Fast memory copy and realloc()

**intmail** · 10-27-2006

Hello,

Could you tell me how to build a grid of data that can be update very fast.
For instance I would like to cut the top of the grid and move up all the rest
of the data. I do not use memcpy() because it wast time.

I give bellow a sample of program and it gives me an "segmentation fault".

What is wrong in my code ?

Thanks.

Code:

// The goal of this program is to remove
// the head of an int array structure. At first the structure have 10 lines of int
// , at end it must have 9 lines.
//The old location  [0] will be filled by value of [1] and so on.
// This works under windows/Watcom compiler not for linux GCC/GLIBC. Why ?

#include <stdio.h>
#include <stdlib.h>

void **data;
void *temp;

int main(void)
{
 data  = (void**) malloc(10);
 data[0]= (int*)malloc(sizeof(int)); 
 data[1]= (int*)malloc(sizeof(int)); 
 //****************** Set data ***********
 data[0]=100; data[1]=200;
 //*********** Cut the head of array ***** 
 temp=data[1];
 data[0]= (int*) realloc(temp, sizeof(int));
	
 printf("%d\n", data[0]); // I expect to obtain 200 here as result not 100.

 return(0);
}

**vart** · 10-27-2006

Code:

 
data  = (void**) malloc(10);//why do you need 10 bytes of memory there is no pointer that can take 5 bytes ;)
 
//data can be NULL - why not check it before using this pointer?

 data[0]= (int*)malloc(sizeof(int)); //data[0] is now a pointer to the allocated memory
//and can be NULL also...

 //****************** Set data ***********
 data[0]=100; //and now your pointer is equal to 100 good address, but do you know what is stored there?
//and what about your allocated memory - you get a memory leak
 
data[1]=200;
 //*********** Cut the head of array ***** 
 temp=data[1];
 data[0]= (int*) realloc(temp, sizeof(int));//realloc awaits a pointer allocated by malloc
//but you give 200 - maybe its a good address ;) but it obviously was not allocated by
//malloc...
//there are other issues - but you should fix these first...

**jim mcnamara** · 10-27-2006

And why are you so sure memcpy wastes time? Since your code can't possibly work, how did you profile it to find out that it's faster than memcpy?

Don't assume something is faster or slower until your test it. Make it work first, worry about fast later on.

**Salem** · 10-27-2006

> This works under windows/Watcom compiler not for linux GCC/GLIBC. Why ?
Luck?

Almost every line of your program has issues.

> I do not use memcpy() because it wast time.
What do you think realloc will do if it decides to move the block?

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