sprintf fails spontaneously depending on what printf and NSLog calls there are.

Question

Hello I have a bizarre problem with sprintf. Here's my code:

void draw_number(int number,int height,int xpos,int ypos){
    char string_buffer[5]; //5000 is the maximum score, hence 4 characters plus null character equals 5
    printf("Number - %i\n",number);
    sprintf(string_buffer,"%i",number); //Get string
    printf("String - %s\n",string_buffer);
    int y_down = ypos + height;
    for (int x = 0; x < 5; x++) {
        char character = string_buffer[x];
        if(character == NULL){ //Blank characters occur at the end of the number from spintf. Testing with NULL works
            break;
        }
        int x_left = xpos+height*x;
        int x_right = x_left+height;
        GLfloat vertices[] = {x_left,ypos,x_right,ypos,x_left,y_down,x_right,y_down};
        rectangle2d(vertices, number_textures[atoi(strcat(&character,"\0"))], full_texture_texcoords);
    }
}

With the printf calls there, the numbers are printed successfully and the numbers are drawn as expected. When I take them away, I can't view the output and compare it, of-course, but the numbers aren't rendering correctly. I assume sprintf breaks somehow.

This also happens with NSLog. Adding NSLog's anywhere in the program can either break or fix the function.

What on earth is going on?

This is using Objective-C with the iOS 4 SDK.

Thank you for any answer.

Answer 1

Well this bit of code is definately odd

char character = string_buffer[x]; 
...
... strcat(&character,"\0") ...

Originally I was thinking that depending on when there happens to be a NUL terminator on the stack this will clober some peice of memory, and could be causing your problems. However, since you're appending the empty string I don't think it will have any effect.

Perhaps the contents of the stack actually contain numbers that atoi is interpretting?Either way I suggest you fix that and see if it solves your issue.

As to how to fix it Georg Fritzsche beat me to it.

Answer 2

With strcat(&character,"\0") you are trying to use a single character as a character array. This will probably result in atoi() returning completely different values from what you're expecting (as you have no null-termination) or simply crash.

To fix the original approach, you could use proper a zero-terminated string:

char number[] = { string_buffer[x], '\0' };
// ...
... number_textures[atoi(number)] ...

But even easier would be to simply use the following:

... number_textures[character - '0'] ...

Answer 3

Don't use NULL to compare against a character, use '\0' since it's a character you're looking for. Also, your code comment sounds surprised, of course a '\0' will occur at the end of the string, that is how C terminates strings.

If your number is ever larger than 9999, you will have a buffer overflow which can cause unpredicable effects.

Answer 4

When you have that kind of problem, instantly think stack or heap corruption. You should dynamically allocate your buffer with enough size- having it as a fixed size is BEGGING for this kind of trouble. Because you don't check that the number is within the max- if you ever had another bug that caused it to be above the max, you'd get this problem here.