Thread: Help allocating and reallocating memory

Hello there, long time visitor first time poster.

I have a problem with the following code... it segfaults as soon as my setFT function is called. I must not be allocating the memory correctly, but what's strange is my temp self-testing code (that **(ptrtwo+5000) = 88 part) actually prints out 88 when I ask it what that particular element is. How is that possible if I haven't allocated that much memory for it?

My goal is to create a pointer to a pointer to an array of longs. Initially, the array[0] element should be this "magic number" we have. That's fine. array[1] should be the size of our "flexible table" which is initialized at 0 until the user uses setFT to add elements.

The ultimate goal is have this dynamic array of elements that allocates, deallocated, and reallocates necessary memory. I just get seg faults

I guess my main question(s) is why am I seg faulting, and if I'm seg faulting, how come I can still put long's into my array?! Any suggestions or thoughts would be greatly appreciated. I'm not sure how clear I have been, but if you need clarification on exactly what the heck I'm talking about, I'll try to do just that. Thank you very much!

Code:

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

FlexTab newFT(){

	long **ptrone;
	long *ptrtwo;

	ptrone = malloc(sizeof(long));
	*ptrone = malloc(sizeof(long));

	**ptrone = (long)calloc(2, sizeof(long));
	ptrtwo = malloc(sizeof(long *));

	**(ptrone) = FT_MAGIC;

	ptrtwo = *ptrone;


	
	*(ptrtwo+1) = 0;
	
	*(ptrtwo+5000) = 88; //just a tester
	
	printf("*(ptrtwo+1) is %ld\n", *(ptrtwo+1));
	printf("**ptrone is %ld\n",**ptrone);
	printf("*ptrtwo is %ld\n",*ptrtwo);

	printf("Ptrtwo @ element 5000 is %ld\n", *(ptrtwo+5000)); //it works?!

	return ptrone;

	}

int disposeFT( FlexTab ft ){

		free(*ft);
		free(ft);
		ft = NULL;
		return 0;
	
}

int setFT( FlexTab ft, const int index, const long item ){

	if (index > (**(ft+1)) ){
		**ft = (long)realloc(*ft, (2 * sizeof(long)) + (index * sizeof(long)) );
		**(ft+1) = index;
		**(ft+index+2)=item;
		}
	else if (index < (**(ft+1)) ){
		**(ft+index+2)=item;
		}
		return **(ft+1);
}

int getFT( FlexTab ft, const int index, long *const itemp ){

	if (**ft == FT_MAGIC && *ft != NULL && ft != NULL){
	*itemp = **(ft + index + 2);
	return ( (sizeof(ft)) / (sizeof(long)) );
	}
	
	else if (index >= FT_MAX){
		return -2;
	}
	return 88;
}

ptrone is a 2 dimensional array of pointers to longs. The first dimension indicates the number of pointers to longs that you want -- for example is you want an array of 10 pointers then you could write it like this

Code:

long *ptrone[10];

The same thing using malloc() to allocate the 10 pointers looks like this

Code:

long **ptrone;
ptrone = malloc( 10 * sizeof(long*));

Once you have that, you can allocate each of the 10 arrays. For example, if each array element is a pointer to 15 longs

Code:

int i,j;

...
for(i = 0; i < 10; i++)
   ptrone[i] = malloc( 15 * sizeof(long) );

//or if you prefer to use calloc
for(i = 0; i < 10; i++)
   ptrone[i] = calloc( 15, sizeof(long) );

I'm supprised your compiler didn't complain about the return value of function newFT().