Read Access Violation in C, trying to find the source - c

Question

guys. I've got a project where I'm given a number of chairs, a number of parasols, and the locations of the chairs. I think have to find the optimal width of all parasols (all equal to each other) to cover each chair. I crafted this bit of code that works for the sample cases in the assignment file (it's messy, but it works):
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
void search();
int* constructParasols();
int check();
void draw();
int main(void)
{
int n, k, totalSize;
scanf("%d", &n);
printf("\n");
scanf("%d", &k);
printf("\n");
int* chairs = (int*)calloc(n + 1, sizeof(int));
for (int i = 0; i < n; i++)
{
scanf("%d", &chairs[i]);
}
printf("\n");
totalSize = chairs[n - 1];
search(n, k, chairs, totalSize, 0, totalSize/k);
free(chairs);
chairs = NULL;
system("pause");
}
void search(int n, int k, int* chairs, int totalSize, int low, int hi)
{
int mid = (low + hi)/2;
int tempPos = 0;
if (low >= hi - 1)
{
if (check(n, k, low, chairs, constructParasols(k,totalSize,low), k - 2, totalSize - (low * 2), totalSize) == 1)
{
printf("width %d is optimal\n", low);
}
else if (check(n, k, hi, chairs, constructParasols(k, totalSize, hi), k - 2, totalSize - (low * 2), totalSize) == 1)
{
printf("width%d is optimal\n", hi);
}
else
{
printf("no solution found\n");
}
}
else if (check(n, k, mid, chairs, constructParasols(k, totalSize, mid), k - 2, totalSize - (mid * 2), totalSize) == 1)
{
search(n, k, chairs, totalSize, low, mid-1);
}
else
{
search(n, k, chairs, totalSize, mid+1, hi);
}
}
int* constructParasols(int k, int totalSize, int width)
{
int* parasols = (int*)calloc(k + 1, sizeof(int));
int tempPos = 0;
for (int i = 0; i < k - 1; i++)
{
parasols[i] = tempPos;
tempPos += width;
}
parasols[k - 1] = totalSize - width;
return parasols;
}
int check(int n, int k, int width, int* chairs, int* parasols, int pNum, int maxPos, int totalSize)
{
int* cover = (int*)calloc(totalSize + 1, sizeof(int));
int failed = 0;
for (int i = 0; i < totalSize; i++)
{
cover[i] = 0;
}
if (k<=2)
{
draw(n, k, width, chairs, parasols, totalSize);
for (int i = 0; i < width; i++)
{
cover[i] = 1;
if(k==2) cover[parasols[1] + i] = 1;
}
for (int i = 0; i < n; i++)
{
if (cover[chairs[i] - 1] == 0)
{
failed = 1;
}
}
if (failed == 0)
{
free(cover);
cover = NULL;
free(parasols);
parasols = NULL;
return 1;
}
else
{
free(cover);
cover = NULL;
free(parasols);
parasols = NULL;
return 0;
}
}
draw(n, k, width, chairs, parasols, totalSize);
for (int i = 0; i < k; i++)
{
for (int j = 0; j < width; j++)
{
cover[parasols[i] + j] = 1;
}
}
for (int i = 0; i < n; i++)
{
if (cover[chairs[i]-1] == 0)
{
failed = 1;
}
}
if (failed == 0)
{
free(cover);
cover = NULL;
free(parasols);
parasols = NULL;
return 1;
}
if (pNum <= 1 && parasols[pNum] == maxPos)
{
free(cover);
cover = NULL;
free(parasols);
parasols = NULL;
return 0;
}
else if (parasols[pNum] == maxPos)
{
free(cover);
cover = NULL;
return check(n, k, width, chairs, parasols, pNum - 1, maxPos - width, totalSize);
}
else
{
free(cover);
cover = NULL;
parasols[pNum]++;
return check(n, k, width, chairs, parasols, pNum, maxPos, totalSize);
}
}
void draw(int n, int k, int width, int* chairs, int* parasols, int totalSize)
{
int* dCover = (int*)calloc(totalSize, sizeof(int));
int* chairPos = (int*)calloc(totalSize, sizeof(int));
for (int i = 0; i < totalSize; i++)
{
dCover[i] = 0;
chairPos[i] = 0;
}
for (int i = 0; i < k; i++)
{
for (int j = 0; j < width; j++)
{
if (dCover[parasols[i] + j] >= 1) dCover[parasols[i] + j] = 2;
else dCover[parasols[i] + j] = 1;
}
}
for (int i = 0; i < n; i++)
{
chairPos[chairs[i]-1] = 1;
}
for (int i = 0; i <= totalSize; i++)
{
if (dCover[i] == 2)
{
printf("=");
}
else if (dCover[i] == 1)
{
printf("-");
}
else
{
printf(" ");
}
}
printf("\n");
for (int i = 0; i <= totalSize; i++)
{
if (chairPos[i] == 1)
{
printf("|");
}
else
{
printf(" ");
}
}
printf("\n");
free(dCover);
dCover = NULL;
free(chairPos);
chairPos = NULL;
return;
}
(removed some comments and debug prints for clarity and reduced size)
However, these were only two samples shown out of the ten he will attempt. I want to get them all correct because I'm a perfectionist about programming, and I noticed a blind spot in the way I check if the cover works. Right now, it'll take the second parasol from the right and drag it space by space and check each time. Once it touches the parasol to the right of it, it'll start on the next parasol to the left, and do this until just the leftmost is remaining and conclude that this configuration won't succeed.
This means that if I give a sample input, such as 8 4 1 3 6 9 12 15 17 20,
it'll fail to check the situation in which the parasols are a width of 4 and evenly spaced out. I set out to rectify this, by changing this:
else
{
free(cover);
cover = NULL;
parasols[pNum]++;
return check(n, k, width, chairs, parasols, pNum, maxPos, totalSize);
}
(which is where it tries to push the parasol to the left and try again) to this:
else
{
printf("freeing cover\n");
free(cover);
cover = NULL;
printf("iteration failed\n");
//parasols[pNum]++;
//return check(n, k, width, chairs, parasols, pNum, maxPos, totalSize);
if (parasols[pNum] != parasols[pNum - 1] + width && check(n, k, width, chairs, parasols, pNum - 1, parasols[pNum]-width, totalSize) == 1)
{
return 1;
}
else
{
parasols[pNum]++;
return check(n, k, width, chairs, parasols, pNum, maxPos, totalSize);
}
}
Seemed to make sense. Only when I run it now, I get a read access violation. Right here:
else dCover[parasols[i] + j] = 1;
dCover. Under the draw command.
It'll grab the parasol and move it to the right, check it, then grab the parasol to the left of that and move it to the right. Just as I expected. It's only after
that in which it throws the exception.
If I revert the code, everything works perfectly. I have no idea how that relates to this - they're two different pointers entirely. I'm using Visual Studio 2017 to build this, if that makes a difference. Can someone help me out?

Answer 1

When you free already freed pointer you get this kind of crush.
You free parasols in check and continue to use it without reconstruct it.
in this part of your code:
if (parasols[pNum] != parasols[pNum - 1] + width && check(n, k, width, chairs, parasols, pNum - 1, parasols[pNum]-width, totalSize) == 1)
{
return 1;
}
else
{
parasols[pNum]++;
return check(n, k, width, chairs, parasols, pNum, maxPos, totalSize);
}
check was failed you free(parasols) and returned 0, so you get into the else and used it again in parasols[pNum]++ and also in the return check(... without reconstruct the parasols.
The program crash in the second free but may crush also when accesing the freed pointer.