UUID Generator — C source

/* RFC 4122 v4 UUID generator (cryptographically secure).
 *
 * Language: C (C11, POSIX)
 * CosmoDev polyglot showcase port of the `uuid` tool.
 * Ported from src/lib/uuid.ts — display source, part of CosmoDev's
 * polyglot tool pages.
 *
 * Produces a random (v4) UUID: 36 characters, lowercase, hyphenated as
 * 8-4-4-4-12 hex digits, with the RFC version (4) and variant (10xx)
 * bits set. Entropy comes from /dev/urandom — the host CSPRNG on every
 * POSIX system — so the "no entropy source" failure is an explicit
 * error path instead of a silent fallback.
 */

#include <stdint.h>
#include <stdio.h>

/* Fill buf with 16 bytes from the OS CSPRNG; returns 0 on success. */
static int random_bytes(uint8_t buf[16])
{
    FILE *f = fopen("/dev/urandom", "rb");
    if (f == NULL)
        return -1;
    size_t got = fread(buf, 1, 16, f);
    fclose(f);
    return got == 16 ? 0 : -1;
}

/* Write a fresh RFC 4122 version-4 UUID into out (36 chars, no NUL). */
static void uuid_v4(char out[36])
{
    uint8_t b[16];
    if (random_bytes(b) != 0) {
        snprintf(out, 36, "error: entropy source unavailable");
        return;
    }

    b[6] = (uint8_t)((b[6] & 0x0f) | 0x40); /* version 4: high nibble of byte 6 */
    b[8] = (uint8_t)((b[8] & 0x3f) | 0x80); /* variant  : high bits of byte 8  */

    /* Render lowercase hex, hyphenating at the 8-4-4-4-12 boundaries. */
    static const char hex[] = "0123456789abcdef";
    int p = 0;
    for (int i = 0; i < 16; i++) {
        if (i == 4 || i == 6 || i == 8 || i == 10)
            out[p++] = '-';
        out[p++] = hex[b[i] >> 4];
        out[p++] = hex[b[i] & 0x0f];
    }
}

int main(void)
{
    char id[37];
    uuid_v4(id);
    id[36] = '\0';
    puts(id);
    return 0;
}