Segmentation Fault in C++ With for Loop [closed] - c++

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I am making an Ant simulation with SDL2 and when I run my code it soon crashed giving me a segmentation fault. I believe this is from trying to access negative values in an array. I tried to make it so the Ant can't do this by giving an if statement checking its value on the "grid" I made out of squares. I added an if statement on line 420 to try and stop my theory but it didn't do anything. Does anyone see the issue here?
My Grid Initialiation:
struct Wall
{
bool IsAlive;
int Health = 10;
SDL_Rect Rect;
bool isAnt;
bool IsHome;
bool IsFood;
};
This is where I set up the rects:
int count = 0;
for (int i = 0; i < 1440; i += 4)
{
for (int j = 0; j < 795; j += 4)
{
SDL_Rect Rect = {i, j, 4, 4};
wall[count].Rect = Rect;
wall[count].IsAlive = true;
if (i < 10)
{
if (j < 10)
{
wall[count].IsAlive = false;
}
}
count++;
}
}
And this is where I do the Ant Digging:
void Break(int AntID, int SurroundingSqr /*Clockwise*/)
{
if (SurroundingSqr == -199)
{
if (A[AntID].PosID > 100)
{
wall[A[AntID].PosID + SurroundingSqr].Health--;
if (wall[A[AntID].PosID + SurroundingSqr].Health == 0)
{
wall[A[AntID].PosID + SurroundingSqr].IsAlive = false;
}
}
}
else
{
wall[A[AntID].PosID + SurroundingSqr].Health--;
if (wall[A[AntID].PosID + SurroundingSqr].Health == 0)
{
wall[A[AntID].PosID + SurroundingSqr].IsAlive = false;
}
}
}
If you want to see all the code you can go to this Gist

Answer 1

You should include all the headers you need. You are missing #include <ctime>.
You have undefined behavior because of indices running out of bounds, as can be seen when running your program compiled with -g -fsanitize=address,undefined:
ant.cpp:350:34: runtime error: index -199 out of bounds for type 'Wall [572397]'
ant.cpp:356:32: runtime error: index -1 out of bounds for type 'Wall [572397]'
=================================================================
==3989611==ERROR: AddressSanitizer: global-buffer-overflow on address 0x00000042fae4 at pc 0x000000405540 bp 0x7ffe896fa2e0 sp 0x7ffe896fa2d8
READ of size 1 at 0x00000042fae4 thread T0
#0 0x40553f in MoveLeft(int) /home/ted/proj/stackoverflow/ant.cpp:356
#1 0x407f87 in MakeAntActions() /home/ted/proj/stackoverflow/ant.cpp:213
#2 0x402687 in main /home/ted/proj/stackoverflow/ant.cpp:102
#3 0x7ff7a0671b74 in __libc_start_main (/lib64/libc.so.6+0x27b74)
#4 0x40293d in _start (/home/ted/proj/stackoverflow/ant-g+++0x40293d)
I would start nesting out the problem by following the call chain from line
102: MakeAntActions(); ➔
213: MoveLeft(i); ➔
350 and 356 (where it croaks)
if (wall[A[AntID].PosID - 199].IsAlive) // line 350 index -199
if (wall[A[AntID].PosID - 1].IsAlive) // line 356 index -1
So, A[AntID].PosID is 0 at the time the program reaches these lines which creates these negative indices - and negative indices are not allowed.
Compile with -g -fsanitize=address,undefined to get hints like this.