3

I want to move a pointer forward one byte. But I get this error:

lvalue required as increment operand

With this code:

int **test = 0;
((char *) *test)++;

But it is fine with this:

int **test = 0;
char **t2 = (char **) test;
(*t2)++;

How do I do the latter concisely?

donkon
  • 909
  • 9
  • 23
  • possible duplicate of [How to make a pointer increment by 1 byte, not 1 unit](http://stackoverflow.com/questions/16578054/how-to-make-a-pointer-increment-by-1-byte-not-1-unit) – Tacet Feb 28 '14 at 04:09

3 Answers3

3

If you want to increment the value pointed to by a double pointer:

(* (char **) test)++;

which means de-refrence the double pointer named "test" and then increment the value it is currently pointing to.

That will increment the value the pointer is pointing too, not advance the pointer itself. That's what I think you are trying to do in your sample code.

Rami
  • 7,162
  • 1
  • 22
  • 19
2

Try to use

(*(char **)test)++;
herohuyongtao
  • 49,413
  • 29
  • 133
  • 174
  • I tried it and it seems to work. So why is it necessary to first cast to the double pointer? – donkon Feb 28 '14 at 04:01
  • 1
    Simple explanation: You can't do `5++;`, or `(1+2)++;`, or `calculate_pi()++;`. You can only increment a variable (`variable++;`), or the value at an address (`(*something)++;`) or a field of a structure which is one of the above (`structure_variable.field++;` or `(*pointer_to_structure).field++;` or `pointer_to_structure->field++;`. Doing anything else is invalid. – user253751 Feb 28 '14 at 05:26
  • So, with your example: `(*something)++;` This means that dereferencing is ok, but dereferencing and then casting then incrementing is not? – donkon Mar 04 '14 at 16:55
0

Contrary to the accepted answer, I believe the only way to do this is:

int **test = 0;
test = (int **)((char *)test + 1);

Any expression with a dereference *, such as (*(char **)test)++, while it may compile will generally give a segmentation fault, because it is accessing memory location 0.

Joseph Quinsey
  • 9,553
  • 10
  • 54
  • 77