Move pointer and realloc, C

Question

I'm trying to code a buffer for an input file. The Buffer should always contain a defined amount of data. If a few bytes of the data were used, the buffer should read data from the file until it has the defined size again.

const int bufsize = 10;
int *field = malloc(bufsize*sizeof(int)); //allocate the amount of memory the buffer should contain
for(i=0;i<bufsize;++i) //initialize memory with something
    *(field+i) = i*2; 

field += 4; //Move pointer 4 units because the first 4 units were used and are no longer needed

field= realloc(field,bufsize*sizeof(int)); //resize the now smaller buffer to its original size
//...some more code were the new memory (field[6]-field[9]) are filled again...

Here is a short example of how I'm trying to do it at the moment (without files, because this is the part thats not working), but the realloc() always returns NULL. In this example, the first 4 units were used, so the pointer should move forward and the missing data at the end of the memory (so that it will again contain 10 elements) should be allocated. What am I doing wrong?

I would be very thankful if someone could help me

Answer 1

You need memmove() instead

memmove(field, field + 4, (bufsize - 4) * sizeof(*field));

you don't need to realloc() because you are not changing the size of the buffer, just think about it.

1. If you do this

   field += 4;
   

   now you lost the reference to the begining of field so you can't even call free on it, nor realloc() of course. Read WhozCraig comment for instance.

2. Doing realloc() for the same size doesn't make that much sense.

3. Using realloc() the way you do causes some other problems, for instance when it fails you also run into the same problem, you loose reference to the original pointer.

   So the recommended method is

   void *pointer;
   pointer = realloc(oldPointer, oldSize + nonZeroNewSize);
   if (pointer == NULL)
       handleFailure_PerhapsFree_oldPointer();
   oldPointer = pointer;
   

So the title of your question contains the answer to it, what you need is to move the data from offset 4 * sizeof(int) bytes to the begining of the pointer, for which memmove() is the perfect tool, notice that you could also think of using memcpy() but memcpy() cannot handle the case of overlapping data, which is your case.