Comment by dzaima

You might want to move around some file descriptors if you don't want the child process to inherit your stdin/stdout/stderr (e.g. if you want to read the stdout of the process you launched, or give it some stdin).

And there does exist such a fork-exec combo - posix_spawn. It allows adding some "commands" of what file descriptor operations to do between the fork & exec before they're ever done, among some other things. But, as the article mentions, using it is annoying - you have to invoke various posix_spawn_file_actions_* functions, instead of the regular C functions you'd use.