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#1
May 30th, 2003, 11:51 AM
 kavi_s
Junior Member

Join Date: May 2003
Posts: 24
Time spent in forums: < 1 sec
Reputation Power: 0
realloc and pointers are driving me crazy!!

I have this code, and the problem is that every time I add a new element to my big list, i keep overwriting what I had before, and the tail never moves to the end of the list...........does anyone know what I am doing wrong?? Please help because I have looked at this thing too long, and I can't figure it out...it might be really simple.......thanks very much in advance...............

#include <stdlib.h>

typedef struct{
int num;
int val;
}blah;

void printBig(blah * start, blah * end, int size); //prints a list of blah variables
void printOne(blah T); //prints one blah varaible

int main()
{
blah * big=NULL; //head of list of blahs
blah * bigTail; //tail of list of blahs
blah * bigptr; //another pointer
int i=0;
int bigsize=0; //total size of big
int onesize=sizeof(blah); //size of one blah
blah temp;

while(i<3)
{
temp.num = i+10;
temp.val = i+10;
bigsize += onesize; //recalculate size of big
big = realloc(big, bigsize); //allocate that much space for the big

bigTail = big + bigsize - onesize; //make tail point to end of list - size of one
//blah
memcpy(bigTail, &temp, onesize); //copy this blah to list
i++;
}

printBig(big, bigTail, onesize); //print list
return 0;
}

void printBig(blah * start, blah * end, int size)
{
int i;
blah * tail = end;

printf("{ \n");
while(start!=end)
{
printOne(*start);
start+=size;
}

printf(" }\n");
}

void printOne(blah T)
{
int i;
printf("(%d, %d) ", T.num, T.val);
}

#2
May 30th, 2003, 02:06 PM
 3dfxMM
Contributing User

Join Date: Aug 2002
Posts: 272
Time spent in forums: 6 Days 17 h 57 m 24 sec
Reputation Power: 18
The problem lies in your pointer math. In the following line, you are actually adding (bigsize * sizeof (blah)) and then subtracting (onesize * sizeof (blah)). The compiler knows that you are working with a pointer to a blah so it assumes that increments and decrements of that pointer are in sizeof (blah) units.

Code:
`bigTail = big + bigsize - onesize; //make tail point to end of list - size of one `

try

Code:
`bigtail = big + (bigsize/onesize) - 1; //make tail point to end of list - size of one`

You will also need to make a similar change to the following line:

change
Code:
`bigtail += onesize;  // adjust tail`

to
Code:
`bigtail++;`

I almost forgot one. You don't need to pass the size into the printBig function. You can just change

Code:
`start += size;`

to

Code:
`start++;`

#3
May 30th, 2003, 03:53 PM
 kavi_s
Junior Member

Join Date: May 2003
Posts: 24
Time spent in forums: < 1 sec
Reputation Power: 0
Thank you soooo much

I don't know how to thank you enough.......Your explanation was wonderful! Hehehe, and I hate pointers to death.....thanks again, you have helped so much!

Kavi

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