If a file-scope object or a function does not need to be visible outside of the file, it should be hidden by being declared as static
. This practice creates more modular code and limits pollution of the global name space.
Subclause 6.2.2 of the C Standard [ISO/IEC 9899:2011] states:
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.
and
If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.
This noncompliant code example includes a helper()
function that is implicitly declared to have external linkage:
enum { MAX = 100 }; int helper(int i) { /* Perform some computation based on i */ } int main(void) { size_t i; int out[MAX]; for (i = 0; i < MAX; i++) { out[i] = helper(i); } /* ... */ } |
This compliant solution declares helper()
to have internal linkage, thereby preventing external functions from using it:
enum {MAX = 100}; static int helper(int i) { /* Perform some computation based on i */ } int main(void) { size_t i; int out[MAX]; for (i = 0; i < MAX; i++) { out[i] = helper(i); } /* ... */ } |
Allowing too many objects to have external linkage can use up descriptive identifiers, leading to more complicated identifiers, violations of abstraction models, and possible name conflicts with libraries. If the compilation unit implements a data abstraction, it may also expose invocations of private functions from outside the abstraction.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
DCL15-C | Low | Unlikely | Low | P3 | L3 |
Tool | Version | Checker | Description |
---|---|---|---|
Astrée | global-object-scope | Fully checked | |
Axivion Bauhaus Suite | CertC-DCL15 | ||
CodeSonar | LANG.STRUCT.SCOPE.FILE | Scope could be file static | |
CC2.DCL15 | Fully implemented | ||
Helix QAC | C1504, C1531 | ||
LDRA tool suite | 27 D | Fully implemented | |
Parasoft C/C++test | CERT_C-DCL15-a | Objects or functions with external linkage shall be declared in a header file | |
PC-lint Plus | 765 | Fully supported | |
Polyspace Bug Finder | Checks for situations where function or object with external linkage is referenced in only one translation unit (rec. fully covered) | ||
RuleChecker | global-object-scope | Fully checked | |
Splint |
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
SEI CERT C++ Coding Standard | VOID DCL15-CPP. Declare file-scope objects or functions that do not need external linkage in an unnamed namespace |
MISRA C:2012 | Rule 8.7 (advisory) |
ISO/IEC 9899:2011 | Subclause 6.2.2, "Linkages of Identifiers" |