Question

I have a struct like the following:

typedef struct {
    player *lastmover;
    player *previous;
} lastmove;


typedef struct {
     int moves;
     char *name;
} player;

I try do a memory alloc like this so the :

lastmove lmv;
lmv.lastmover=malloc(sizeof(player *));
lmv.previous=malloc(sizeof(player *));
.....
callfunction(&lmv);
.....

then in another place i use a pointer lmvp (lastmover *) and do assignment like this:

void
callfuntion(lastmove *)
{
      .....
      lmvp->previous=lmvp->lastmover;
      lmvp->lastmover=p;     //where p is of type (player *)
      .....
}

this all works fine, but I don’t know how to control weather previous mover is initialized. In the first game move in the program the lastmover variable (player *) is initialized, but the previous variable that is assigned to NULL(or garbage?) lmvp->previous=lmvp->lastmover;. But I want somehow to check if the previous mover is initialized or not. Im trying this:

void
callfunction(lastmove *)
{
     ......
     ......
     if(lmvp->previous!=NULL)
     ......
}

But im quite certain it will not do.. How is the best way to control this?

Answer 1

you’re only allocating size of pointer, this

lmv.lastmover=malloc(sizeof(player *));

Should be

lmv.lastmover=malloc(sizeof(player));

this all works fine

it shouldn’t 🙂

Edit:
if you only assign a pointer to lastmover then you don’t need to allocate memory at all, otherwise it’s a memory leak, now if you want to check if it’s initialized then initialize it to NULL

lastmove lmv = {0};
//or
lmv.lastmover= NULL;
lmv.previous= NULL;
...
//later
if(lmv->previous!=NULL)

A third way to do it, in C99, is using designated initializers:

lastmove lmv = {
   .lastmover= NULL;
   .previous= NULL;
};