Thread: Printf: somehow it changed a structure field.

Hello!

It may be possible that I made a stupid mistake somewhere in the code, but I just can't find it. I can bypass this problem but I'm afraid that it will arise in a different part of the program.

I'm using cygwin & gcc 3.4.4.

With the help of gdb I've managed to localize the problem to the following segment:

Code:

// Disregard some unimportant silliness - it is only for debugging purposes.
// maxcrd=36
// N=6

struct player
{
	int afd;
	char name[maxname];
	int ncards;
	int hand[maxcrd];
	int beat;
	int con;
	int out;
};

struct gamestate
{
	int started;
	int nplayers;
	struct player player[N+1];
	int current;
	int previous;
	int trump;
	int top;
	int deck[maxcrd];
	int field[2][maxcrd];
};


void printstate(gamestate *gs)
{
	printf("Gamestate;\n");
	printf("Cur: %d Prv: %d\n", gs->current, gs->previous);
	printf("Trump: %d", gs->trump);
	printf("\n");
	printf("Top: %d\n", gs->top);
	printf("\n");
	
	int i;
	for(i=0; i<6; i++)
	{
		if(gs->field[1][i]==0)
			printf("   | ");
		else
			printf("%s | ", cid(gs->field[1][i]));
	}
	printf("\n");
	for(i=0; i<6; i++)
	{
		if(gs->field[0][i]==0)
			printf("   | ");
		else
			printf("%s | ", cid(gs->field[0][i]));
	}
	printf("\n");
}

// ...

int main(void)
{
	gamestate *gs;
	gs=(gamestate *)malloc(sizeof(gs));
// ...
}

The situation's the following: there is a 'gamestate' struct. In a function (namely, 'printstate') using 'printf' I print out some data. At some point for a reason I can't yet understand the function changes the value of (gamestate *) 'gs->player[0].ncards' either to some close value or just to a random byte.

For example, if 'gs->player[0].ncards' is 9, at some line in the function (after several 'for' iterations) it turns into 545005600.

If I change the function as follows:

Code:

void printstate(gamestate *gs)
{
	printf("Gamestate;\n");
	printf("Cur: %d Prv: %d\n", gs->current, gs->previous);
	printf("Trump: %d ", gs->trump);
	printf("\n");
	printf("Top: %d\n", gs->top);
	printf("\n");
	
	
	int i;
	for(i=0; i<6; i++)
	{
		if(gs->field[1][i]==0)
			write(1, "   | ", strlen("   | "));//printf("   | ");
		else
			printf("%s | ", cid(gs->field[1][i]));
	}
	printf("\n");
	for(i=0; i<6; i++)
	{
		if(gs->field[0][i]==0)
			write(1, "   | ", strlen("   | "));//printf("   | ");
		else
			printf("%s | ", cid(gs->field[0][i]));
	}
	printf("\n");
}

...everything is fine, no sudden variable changes.

Please help me understand the issue, thank you in advance,

~Artemiy.

You're not allocating enough memory:

Code:

gamestate *gs;
gs=(gamestate *)malloc(sizeof(gs));

gs is a pointer, so applying sizeof to it yields the size of a pointer, which is likely 4. You instead want something like:

Code:

gs = malloc(sizeof *gs);

Originally Posted by cas

You're not allocating enough memory.
gs is a pointer, so applying sizeof to it yields the size of a pointer.

Thank you very much, that solved it! I didn't notice that I wrote the pointer name instead of the type name at all...

Again, thanks a lot,
~Artemiy.