Simple random string generator in C

Question

It is just a simple piece of C code that generates random lenght random HEX srings. Any ideas how this code can be improved?

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define MAX_SIZE 256

int main() {
    time_t t;
    srand((unsigned) time(&t));

    const char* digits = "0123456789ABCDEF";

    unsigned char str_len = rand() % MAX_SIZE;


    char* str = malloc((str_len + 1) * sizeof(char));
    if (str == NULL) {
        fprintf(stderr, "malloc failed\n");
        return -1;
    }

    char* init_ptr = str;
    char tmp_char;

    for (int i = 0; i < str_len; i++) {
        if (i == 0) {
            do {
                tmp_char = *(digits + (rand() % 16));
            } while (tmp_char == '0');
        }
        *str = tmp_char;
        tmp_char = *(digits + (rand() % 16));
        str++;
    }

    *str = '\0';

    printf("%s", init_ptr);
    free(init_ptr);

    return 0;
}

Answer 1

You are effectively picking your first hex digit from "123456789ABCDEF" (no zero) and the subsequent digits from "0123456789ABCDEF". Hence it might be better to adjust the first random pick:

// This assumes that str_len is at least 1.
*str = *(digits + (1 + rand() % 15));  // Pick first char in range 1..15
str++;

// For loop starts at 1 as str[0] already picked.
for (int i = 1; i < str_len; i++) {
    *str = *(digits + (rand() % 16));  // Following chars in range 0..15
    str++;
}

That eliminates the tmp_char variable and the while loop, which makes the code a lot cleaner, to my eye anyway.

Answer 2

Looks pretty good although I think the if statement inside the for loop is redundant, i is always going to be 0 initially so you can just go straight into the do{} while()

Answer 3

Your do/while loop is redundant. You can simply generate a smaller random number and increment by 1:

tmp_char = *(digits + (rand() % 15) + 1);

Using malloc for this also seems like overkill. I'd simply declare a char array of size MAX_SIZE. The buffer is only used locally to the main function and is relatively small so can be declared safely on the stack.