#include <stdio.h>
#include <stdlib.h>
#include <openssl/rand.h>

#define IDLN 8
#define CHARSET \
    "abcdefghijklmnopqrstuvwxyz" \
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ" \
    "0123456789" \
    "-_!~"
#define CHARSETLN (sizeof(CHARSET) - 1)

char* gen() {
    char* id = malloc(IDLN + 1);
    if (id == NULL) {
        perror("malloc");
        exit(1);
    }

    unsigned char buf[IDLN];
    if (RAND_bytes(buf, IDLN) != 1) {
        perror("RAND_bytes");
        free(id);
        exit(1);
    }

    for (
        int i = 0;
        i < IDLN || (id[IDLN] = '\0');
        id[i++] = CHARSET[buf[i] % CHARSETLN]
    );

    return id;
}

int main(void) {
    char* id = gen();
    printf("%s", id);
    free(id);
}