Generating a random UUID in C

Question

How would I go about generating an entropy-based UUID in C and storing it as a string (char pointer)?

I'm hoping that there is an easy way to do this internally, but system("uuidgen -r") will work if not.

Answer 1

This functionality is provided by libuuid. (Packages libuuid1 and uuid-dev on Debian.)

This is a simple program that generates an entropy-based (random) UUID and writes it to stdout, then exits with status 0.

/* For malloc() */
#include <stdlib.h>
/* For puts()/printf() */
#include <stdio.h>
/* For uuid_generate() and uuid_unparse() */
#include <uuid/uuid.h>


/* Uncomment to always generate capital UUIDs. */
//#define capitaluuid true

/* Uncomment to always generate lower-case UUIDs. */
//#define lowercaseuuid true

/*
 * Don't uncomment either if you don't care (the case of the letters
 * in the 'unparsed' UUID will depend on your system's locale).
 */


int main(void) {
    uuid_t binuuid;
    /*
     * Generate a UUID. We're not done yet, though,
     * for the UUID generated is in binary format 
     * (hence the variable name). We must 'unparse' 
     * binuuid to get a usable 36-character string.
     */
    uuid_generate_random(binuuid);

    /*
     * uuid_unparse() doesn't allocate memory for itself, so do that with
     * malloc(). 37 is the length of a UUID (36 characters), plus '\0'.
     */
    char *uuid = malloc(37);

#ifdef capitaluuid
    /* Produces a UUID string at uuid consisting of capital letters. */
    uuid_unparse_upper(binuuid, uuid);
#elif lowercaseuuid
    /* Produces a UUID string at uuid consisting of lower-case letters. */
    uuid_unparse_lower(binuuid, uuid);
#else
    /*
     * Produces a UUID string at uuid consisting of letters
     * whose case depends on the system's locale.
     */
    uuid_unparse(binuuid, uuid);
#endif

    // Equivalent of printf("%s\n", uuid); - just my personal preference
    puts(uuid);

    return 0;
}

uuid_unparse() doesn't allocate it's own memory; to avoid a segmentation fault upon execution you must do that with manually with uuid = malloc(37); (you can also store the UUID in a char array of that length: char uuid[37];). Make sure to compile with -luuid so that the linker knows that uuid_generate_random() and uuid_unparse() are defined in libuuid.