void (*signal(int sig, void (*func)(int)))(int)

Everything in computers (and I may say, more in general in life) is based on assumptions made on knowledge we implicitly have. After a while, writing eq or == becomes natural and you stop mixing them up. These kinds of errors are frequent when you don't use something daily; for instance, after more than a decade I still keep forgetting Powershell's syntax because I don't use it often and when I do, I tend to do basic things.