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If an a file-scope object or a function does not need to be visible outside of the current scopefile, it should be hidden by being declared as static. This practice creates more modular code and limits pollution of the global name space.

Wiki MarkupSection Subclause 6.2.2 of C99 \[[of the C Standard [ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] states that2011] 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.

...

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.:

Code Block

bgColor	#ffcccc
lang	c


enum { MAX = 100 };

int helper(int i) {
  /* performPerform 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.:

Code Block

bgColor	#ccccff
lang	c


enum {MAX = 100};

static int helper(int i) {
  /* performPerform 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.