When using vfork() what are you allowed to do? Can you even do IO redirection (piping)? The man page says:
> ... the behavior is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork() ...
But the time between fork and exec is exactly where you do a lot of setup, like IO redirection, dropping privileges, setuid(), setting a signal mask (nohup) etc. and I don't think you can do that without setting any variables. You certainly write to the stack when calling a function.
If you can't do these things you can't really use it to implement posix_spawn(). I guess it could use vfork() in the case no actions are required, but only then.
> ... the behavior is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork() ...
But the time between fork and exec is exactly where you do a lot of setup, like IO redirection, dropping privileges, setuid(), setting a signal mask (nohup) etc. and I don't think you can do that without setting any variables. You certainly write to the stack when calling a function.
If you can't do these things you can't really use it to implement posix_spawn(). I guess it could use vfork() in the case no actions are required, but only then.