哪一个选择waitpid/wait/waitid?

I want to use execl in child process after doing fork. execl will execute the script which will take around 120 seconds of time. I tried almost all the combination with waitpid, wait and waitid with different argument (0, WNOHANG, etc.,) but in all case I am getting -1 return value. So I want to know in which wait function I need to use when? So I can concentrate in one wait function to make it work. I want to use execl in child process after doin