I'm going to guess that most of those issues were caused by implicit declarations no longer being valid. int main() { printf("Sans headers, this is

I'm going to guess that most of those issues were caused by implicit declarations no longer being valid. int main() { printf("Sans headers, this is valid C89."); return 0; } Without an explicit declaration, C will consider this function to have the following signature based on the call site: int printf(); By the way, in C () doesn't mean "no parameters" it means "any parameters, which the compiler will infer from the call site and pass to the function."