An identifier can be 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:
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 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.
If, within a translation unit, the same identifier appears with both internal and external linkage, the behavior is undefined.
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.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
DCL07-A |
1 (low) |
2 (probable) |
3 (low) |
P6 |
L2 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
[[ISO/IEC 9899-1999:TC2]] Section 6.2.2, "Linkages of identifiers"