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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.

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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);
  }

  /* ... */

}

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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);
  }

  /* ... */

}

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

Related Guidelines

Bibliography

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