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 creates more modular code and limits pollution of the global name space.

Section 6.2.2 of C99 \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] states that

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.

Noncompliant Code Example

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);
  }

  /* ... */

}

Compliant Solution

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);
  }

  /* ... */

}

Risk Assessment

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.

Automated Detection

Tool	Version	Checker	Description
Splint
LDRA tool suite

Related Vulnerabilities

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

Related Guidelines

CERT C++ Secure Coding Standard: DCL15-CPP. Declare file-scope objects or functions that do not need external linkage in an unnamed namespace

ISO/IEC 9899:1999 Section 6.2.2, "Linkages of identifiers"

Bibliography

      02. Declarations and Initialization (DCL)      DCL16-C. Use 'L', not 'l', to indicate a long value