From: "Philip Bock" <philipb AT escape DOT ca>
Newsgroups: comp.os.msdos.djgpp
Subject: Memory access error?
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Date: Mon, 08 Nov 1999 14:37:51 GMT
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

I have posted several questions about a program I am working on to this
group over the last few weeks, and I just wanted to thank you all for the
great help you've been! I finally got my program to compile without any
errors, which is a bit of a feat considering it's the first time I've tried
all this object-oriented stuff.

However, when I run it, it crashes with this error:

Shutting down Allegro
Exiting due to signal SIGSEGV
General Protection Fault at eip=000018e0
eax=65726168 ebx=00008394 ecx=00000000 edx=00000000 esi=00000054
edi=0005c6e8
ebp=000dc688 esp=000dc670 program=C:\DEVELOP\VECTOR2D\VECTOR.EXE
cs: sel=00a7  base=83949000  limit=000fffff
ds: sel=00af  base=83949000  limit=000fffff
es: sel=00af  base=83949000  limit=000fffff
fs: sel=00c7  base=00000000  limit=0010ffff
gs: sel=00c7  base=00000000  limit=0010ffff
ss: sel=00af  base=83949000  limit=000fffff
App stack: [000dc6e8..0005c6e8]  Exceptn stack: [0005c5c8..0005a688]

Call frame traceback EIPs:
  0x000018e0
  0x000015da
  0x000355ae

Am I correct in assuming this has something to do with my constructors, or
what? (Note: the program doesn't include proper destructors, meaning that
some allocated memory is not explicitly deallocated. I was saving that for
version 2.0 : )  )

Code:

#include <allegro.h>
#include <iostream.h>
#include <conio.h>
#include <stdlib.h>

struct coord
{
 int x, y;
};

struct vector
{
 coord c1, c2;
 int color;
};

struct shape
{
 int numvects;
 int stype;
 int sfill;
 vector **vects;
 shape(int newnumvects, int newstype, int newsfill);
};

struct vimg
{
 int numvectors, numshapes;
 vector *vectors;
 shape **shapes;
 vimg(int nnumv, int nnums, int vpers[]);
};

void program_init(char program_title[]);
void draw_vimg(vimg *vectimg, int x, int y, int xscale, int yscale);
vimg *gen_vimg(void);

int main(void)
{
 program_init("Vector Graphics Rendering Demo");
 getch();
 set_gfx_mode(GFX_VESA1, 640, 480, 0, 0);

 vimg *mygraphic;
 mygraphic = gen_vimg();
 draw_vimg(mygraphic, 200, 100, 1, 1);

 getch();

 set_gfx_mode(GFX_TEXT, 0, 0, 0, 0);
}

void program_init(char program_title[])
{
 clrscr();

 textcolor(BLACK);
 textbackground(LIGHTGRAY);
 for(int c = 1; c != 81; c++)
 {
  gotoxy(c, 1);
  cprintf(" ");
 }
 for(int x = 0; program_title[x] != '\0'; x++)
 gotoxy(40 - (x / 2), 1);
 cout << program_title << '\n';
 textcolor(LIGHTGRAY);
 textbackground(BLACK);

 cout << "Initializing Allegro library...\n";
 allegro_init();

 cout << "Setting up timer...\n";
 install_timer();

 cout << "Initializing mouse routines...";
 if (install_mouse() == -1)
  cout << "failed\n";
 else
  cout << '\n';
 cout << "Detected Operating System: ";

 if (os_type == OSTYPE_UNKNOWN)
  cout << "MS-DOS\n";
 if (os_type == OSTYPE_WIN95)
  cout << "Windows 95\n";
 if (os_type == OSTYPE_DOSEMU)
  cout << "Linux DOSEMU\n";
}

void draw_vimg(vimg *vectimg, int x, int y, int xscale, int yscale)
{
 for(int a = 0; a != vectimg->numshapes; a++)
 {
  file://Code to draw one shape's outline
  for(int b = 0; b != vectimg->shapes[a]->numvects; b++)
   line(screen, (vectimg->shapes[a]->vects[b]->c1.x*xscale)+x,
(vectimg->shapes[a]->vects[b]->c1.y*yscale)+y,
(vectimg->shapes[a]->vects[b]->c2.x*xscale)+x,
(vectimg->shapes[a]->vects[b]->c2.y*yscale)+y,
vectimg->shapes[a]->vects[b]->color);
  file://Code to fill shape goes here.
 }
}

vimg *gen_vimg(void)
{
 int vpers[]={6};
 vimg newvimg(6, 1, vpers);

 newvimg.vectors[0].c1.x = 50;
 newvimg.vectors[0].c1.y = 1;
 newvimg.vectors[0].c2.x = 100;
 newvimg.vectors[0].c2.y = 1;
 newvimg.vectors[0].color = 1;
 newvimg.vectors[1].c1.x = 100;
 newvimg.vectors[1].c1.y = 1;
 newvimg.vectors[1].c2.x = 150;
 newvimg.vectors[1].c2.y = 50;
 newvimg.vectors[1].color = 1;
 newvimg.vectors[2].c1.x = 150;
 newvimg.vectors[2].c1.y = 50;
 newvimg.vectors[2].c2.x = 100;
 newvimg.vectors[2].c2.y = 100;
 newvimg.vectors[2].color = 1;
 newvimg.vectors[3].c1.x = 100;
 newvimg.vectors[3].c1.y = 100;
 newvimg.vectors[3].c2.x = 50;
 newvimg.vectors[3].c2.y = 100;
 newvimg.vectors[3].color = 1;
 newvimg.vectors[4].c1.x = 50;
 newvimg.vectors[4].c1.y = 100;
 newvimg.vectors[4].c2.x = 1;
 newvimg.vectors[4].c2.y = 50;
 newvimg.vectors[4].color = 1;
 newvimg.vectors[5].c1.x = 1;
 newvimg.vectors[5].c1.y = 50;
 newvimg.vectors[5].c2.x = 50;
 newvimg.vectors[5].c2.y = 1;
 newvimg.vectors[5].color = 1;

 newvimg.shapes[0]->vects[0] = &newvimg.vectors[0];
 newvimg.shapes[0]->vects[1] = &newvimg.vectors[1];
 newvimg.shapes[0]->vects[2] = &newvimg.vectors[2];
 newvimg.shapes[0]->vects[3] = &newvimg.vectors[3];
 newvimg.shapes[0]->vects[4] = &newvimg.vectors[4];
 newvimg.shapes[0]->vects[5] = &newvimg.vectors[5];
}

shape::shape(int newnumvects, int newstype, int newsfill)
{
 numvects = newnumvects;
 stype = newstype;
 sfill = newsfill;
 vects = new vector *[newnumvects];
}

vimg::vimg(int nnumv, int nnums, int vpers[])
{
 numvectors = nnumv;
 numshapes = nnums;
 vectors = new vector [nnumv];
 shapes = new shape *[nnums];
 for (int c = 0; c != nnums; c++)
  shapes[c] = new shape(vpers[c], 0, 0);
}