, that is, in all compilation units and libraries belonging to the program. The identifier is available to the linker. When a second declaration of the same identifier with external linkage occurs, the linker associates the identifier with the same object or function.

• Internal linkage: An identifier with internal linkage represents the same object or function within a given translation unit. The linker has no information about identifiers with internal linkage. Consequently, these identifiers are internal to the translation unit.

• No linkage: If an identifier has no linkage, then any further declaration using the identifier declares something new, such as a new variable or a new type.

According to the C Standard, 6.2.2 paragraph 3 [ISO/IEC 9899:2024], linkage is determined as follows:

An identifier declared in different scopes or in the same scope more than once can be made to refer to the same object or function by a process called linkage. There are three kinds of linkage: external, internal, and none.

In the set of translation units and libraries that constitutes an entire program, each declaration of a particular identifier with external linkage denotes the same object or function. Within one translation unit, each declaration of an identifier with internal linkage denotes the same object or function. Each declaration of an identifier with no linkage denotes a unique entity.

If the declaration of a file scope identifier for:
    - an object contains any of the storage-class specifiers static  or contexpr;

    - or, a function contains the storage-class specifier static,

 then the identifier has internal linkage.

For an identifier declared with the storage-class specifier extern in a scope in which a prior declaration of that identifier is visible, if the prior declaration specifies internal or external linkage, the linkage of the identifier at the later declaration is the same as the linkage specified at the prior declaration. If no prior declaration is visible, or if the prior declaration specifies no linkage, then the identifier has external linkage.

If the declaration of an identifier for a function has no storage-class specifier, its linkage is determined exactly as if it were declared with the storage-class specifier extern. If
the declaration of an identifier for an object has file scope and no does not contain the storage-class specifier static or contexpr, its its linkage is external.

The following identifiers have no linkage: an identifier declared to be anything other than an object or a function; an identifier declared to be a function parameter; a block scope
identifier for an object declared without the storage-class specifier extern.

If, within a translation unit, the same identifier appears with both internal and external linkage, the behavior is undefined.

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Use of an identifier (within one translation unit) classified as both internally and externally linked is undefined behavior. (See also undefined behavior 8.) A translation unit includes the source file together with its headers and all source files included via the preprocessing directive #include.

The following table identifies the linkage assigned to an object that is declared twice in a single translation unit. The column designates the first declaration, and the row designates the redeclaration.

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Noncompliant Code Example

In this non-compliant noncompliant code example, i2 and i5 is are defined as having both internal and external linkage. Future use of either identifier results in undefined behavior.

int i1 = 10;         /* definitionDefinition, external linkage */
static int i2 = 20;  /* definitionDefinition, internal linkage */
extern int i3 = 30;  /* definitionDefinition, external linkage */
int i4;              /* tentativeTentative definition, external linkage */
static int i5;       /* tentativeTentative definition, internal linkage */

int i1;              /* validValid tentative definition */
int i2;              /* not legalUndefined, linkage disagreement with previous */
int i3;              /* validValid tentative definition */
int i4;              /* validValid tentative definition */
int i5;  /* Undefined, linkage disagreement with previous */

int   main(void) {
   /* not legal, linkage disagreement with previous */
... */
  return 0;
}

Implementation Details

Microsoft Visual Studio 2013 issues no warnings about this code, even at the highest diagnostic levels.

GCC and Clang 14 both generate fatal diagnostics for the conflicting definitions of i2 and i5.

Compliant Solution

This compliant solution does not include conflicting definitions.:

int i1 = 10;         /* definitionDefinition, external linkage */
static int i2 = 20;  /* definitionDefinition, internal linkage */
extern int i3 = 30;  /* definitionDefinition, external linkage */
int i4;              /* tentativeTentative definition, external linkage */
static int i5;       /* tentativeTentative definition, internal linkage */

int main(void) {
  /* ... */
  return 0;
}

Risk Assessment

Use of an identifier classified as both internally and externally linked causes is undefined behavior. However, it is unlikely that an attacker could exploit this behavior to run arbitrary code.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

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Bibliography

References

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