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This noncompliant code example omits the type specifier.:

extern foo;

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This compliant solution explicitly includes a type specifier.:

extern int foo;

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However, to conform with the C Standard, you must explicitly prototype every function before invoking it. This noncompliant example fails to prototype the foo() function before invoking it in main().:

int main(void) {
  int c = foo();
  printf("%d\n", c);
  return 0;
}

int foo(int a) {
  return a;
}

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In this compliant solution, a prototype for foo() appears before the function invocation.:

int foo(int);

int main(void) {
  int c = foo(0);
  printf("%d\n", c);
  return 0;
}

int foo(int a) {
  return a;
}

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This compliant solution explicitly defines the return type of foo() as unsigned int.:

unsigned int foo(void) {
  return UINT_MAX;
}

int main(void) {
  long long c = foo();
  printf("%lld\n", c);
  return 0;
}

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Related Vulnerabilities

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

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Bibliography

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