How to not "return address of local variable" when modifying a copy of a struct?

I have the struct:

struct mystruct {        int a;};

If I create a function with the struct as an argument,and try to directly return its address:

struct mystruct *modifystruct1(struct mystruct s){        s.a = 5;        return &s;}

Compiling with c99 -Wall -Wextra -pedantic will warn warning: function returns address of local variable [-Wreturn-local-addr],which I know I shouldn't do.

However, if I save the address to another variableand try to return that, the warning disappears:

struct mystruct *modifystruct2(struct mystruct s){        struct mystruct *sptr = &s;        sptr->a = 5;        return sptr;}

Is this okay to do, or is it no different from the above?(and if so, why is there no more warning?)

If not, how can I modify a copy of a struct inside a functionand return a pointer to that struct,safely, preferably without using malloc?