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If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.

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Noncompliant Code Example

This non-compliant 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 objects from other scopes 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.

Automated Detection

Splint Version 3.1.1 can detect violations of this rule.

Related Vulnerabilities

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

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.2.2, "Linkages of identifiers"

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