How to duplicate a struct with char* and point on

问题:

I'm new in C programming and would love to get some help.

I have this struct :

typedef struct house
{
   int numOfRooms;
   char* houseName;
}HOUSE,*pHOUSE;

I want to create a function that gets a pointer to HOUSE and returns a new pointer to same HOUSE that is located in a different place in memory - the purpose is to be able to change one pointer without changing both : I will try to be clearer :

pHOUSE duplicate_house(pHOUSE house)
{
  pHOUSE newh = (pHOUSE)calloc(1,sizeof(HOUSE));
  newh = house 
 //I get here a pointer that points on the same house.. so If I change for exmample:
 //  newh->numOfRooms = 9 - > both will change and I don't want it to happen!


}

I read that we can use : memcpy_s but here If I only had integers inside the struct it could be easy , here I have char * , so means I need also to copy the char * houseName separately? what can I do? How can I copy an object that has multiply types like char *? and If I had an array ? what could I do ?

typedef struct house
{
    int numOfRooms;
     char* houseName;
     struct house *houses[10];
}HOUSE,*pHOUSE;

how can I copy that?

thank u very much!

回答1:

You need to copy both the structure as well as all memory that's managed by the structure. Like so:

pHOUSE copy(pHOUSE house)
{
    pHOUSE newHouse = malloc(sizeof *newHouse);      // allocate

    if (pHOUSE)
    {
        memcpy(newHouse, house, sizeof *newHouse);   // or "*newHouse = *house;"
        size_t const len = strlen(house->houseName);
        newHouse->houseName = malloc(len + 1);

        if (!newHouse->houseName) { free newHouse; return NULL; }

        strncpy(newHouse->houseName, house->houseName, len + 1);
    }
    return pHOUSE;
}

As you can see, the error handling with two allocations is already getting very cumbersome. If you had multiple internal allocations, the only way to remain sane is to use gotos systematically to create the appropriate cleanup points.

---

Example to illustrate the last point:

struct FooLish
{
    char * p1;
    char * p2;
    char * p3;
};

struct FooLish * copy(struct FooLish const * oldFoo)
{
    struct FooLish * newFoo = malloc(sizeof *newFoo);
    if (!newFoo) { goto end0; }

    {
        size_t const len = strlen(oldFoo->p1);
        newFoo->p1 = malloc(strlen(len + 1);
        if (!newFoo->p1) { goto end1; }
        strncpy(newFoo->p1, oldFoo->p1, len + 1);
    }
    {
        size_t const len = strlen(oldFoo->p2);
        newFoo->p2 = malloc(strlen(len + 1);
        if (!newFoo->p2) { goto end2; }
        strncpy(newFoo->p2, oldFoo->p2, len + 1);
    }
    {
        size_t const len = strlen(oldFoo->p3);
        newFoo->p3 = malloc(strlen(len + 1);
        if (!newFoo->p3) { goto end3; }
        strncpy(newFoo->p3, oldFoo->p3, len + 1);
    }

    return newFoo;

end3:
   free(newFoo->p2);
end2:
   free(newFoo->p1);
end1:
   free(newFoo);
end0:
    return NULL;
}