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Use of an identifier (within one translation unit) classified as both internally and externally linked causes undefined behavior. A translation 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 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;              /* not valid, linkage disagreement with previous */
int i3;              /* valid tentative definition */
int i4;              /* valid tentative definition */
int i5;              /* not valid, linkage disagreement with previous */

int main(void) {
  /* ... */
}

Implementation Details

Both Microsoft Visual Studio 2003 and Microsoft Visual Studio 2005 compile this non-compliant code example without warning even at the highest diagnostic levels. Microsoft Visual Studio 2008 does provide warnings at the default warning level. The GCC compiler generates a fatal diagnostic 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) {
  /* ... */
}

Risk Assessment

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

Related Vulnerabilities

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

References

\[[Banahan 03|AA. C References#Banahan 03]\] [Section 8.2, "Declarations, Definitions and Accessibility"|http://publications.gbdirect.co.uk/c_book/chapter8/declarations_and_definitions.html]
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.2.2, "Linkages of identifiers"
\[[Kirch-Prinz 02|AA. C References#Kirch-Prinz 02]\]
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 5.5, "No object or function identifier with static storage duration should be reused"

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