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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 \[[Kirch-Prinz 02|AA. C References#Kirch-Prinz 02]\]: |
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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 */
int main(void) {
/* // ... */
}
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Implementation Details
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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) {
/* // ... */
}
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Risk Assessment
Use of an identifier classified as both internally and externally linked causes undefined behavior.
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Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
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\[[Banahan 03|AA. C References#Banahan 03]\] [Section 8.2, "Declarations, Definitions and Accessibility"|http://publications.gbdirect.co.uk/c_book/chapter8/declarations_and_definitions.html]
\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 6.2.2, "Linkages of identifiers"
\[[Kirch-Prinz 02|AA. C References#Kirch-Prinz 02]\] |
