Page History

...

Most C90 implementations do not issue a diagnostic for the violation of this C99 constraint. Many C99 translators will continue to treat such declarations as implying the type int.

Compliant Solution

This compliant solution explicitly declares foo to be of type int.

extern int foo;

...

extern int func();

However, for conformance to conform with C99, you should must explicitly prototype every function before making a call to itinvoking it. This non-compliant 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;
}

Because the compiler assumes foo() to have type extern int foo(), it cannot diagnose the missing argument and a bogus value is printed.

Compliant Solution (implicit function declaration)

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;
}

...

Non-Compliant Code Example (implicit return type)

Similarly, don't do not declare a function with implicit return type. If it returns a meaningful integer value, declare it int. If it returns no meaningful value, declare it void.

...