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 paragraph 3 [ISO/IEC 9899:2024], linkage is determined as follows:
If the declaration of a file scope identifier for:
- an object contains any of the storage-class specifiersstatic
orcontexpr;
- or, a function contains the storage-class specifier
static
,then 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 does not contain the storage-class specifierstatic
orcontexpr
, 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.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
DCL36-C | Medium | Probable | Medium | P8 | L2 |
Automated Detection
Tool | Version | Checker | Description |
---|---|---|---|
Astrée | 24.04 | static-function-declaration static-object-declaration | Partially checked |
Axivion Bauhaus Suite | 7.2.0 | CertC-DCL36 | Fully implemented |
CodeSonar | 8.1p0 | LANG.STRUCT.DECL.NOEXT | Missing External Declaration |
Coverity | 2017.07 | PW.LINKAGE_CONFLICT | Implemented |
Cppcheck Premium | 24.11.0 | premium-cert-dcl36-c | |
1.2 | CC2.DCL36 | Fully implemented | |
GCC | 4.3.5 | ||
Helix QAC | 2024.4 | C0625 | Fully implemented |
Klocwork | 2024.4 | MISRA.FUNC.STATIC.REDECL | Fully implemented |
LDRA tool suite | 9.7.1 | 461 S, 575 S, 2 X | Fully implemented |
PC-lint Plus | 1.4 | 401, 839, 1051 | Fully supported |
Splint | 3.1.1 | ||
Parasoft C/C++test | 2023.1 | CERT_C-DCL36-a | Identifiers shall not simultaneously have both internal and external linkage in the same translation unit |
Polyspace Bug Finder | R2024a | Checks for inconsistent use of static and extern in object declarations (rule partially covered) | |
RuleChecker | 24.04 | static-function-declaration static-object-declaration | Partially checked |
TrustInSoft Analyzer | 1.38 | 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)
Taxonomy | Taxonomy item | Relationship |
---|---|---|
MISRA C:2012 | Rule 8.2 (required) | Prior to 2018-01-12: CERT: Unspecified Relationship |
MISRA C:2012 | Rule 8.4 (required) | Prior to 2018-01-12: CERT: Unspecified Relationship |
MISRA C:2012 | Rule 8.8 (required) | Prior to 2018-01-12: CERT: Unspecified Relationship |
MISRA C:2012 | Rule 17.3 (mandatory) | Prior to 2018-01-12: CERT: Unspecified Relationship |
Bibliography
[Banahan 2003] | Section 8.2, "Declarations, Definitions and Accessibility" |
[ISO/IEC 9899:2024] | 6.2.2, "Linkages of Identifiers" |
[Kirch-Prinz 2002] |
10 Comments
Abhijit Rao
When I compiled the Non-compliant code example using visual studio 2008 - I get
The same message is displayed for the other 4 variables too.
Please consider revising the statement "...Microsoft Visual Studio compile this non-compliant code example without warning..." I did not have to change the warning level.
Robert Seacord
I only have access to MSVS 2005 right now. At Level 3 (/W3) and at Level 4 (/W4) I get:
------ Build started: Project: DCL36-C, Configuration: Debug Win32 ------
Compiling...
stdafx.cpp
Compiling...
DCL36-C.cpp
Compiling manifest to resources...
Linking...
Embedding manifest...
Build log was saved at "file://c:\Documents and Settings\rcs\My Documents\Visual Studio\Projects\DCL36-C\DCL36-C\Debug\BuildLog.htm"
DCL36-C - 0 error(s), 0 warning(s)
========== Build: 1 succeeded, 0 failed, 0 up-to-date, 0 skipped ==========
But I'll take you word for 2008 and update.
Aaron Ballman
It only provides those warnings when compiling in C++ mode (/TP), but will not warn even with highest warnings (/Wall) enabled when compiling in C mode (/TC).
David Svoboda
Studying the standard, I disagree with the assessment of the NCCE. According to the standard, if a variable is declared static, and then declared with no storage-class specifier, it remains static. Thus in the NCCE both
i2
andi5
should be valid, not invalid.David Svoboda
David Keaton saez:
Kristen Newbury
the NCCE seems to come almost verbatim from the standard, see [6.9.2 Example 1](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf)
(edit: sorry posted then saw that actually MVSC is the one that allows this)
additionally - compiling the example (with clang version 14.0.0) - yields (so seems correct at least with clang):
David Svoboda
Kristen: Thanks. I've added Clang's behavior to the implementation details.
Robert C. Seacord
Kristen Newbury
hi ya'll , quick question when trying to create a NCCE from the table...
is the column/row heading that says "no linkage" truly meant to say "no linkage"?
I think it should possibly instead say "no specifier"?
from the standard: "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."
so in the current NCCE - int i5; actually has linkage - external linkage (via no specifier) : " If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external."
this would agree with the table cell (0,1) (row,column) if it were to say (no specifier, static) == undefined.
following this logic a fully comprehensive NCCE would be (if someone wouldn't mind checking this - thanks!!!):
David Svoboda
Kristen:
"No specifier" would also be a decent entry for the table. However there are specifiers other than linkage specifiers, so "no linkage" is more precise.
In the NCCE i5 has no linkage...remember it is outside of any block. If it were defined within main() it would have local linkage; that is, it would live on the stack and only persist until its function (main()) ended. The re-declaration of i5 in the NCCE is truly undefined, since it was static in its first declaration and had no linkage in its second.
Your code example does correctly represent the table, including which declarations are compliant and which are non-compliant.