An identifier declared in different scopes or multiple times within the same scope can be made to refer to the same object or function by linkage. An identifier can be classified as externally linked, internally linked, or not-linked. These three kinds of linkage have the following characteristics:

• 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 C99:

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

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.

Non-Compliant Code Example

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

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

int i1;              /* valid tentative definition */
int i2;              /* not legal, linkage disagreement with previous */
int i3;              /* valid tentative definition */
int i4;              /* valid tentative definition */
int i5;              /* not legal, linkage disagreement with previous */

Compliant Solution

This compliant solution does not include conflicting definitions.

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

Risk Assessment

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

Related Vulnerabilities

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

References

\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 6.2.2, "Linkages of identifiers"