I'm trying to figure out the correct syntax in this situation:

• Assign double pointer to an object
• Get the original address of that double pointer back

Here's a code example:

typedef struct obj2{
  obj1 **ptr;
} obj2;

void func(obj1 **dbl)
{
    obj2 *tmp = malloc(sizeof(obj2));

    printf("before; dbl address is %p\n", (void*)&dbl); // 0x7fff6a1362a0
    tmp->ptr = dbl;

    // here; trying to print the same address as the first printf
    printf("after; dbl address is %p\n", (void*)(tmp->ptr)); // 0x7fff6a136280
}

What am I doing wrong?

EDIT: The use case for this is so I can use dbl elsewhere in the codebase, so the ultimate goal here is just to be able to correctly pass around a double pointer.

CodePudding user response：

&dbl is the address of the parameter dbl which is a local variable.

tmp->ptr = dbl;

Assigns the tmp->ptr with the value dbl holds, not the address of the variable dbl. Those addresses most likely to be different inless you assign dbl with &dbl (it makes no sense at all).

CodePudding user response：

In this statement

tmp->ptr = dbl;

you are not assigning the address of the pointer db1 to the object tmp->ptr. You are assigning the value stored in the pointer.

So these two calls of printf output different values

printf("before; dbl address is %p\n", (void*)&dbl); // 0x7fff6a1362a0

// here; trying to print the same address as the first printf
printf("after; dbl address is %p\n", (void*)(tmp->ptr)); // 0x7fff6a136280

You should change the first call of printf the following way

printf("before; dbl address is %p\n", (void*)dbl);