Invalid lvalue in assignment error when trying to make a pointer NULL

Question

I have a pointer of a structure type that I made. On program start it begins as NULL and I then malloc/realloc as I need to add/remove these structures and I was just gonna use my pointer to point at the first structure and move through it like an array.

When I malloc/realloc I always make the size of the "array"/area in memory one larger than it needs to be. I do this so I can set the "last index"/area in memory to NULL so I can say something like while (pointer != NULL).

I get the error: invalid lvalue in assignment when I try to assign NULL to the last position in the array/area of memory with the lines:

  // Realloc remotelist by adding one to connrhosts
  connrhosts++;
  remotelist = realloc(remotelist, sizeof(rhost)*(connrhosts + 1));
  (remotelist + connrhosts) = NULL;

What I think I am saying is:

• Its time to add a new structure to my array so I will increase connrhosts by one.
• Realloc the memory that is pointed at remotelist to a new area of memory that is the size of connrhosts (how many structures I will use) as well as one additional space so I can make it NULL
• Point remotelist to the new area of memory
• Use my pointer remotelist and add the offset connrhosts which will now point to the last index of the area of memory and make that pointer NULL.

As far as I can tell (or feel) I did everything correctly, but I have been working on this project for sometime now and am under the impression I have tunnel vision. I would love to have a fresh set of eyes take a look at my logic/code and let me know what they think and what I did wrong. Thanks again. :D

Edit - Part of my problem is I think I have a misunderstanding of what I can do with pointers.

Here is my structure:

typedef struct {
  char address[128]; // Buffer that holds our address of the remote host
  int port; // Port of the remote host
  int conn; // FD to the connection of our remote host
  int ofiles; // Open files associated with the remote host
} rhost;

What I was hoping I could do was loop through my array/area of memory and say if its not NULL then do something with it. So my original loop statement is while (NULL != remotelist). Now I believe are reading responses and comments that this logic is wrong because I am checking if a pointer is null? I ought to be checking if the area of memory/structure that the pointer is pointing is null? If this is the case it ought to be something like while (NULL != *(remotelist + someoffset))?

I am doing it this way as my teacher suggested it/talked about it in class.

My initial declaration/initialization of remotelist was: rhost *remotelist = NULL;

Answer 1

Errornous lvalue assignments occur when the LHS is an evaluated expression that does not become a variable that can be assigned. What you're doing looks like an operation (pointer arithmetic) which should be on the RHS.

What you can do is:

remotelist[connrhosts] = NULL;  // array notation asuming 
                                // remotelist is an array of pointers

assuming connrhosts is a int or size_t or you could do:

remotelist += connrhost; // pointer arithmetic
*remotelist = NULL; // assuming remotelist is an array of pointers.

Answer 2

You also need to dereference your pointer.

*(remotelist + connrhosts) = NULL;

Although I think

 remotelist[connrhosts] = NULL; 

is clearer.