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The previous standard of C90 allowed for implicit typing of variables and functions. Since Because implicit declarations lead to less stringent type checking, they can often introduce unexpected and erroneous behavior or even security vulnerabilities.

...

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

int foo(int a) {
  return a;
}

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

...

int foo(int);

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

int foo(int a) {
  return a;
}

Wiki MarkupFor more information on function declarations see \[[DCL07-A. Include the appropriate type information in function declarators]\].

Non-Compliant Code Example (implicit return type)

...

foo() {
  return UINT_MAX;
}

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

Since Because the compiler assumes that foo() returns a value of type int, UINT_MAX is incorrectly converted to -1.

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