My own snprintf implementation in C

Question

I decided to make my own version of snprintf in C. I intentionally changed some things though. My version guarantees the buffer printed to will be null-terminated, and it returns the number of characters printed to the buffer, not the number that would have been printed if the buffer's size was not limited. And I only worried about some of the main formatting features, like %s, %c, %d, %h, and %H.

I would love to know what I could do better in this and improve, or what aspects of it I did or did not implement well.

#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>

int INT_TO_STR_DIGITS_L[16] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
int INT_TO_STR_DIGITS_U[16] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F' };

int int_to_str(int x, char *buf, size_t size, int base, int uppercase) {
    int length = (int)ceil(log((double)x)/log((double)base));
    int r, i = 0;
    char c;

    if (size < length) {
        x /= (int)pow(base, (float)(length - size));
        length = size;
    }

    do {
        if (i >= size) break;
        r = x % base;
        if (uppercase) {
            c = INT_TO_STR_DIGITS_U[r];
        } else {
            c = INT_TO_STR_DIGITS_L[r];
        }
        buf[length-i-1] = c;
        x /= base;
        i++;
    } while (x != 0);

    return i;
}

int my_snprintf(char *str, size_t max_size, const char *fmt, ...) {

    va_list arg_list;
    va_start(arg_list, fmt);

    int chars_printed = 0;
    char *start_str = str;

    char c, *str_arg;
    int num, len;
    int uppercase = 0, base = 10;

    for (int i = 0; fmt[i] != 0; i++) {
        if (max_size - chars_printed <= 0) {
            break;
        } else if (fmt[i] == '%') {
            i++;
            switch (fmt[i]) {
            case 'c':
                c = va_arg(arg_list, int);
                str[chars_printed++] = c;
                break;
            case '%':
                str[chars_printed++] = '%';
                break;
            case 's':
                str_arg = va_arg(arg_list, char *);
                len = strnlen(str_arg, max_size - chars_printed);
                strncpy(str+chars_printed, str_arg, len);
                chars_printed += len;
                break;
            case 'H':
                uppercase = 1;
            case 'h':
                base = 16;
            case 'd':
                num = va_arg(arg_list, int);
                len = int_to_str(num, str+chars_printed, max_size - chars_printed, base, uppercase);
                chars_printed += len;
                break;
            default:
                printf("Invalid format.\n");
                va_end(arg_list);
                return -1;
            }
        } else {
            str[chars_printed++] = fmt[i];
        }
    }

    if (chars_printed == max_size) chars_printed--;
    str[chars_printed] = 0;

    va_end(arg_list);
    return chars_printed;
}

Answer 1

Bug

Your int to string conversion isn't working correctly when the number being printed is an exact power of the base. Here is a program that demonstrates the bug:

int main(void)
{
    char buf[256];

    memset(buf, 'z', 256);
    my_snprintf(buf, 256, "abc%ddef", 1000);
    printf("%s\n", buf);
}

Expected output:

abc1000def

Actual output:

ab1000zdef

As you can see, the 1000 portion was written one too far to the left. The problem is that your number length computation is off by one for exact powers of the base.

Unnecessary and unsafe floating point operations

The Floating Point Police™ would like to point out that the use of floating point in int_to_str() is both unnecessary and dangerous. First of all, this line:

int length = (int)ceil(log((double)x)/log((double)base));

could be rewritten to use a loop to count the number of digits. By using floating point, you open yourself up to rounding errors. For example, if x were 125 and base were 5, you would expect length to be 3. However, when I ran the above code using 125 and 5 on my x86 machine, I got a length of 4 instead. This is because the division evaluated to something like 3.00000001 and ceil rounded it up to 4. (Of course there is already an unrelated off by one bug mentioned in the previous section. This floating point use is a separate cause of concern).

The same thing applies to this line:

    x /= (int)pow(base, (float)(length - size));

This could be rewritten to be a loop where you divide by base once per loop iteration. By using pow() and casting to int, you run the risk of the result of pow erroneously rounding down to the previous int.

Answer 2

1. Bug: int_to_str() fails for negative int.

2. Undefined specifiers: %h and %H are not part of the standard library. So without a specification, hard to know if they are performing correctly. Did you mean %x and %X?

3. Mixing int and size_t math. This is pedantic point. As the max of those 2 types are not specified to which is larger, there is a worst case chance max_size - chars_printed <= 0 will never be true should max_size > INT_MAX. Suggest adding the ** line below and avoid math that relies on signed math as it is likely unsigned math. or use size_t chars_printed and cope with returning int at the end. (chars_printed should be of the type with the greater positive range.)

   int my_snprintf(char *str, size_t max_size, const char *fmt, ...) {
     if (max_size > INT_MAX) Handle_PathologicalCase_TBD(); // **
     int chars_printed = 0;
       ...
       // if (max_size - chars_printed <= 0) {
       if (max_size <= chars_printed) {
   

4. Bug int length = (int)ceil(log((double)x)/log((double)base)); is not as reliable as hoped for. Detailed well in another answer. The alternative is to convert to a string with an internal max-sized buffer like char buf[34] for int32_t in base 2. Then copy the buffer result.

5. Style: fall through. Cases that drop though look like an error without a break. Add comment to show intent

       case 'H':
           uppercase = 1;
           // fall though
       case 'h':
           base = 16;
           // fall though
       case 'd':
   

6. printf("Invalid format.\n"); and such are better printed to stderr.

   fprintf(stderr, "Invalid format.\n");
   

7. Wrong type for len

   // int len;
   size_t len;
   

8. Style: No need to declare d so soon. Same for c. Suggest type change unsigned char c = va_arg(arg_list, int);. Same for str_arg.

   // int num;
   //  ... ~30 lines
   //    case 'd':
   //      num = va_arg(arg_list, int);
   
       case 'd':
           int num = va_arg(arg_list, int);
   

9. Minor: Code simplification

   // if (uppercase) {
   //     c = INT_TO_STR_DIGITS_U[r];
   // ...
   
   if (uppercase) {
       c = "0123456789ABCDEF"[r];
   } else {
       c = "0123456789abcdef"[r];
   }
   

10. Corner bug: Below code fails (UB). Watch for size == 0

    my_snprintf(str, 0, fmt, ...)
    

11. [Edit] Bug: int uppercase = 0, base = 10; is initialized outside the for() loop. So a "%d" after a "%x" will be treated as hex. A "%h" after a "%H" will be treated as print with uppercase letters. Simple fix, move int uppercase = 0, base = 10; to after else if (fmt[i] == '%'). Better fix: pass base and upper/lower as parameters to new int_to_str().

12. char *start_str unused. Recommend deletion.

13. Variables/functions like INT_TO_STR_DIGITS_L[] and int_to_str() that are only intended for local use should be static. Unclear why all uppercase. Avoid long line that exceed presentation width

    // int INT_TO_STR_DIGITS_L[16] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
    

    static int int_to_str_digits_l[16] = "0123456789abcdef";