pointer assignment.

[denc4]

hi
what i would like to do is store the address contained in a pointer variable at a target address pointed at by another pointer.

typedef struct STRUCTURE
{
  char name[30];
  struct STRUCTURE *Next;
}DATA;

main()
{
  DATA *p0;
  DATA *p1;
  p0 = p0->Next;

  p0 = p1;        /* alt1: overwrites pointer, not target address */
  *p0 = p1;       /* alt2: error. type mismatch */
  *p0 = *p1;      /* alt3: moves data p1 points to, to where p0 points to */
}

i have played around abit with the indirection character, which resulted in the alternatives above. alternative2 is what i want to achieve, but the compiler complains and rejects it.

any idea how to do this properly?
thx.

 

[cpjust]

p0 = p0->Next;

Since you haven't allocated any memory for p0 yet, this code will crash.

*p0 = p1;

This makes no sense. You're basically trying to put a square peg into a round hole.

Maybe if you explained what you are trying to do (instead of how you're trying to do it) we can figure out a better way for you to write it.

 

[denc4]

i know i haven't allocated any memory for it in my example above. The reason for this is that the actual code is quite large so i made a version for this forum which illustrates my problem in a succinct form.

i explained what i am trying to achieve in the first sentence of my first post.

my program manages a single linked list. after p0 = p0->Next, p0 becomes a pointer to a pointer within my linked list. now i want to store the value of a pointer (p1) at the offset p0 points to.

i thought this could be done by using the indirection character on p0 in an assignment:

*p0 = p1;

but the compiler says its a type mismatch. the question is; how do i tell the compiler p0 points at a value compatible with p1?

 

[risby]

You've said p1 is a pointer and we can see that p0 is a pointer to a structure (which contains at least one element called Next, probably also a pointer to a similar structure).

When you dereference p0 (as in *p0) you have a structure on the lefthand side of your assignment not a pointer so it is incompatible with p1 which is a pointer not a structure.

If both p0, p1 and Next are pointers to structures they can each have either of the other assigned to them, i.e.

p0 = p1
p0 = p0 -> Next
p0->Next = p1
p1->Next = p0

In this context your phrase "at the offset p0 points to" is difficult to interpret: it points to a structure, or a location in memory, not an "offset".

==========================================

Some cause happiness wherever they go; others whenever they go.

 

[cpjust]

The only valid things you can do are copy one pointer to another pointer, or copy one structure to another structure. For example:

/* Copy pointers to pointers. */
p0 = p1;
p0 = p0->Next;
p0->Next = p1;
p0->Next = p1->Next;

/* Copy structures to structures. */
*p0 = *p1;
*p0 = *(p0->Next);
*(p0->Next) = *p1;
*(p0->Next) = *(p1->Next);

 

[denc4]

i've tried to cast the receiving pointer:

(DATA*)p0 = p1;

i had hopes it would accept it, but alas.

i found a way around the problem. i have restructered my code so it won't have to do this assigment statement anymore, it's a bit longer now, but it works.

thanks for the help.

 

[chipperMDW]

There's no casting required; that'll just make things worse.

As mentioned, your problem is that you declared the variable p0 to have the wrong type.

You declared it as a DATA*, which is the address of a DATA. From your description, you don't want a DATA*; you want a DATA**, which is the address of the address of a DATA.

 

[cpjust]

I thought he wanted to copy a pointer into an object?

*p0 = p1;       /* alt2: error. type mismatch */

alternative2 is what i want to achieve, but the compiler complains and rejects it.

 

[chipperMDW]

I'm judging by this statement:

what i would like to do is store the address contained in a pointer variable at a target address pointed at by another pointer.

If I'm reading that correctly, I believe he expects [tt]*p0 = p1[/tt] to "store the address contained in a pointer variable [i.e. the value of p1] at a target address pointed at by another pointer [i.e. into *p0]."

If that's the case, then p0 needs to be a DATA** so it can contain the address of a pointer, as in:

p0 becomes a pointer to a pointer within my linked list

I may certainly be reading it incorrectly.

 

[denc4]

i mentioned that after p0=p0->Next, p0 is a pointer to a pointer, but now i think i was wrong.
all it did was copy the value of one pointer to another.