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. 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 [ISO/IEC 9899:2011], linkage is determined as follows:

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 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.

Noncompliant Code Example

In this noncompliant code example, i2 and i5 are 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;  /* Undefined, linkage disagreement with previous */
int i3;  /* Valid tentative definition */
int i4;  /* Valid tentative definition */
int i5;  /* Undefined, linkage disagreement with previous */

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

Risk Assessment

Use of an identifier classified as both internally and externally linked is undefined behavior.

Automated Detection

Tool	Version	Checker	Description
Astrée		static-function-declaration static-object-declaration	Partially checked
Axivion Bauhaus Suite		CertC-DCL36	Fully implemented
CodeSonar		LANG.STRUCT.DECL.NOEXT	Missing External Declaration
Coverity		PW.LINKAGE_CONFLICT	Implemented
ECLAIR		CC2.DCL36	Fully implemented
GCC
Helix QAC		C0625	Fully implemented
Klocwork		MISRA.FUNC.STATIC.REDECL	Fully implemented
LDRA tool suite		461 S, 575 S, 2 X	Fully implemented
PC-lint Plus		401, 839, 1051	Fully supported
Splint
Parasoft C/C++test		CERT_C-DCL36-a	Identifiers shall not simultaneously have both internal and external linkage in the same translation unit
Polyspace Bug Finder		CERT C: Rule DCL36-C	Checks for inconsistent use of static and extern in object declarations (rule partially covered)
RuleChecker		static-function-declaration static-object-declaration	Partially checked
TrustInSoft Analyzer		non-static declaration follows static declaration	Partially verified.

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)

Bibliography