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This noncompliant code example omits the type specifier.

extern foo;

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.

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This compliant solution explicitly includes a type specifier.

extern int foo;

Noncompliant Code Example (Implicit Function Declaration)

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However, to conform with C99, 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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Similarly, 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.

foo(void) {
  return UINT_MAX;
}

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

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This compliant solution explicily 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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