Newsgroups: comp.os.msdos.djgpp
From: m <manuel DOT desbonnet AT removethis DOT ul DOT ie>
Subject: Re: C Newbie - Pointers to Structures Problem
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References: <01bd582b$a4841700$81efd4cf AT gregbl DOT ihug DOT co DOT nz>
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Date: Thu, 26 Mar 1998 15:47:59 GMT
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See program below...

manuel.

Greg Bell wrote:

> Hi,
>
> I'm a newbie to this newsgroup and have just started programming in C/C++.
> At the moment I am learning how to use pointers. I want to write a small
> test application which draws a line from point 1 to point 2. Now I have the
> algorithm for doing that worked out but I am using a small structure to
> hold a new type which consists of two integers x and y (the coordinates of
> the point). I have written a swap routine which uses pointers to allow me
> to swap two sets of points over. The problem is that although the program
> (included below) works it exits from TC3 with a Null Pointer Assignment
> error and from DJGPP with a rather cryptic SIGSEV error which includes
> register values and other such stuff. I use TC3 at the technical institute
> that I attend and DJGPP at home (they are both quite similar esp. when
> using RHIDE). Anyway if anyone can enlighten me it would be much
> appreciated.
>
> Thanks in advance
>
> Greg Bell
>
> Program Follows:
>
> #include <stdlib.h>
> #include <stdio.h>
> #include <conio.h>
>
> typedef struct co_ordinates
> {
>     int x, y;
> }
> POINT;
>
> typedef struct co_ordinates *POINTPTR;
>
> POINT a,b,c,d;
> POINT point_difference();
> void swap_points();
>
> void main(void)
> {
>    clrscr();
>    a.x = 1;
>    a.y = 1;
>    b.x = 5;
>    b.y = 5;
>    c = point_difference(a,b);
>    printf("point a is at %d, %d\n",a.x,a.y);
>    printf("point b is at %d, %d\n",b.x,b.y);
>    printf("the difference between point a and point b is %d,
> %d\n",c.x,c.y);
>    printf("Now I will swap the points\n");
>    swap_points (&a,&b); //Send the addresses of points a and b to
> swap_points
>    printf("point a is now at %d, %d\n",a.x,a.y);
>    printf("point b is now at %d, %d\n",b.x,b.y);
>    c = point_difference(a,b);
>    printf("the difference between point a and point b is now %d,
> %d\n",c.x,c.y);
>
> }
>
> POINT point_difference(POINT a, POINT b)
> {
>    POINT c;
>         c.x = a.x - b.x;
>    c.y = a.y - b.y;
>    return(c);
> }
>
> void swap_points(POINTPTR a, POINTPTR b) //Assign pointer types to a & b
> {

// As you have it temp is an uninitialised pointer, so when you// use it (ie.
temp->x = a->x) you are writing to a random?
// memory location which gives you a segmentation fault.
// The way to do it is to make temp a POINT rather than a POINTPTR.
//    POINT temp;
//    temp.x = a->x; temp.y = a->y;
//    ...

>    POINTPTR temp; //Create a temporary address holder
>    temp->x = a->x; //Shift point a to temp
>    temp->y = a->y;
>    a->x = b->x; //Shift point b to a
>    a->y = b->y;
>    b->x = temp->x; //Shift temp to point b
>    b->y = temp->y;
> }