Completely stumped by stack smash.
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  1. #1
    Join Date
    Nov 2004
    Location
    Lincoln, NE
    Posts
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    [Solved] Completely stumped by stack smash.

    For some reason, I'm getting a segmentation fault when main() exits with the following code:

    Code:
    #include <iostream>
    #include <cstring>
    #include <ctime>
    #include <cstdlib>
    #include <unistd.h>
    
    using std::cout;
    using std::endl;
    using std::flush;
    using std::fill;
    
    void moveTortoise(int *);
    void moveHare(int *);
    void race(char *);
    void checkCollision(int *,int * ,char *);
    bool isWinner(int *, int *);
    
    const int TICK_MS = 10000;
    const int TRACK_LENGTH = 70;
    const int FINISH = TRACK_LENGTH - 1;
    const char HARE_CHAR = 'H';
    const char TORTOISE_CHAR = 'T';
    
    int main() {
    	char track[TRACK_LENGTH + 1] = {0};
    
    	srand(time(0));
    
    	std::fill(track, track + FINISH, 45);
    
    	track[0] = HARE_CHAR;
    	cout << "BANG !!!!!\nAND THEY'RE OFF !!!!!" << endl;
    	race(track);
    	return 0;
    }
    
    void race(char *track) {
    	int hare_position = 0;
    	int tortoise_position = 0;
    
    	 while (!isWinner(&hare_position, &tortoise_position)) {
    		usleep(TICK_MS);
    		track[tortoise_position] = '-';
    		track[hare_position] = '-';
    		moveTortoise(&tortoise_position);
    		moveHare(&hare_position);
    		track[tortoise_position] = TORTOISE_CHAR;
    		track[hare_position] = HARE_CHAR;
    		checkCollision(&hare_position, &tortoise_position, track);
    		cout << "\r" << track << flush;
    	}
    }
    
    void moveTortoise(int *position) {
    	int move  = 0;
    	move = 1 + rand() % 10;
    	move <= 5 ? *position += 3 : (move <= 7 ? *position -= 6 : *position++);
    	*position = *position > 0 ? (*position > FINISH ? FINISH : *position) : 0;
    }
    
    void moveHare(int *position) {
    	int move = 1 + rand() % 10;
    	move == 1 ? *position -= 12 : (move <= 3 ? *position -= 2 : (move <= 6 ? *position++ : (move <= 8 ? *position += 9 : 0)));
    	*position = *position > 0 ? (*position > FINISH ? FINISH : *position) : 0;
    }
    
    void checkCollision(int *hare, int *tortoise, char *track) {
    	static int start = 0;
    
    	if (start > 0)	{
    	   std::fill(track + start, track + start + 7, 45);
    		start = 0;
    	}
    
    	if (*hare == *tortoise) {
    		start = *hare > 15 ? *hare - 8 : *hare + 2;
    		strncpy(track + start, "OUCH!!!", 7);
    	}
    }
    
    bool isWinner(int *hare, int *tortoise) {
    	if ((*hare == FINISH) && (*tortoise == FINISH)) {
    		cout << endl << "It's a tie." << endl;
    		return true;
    	}
    	if (*hare == FINISH) {
    		cout << endl << "Hare wins. Yuch." << endl;
    		return true;
    	}
    	if (*tortoise == FINISH) {
    		cout << endl << "TORTOISE WINS!!! YAY!!!" << endl;
    		return true;
    	}
    	return false;
    }
    I can't find anywhere that I would be overwriting the array bound, and everything runs perfectly until the program attempts to exit. Valgrind reports:
    Code:
    ==5972== Invalid read of size 8
    ==5972==    at 0x400A69: main (in /[path]/race)
    ==5972==  Address 0x6fffffff8 is not stack'd, malloc'd or (recently) free'd
    ==5972== 
    ==5972== 
    ==5972== Process terminating with default action of signal 11 (SIGSEGV)
    ==5972==  Access not within mapped region at address 0x6FFFFFFF8
    ==5972==    at 0x400A69: main (in /[path]/race)
    ==5972==  If you believe this happened as a result of a stack
    ==5972==  overflow in your program's main thread (unlikely but
    ==5972==  possible), you can try to increase the size of the
    ==5972==  main thread stack using the --main-stacksize= flag.
    ==5972==  The main thread stack size used in this run was 8388608.
    ==5972== 
    ==5972== HEAP SUMMARY:
    ==5972==     in use at exit: 0 bytes in 0 blocks
    ==5972==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
    ==5972== 
    ==5972== All heap blocks were freed -- no leaks are possible
    ==5972== 
    ==5972== For counts of detected and suppressed errors, rerun with: -v
    ==5972== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 2 from 2)
    Segmentation fault (core dumped)
    I realize that using vectors instead of pointers would be the definative solution, but I'm a noob taking a C++ class and my assignment calls for use of char arrays and pointer passing. My environment is Linux Mint 13 using g++ 4.6.3-1 with default compile options (g++ file.cpp). I suspect I might be passing a bad array to cout, in that when I display the array positions returned by moveTortoise() and moveHare(), it doesn't seg fault on exit:
    Code:
                    //SNIP
    		moveTortoise(&tortoise_position);
    		moveHare(&hare_position);
    		//Adding the line below equals no segmentation fault.
    		cout << tortoise_position << " " << hare_position;
    		track[tortoise_position] = TORTOISE_CHAR;
    		track[hare_position] = HARE_CHAR;
                    //SNIP
    Any ideas?
    Last edited by Comintern; February 10th, 2013 at 11:07 AM. Reason: Mark as solved.

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