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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 Wiki MarkupSection 6.2.2 of C99 states that: \[of the C Standard [ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]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.

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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.

If an object, such as a function, does not need to be visible outside the current scope, it should be hidden by being declared as static. This helps create more modular code and may expose hidden assumptions about abstraction.

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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) {
  /* performPerform some computation based on i */
}

int main(void) {
  intsize_t i;
  int out[MAX];

  for (i = 0; i < MAX; i++) {
    out[i] = helper(i);
  }

  /* ... */
  return 0;
}

Compliant Solution

This compliant solution declares helper() to have internal linkage, thereby preventing objects external functions from other scopes from using it.:

enum {MAX = 100};

static int helper(int i) {
  /* performPerform some computation based on i */
}

int main(void) {
  intsize_t i;
  int out[MAX];

  for (i = 0; i < MAX; i++) {
    out[i] = helper(i);
  }

  /* ... */
  return 0;
}

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

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Related Vulnerabilities

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

References

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Related Guidelines

Bibliography

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Image Added Image Added identifiers"DCL14-A. Do not make assumptions about the order of global variable initialization across translation units      02. Declarations and Initialization (DCL)       Image Modified