Help memcpy!

Hi all, this is my first post and i'm a beginner in c programming,
i need help to understand what i'm doing wrong.

I'm not getting the expected result:

Code:

---

...
size_t size = sizeof(uint32_t);
...
void *ret = malloc(size * width * height);
if (ret == NULL) return 1;
...
memcpy(ret + (y * width * size) + x, data + (y * width * size) + w, size);
...

---

i'm getting the expected result through, with this piece of code:

Code:

---

...
size_t size = sizeof(uint32_t);
...
void *ret = malloc(size * width * height);
if (ret == NULL) return 1;
...
((uint32_t *)ret)[y * width + x] = ((uint32_t *)data)[y * width + w];
...

---

but as the size is dynamic, i can't use this, and i really need something like the fist case.

Thanks in advance, and sorry if my english is not the best.

What are the results in first case?

Quote:

---

Originally Posted by Kozay Novel

Code:

---

...
memcpy(ret + (y * width * size) + x, data + (y * width * size) + w, size);
...

---

i'm getting the expected result through, with this piece of code:

Code:

---

...
((uint32_t *)ret)[y * width + x] = ((uint32_t *)data)[y * width + w];
...

---

---

Your offset is different between these two cases.For example in 1st case you write (y * width * size) + x but if you compare it with the second one,i believe that you wish to write (y * width + x )* size

My guess is (y * width + x ) * size is closer to [y * width + x].

Tim S.

> memcpy(ret + (y * width * size) + x, data + (y * width * size) + w, size);
Well this shouldn't even compile, since you can't do arithmetic on a void pointer.

It you make ret like this
unsigned char *ret;

then the address calculation you have should work with
(y * width + x ) * size

Thanks std10093 and stahta01,
Sorry but the in the original, i copy wrongly from a commentary in my code (as i try many offsets).

it should be:

Code:

---

...
size_t size = sizeof(uint32_t);
...
void *ret = malloc(size * width * height);
if (ret == NULL) return 1;
...
memcpy(ret + (y * width) + x, data + (y * width) + w, size);
...

---

and:

Code:

---

...
size_t size = sizeof(uint32_t);
...
void *ret = malloc(size * width * height);
if (ret == NULL) return 1;
...
((uint32_t *)ret)[y * width + x] = ((uint32_t *)data)[y * width + w];
...

---

and still does not work.

Thank Salem, it works now, but actually, it works with void* pointers, as it is to represent any type, that is why i needed to use as its is a dynamic array that can be uint32_t, uint64_t, or (sizeof(uint8_t)*3).

Code:

---

...
size_t size = sizeof(uint32_t);
...
void *ret = malloc(size * width * height);
if (ret == NULL) return 1;
...
memcpy(ret + (y * width + x) * size, data + (y * width + w) * size, size);
...

---

It might work with your compiler (today), but the fact remains that you can't rely on void* pointer arithmetic.
Try increasing the warning levels on your compiler.

$ gcc -Wall -Wextra -ansi -pedantic foo.c
foo.c: In function ‘main’:
foo.c:11:13: warning: pointer of type ‘void *’ used in arithmetic [-pedantic]

Thanks Salem, i corrected my code now, through i normally use C99, it's a good call to remember to use, if possible, C89/90.

Quote:

---

Originally Posted by Kozay Novel

through i normally use C99, it's a good call to remember to use, if possible, C89/90.

---

C99 does not define pointer arithmetic with void* either.