How can I safely and quickly extract digits from an int?

Question

We currently have some code to extract digits from an int, but I need to convert this to a platform without snprintf, and I am afraid of a buffer overrun. I have started to write my own portable (and optimized) snprintf but I was told to ask here in case someone had a better idea.

int extract_op(int instruction)
{ 
    char buffer[OP_LEN+1];
    snprintf(buffer, sizeof(buffer), "%0*u", OP_LEN, instruction);
    return (buffer[1] - 48) * 10 + buffer[0] - 48;
}

We are using C strings because Speed is very important.

Answer 1

Using sprintf should be fine. sizeof type * 3 * CHAR_BIT / 8 + 2 is a sufficiently large buffer for printing an integer of type type. You can simplify this expression if you assume CHAR_BIT is 8 or if you only care about unsigned formats. The basic idea behind it is that each byte contributes at most 3 digits in decimal (or octal), and you need space for a sign and null termination.