Generating random values without time.h

Question

I want to generate random numbers repeatedly without using the time.h library. I saw another post regarding use the

srand(getpid()); 

however that doesn't seem to work for me getpid hasn't been declared. Is this because I'm missing the library for it? If it is I need to work out how to randomly generate numbers without using any other libraries than the ones I currently have.

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


int main(void) {
    int minute, hour, day, month, year;
    srand(getpid());
    minute = rand() % (59 + 1 - 0) + 0;
    hour = rand() % (23 + 1 - 0) + 0;
    day = rand() % (31 + 1 - 1) + 1;
    month = rand() % (12 + 1 - 1) + 1;
    year = 2018;

    printf("Transferred successfully at %02d:%02d on %02d/%02d/%d\n", hour, 
    minute, day, month, year);

    return 0;
}

NB: I can only use libraries <stdio.h> and <stdlib.h> and <string.h> — strict guidelines for an assignment.

Answer 1

getpid hasn't been declared.

No, because you haven't included the <unistd.h> header where it is declared (and according to your comment , you cannot use it, because you're restricted to using <stdlib.h> , <string.h> , and <stdio.h> ).

In that case, I would use something like

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

static int randomize_helper(FILE *in)
{
    unsigned int  seed;

    if (!in)
        return -1;

    if (fread(&seed, sizeof seed, 1, in) == 1) {
        fclose(in);
        srand(seed);
        return 0;
    }

    fclose(in);
    return -1;
}

static int randomize(void)
{
    if (!randomize_helper(fopen("/dev/urandom", "r")))
        return 0;
    if (!randomize_helper(fopen("/dev/arandom", "r")))
        return 0;
    if (!randomize_helper(fopen("/dev/random", "r")))
        return 0;

    /* Other randomness sources (binary format)? */

    /* No randomness sources found. */
    return -1;
}

and a simple main() to output some pseudorandom numbers:

int main(void)
{
    int i;

    if (randomize())
        fprintf(stderr, "Warning: Could not find any sources for randomness.\n");

    for (i = 0; i < 10; i++)
        printf("%d\n", rand());

    return EXIT_SUCCESS;
}

The /dev/urandom and /dev/random character devices are available in Linux, FreeBSD, macOS, iOS, Solaris, NetBSD, Tru64 Unix 5.1B, AIX 5.2, HP-UX 11i v2, and /dev/random and /dev/arandom on OpenBSD 5.1 and later.

As usual, it looks like Windows does not provide any such randomness sources: Windows C programs must use proprietary Microsoft interfaces instead.

The randomize_helper() returns nonzero if the input stream is NULL, or if it cannot read an unsigned int from it. If it can read an unsigned int from it, it is used to seed the standard pseudorandom number generator you can access using rand() (which returns an int between 0 and RAND_MAX , inclusive). In all cases, randomize_helper() closes non-NULL streams.

You can add other binary randomness sources to randomize() trivially.

If randomize() returns 0, rand() should return pseudorandom numbers. Otherwise, rand() will return the same default sequence of pseudorandom numbers. (They will still be "random", but the same sequence will occur every time you run the program. If randomize() returns 0, the sequence will be different every time you run the program.)

---

Most standard C rand() implementations are linear congruental pseudorandom number generators, often with poor choices of parameters, and as a result, are slowish, and not very "random".

For non-cryptographic work, I like to implement one of the Xorshift family of functions, originally by George Marsaglia. They are very, very fast, and reasonably random; they pass most of the statistical randomness tests like the diehard tests .

In OP's case, the xorwow generator could be used. According to current C standards, unsigned int is at least 32 bits, so we can use that as the generator type. Let's see what implementing one to replace the standard srand()/rand() would look like:

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

/* The Xorwow PRNG state. This must not be initialized to all zeros. */
static unsigned int  prng_state[5] = { 1, 2, 3, 4, 5 };

/* The Xorwow is a 32-bit linear-feedback shift generator. */
#define  PRNG_MAX  4294967295u

unsigned int  prng(void)
{
    unsigned int  s, t;

    t = prng_state[3] & PRNG_MAX;
    t ^= t >> 2;
    t ^= t << 1;
    prng_state[3] = prng_state[2];
    prng_state[2] = prng_state[1];
    prng_state[1] = prng_state[0];
    s = prng_state[0] & PRNG_MAX;
    t ^= s;
    t ^= (s << 4) & PRNG_MAX;
    prng_state[0] = t;
    prng_state[4] = (prng_state[4] + 362437) & PRNG_MAX;
    return (t + prng_state[4]) & PRNG_MAX;
}

static int prng_randomize_from(FILE *in)
{
    size_t        have = 0, n;
    unsigned int  seed[5] = { 0, 0, 0, 0, 0 };

    if (!in)
        return -1;

    while (have < 5) {
        n = fread(seed + have, sizeof seed[0], 5 - have, in);
        if (n > 0 && ((seed[0] | seed[1] | seed[2] | seed[3] | seed[4]) & PRNG_MAX) != 0) {
            have += n;
        } else {
            fclose(in);
            return -1;
        }
    }

    fclose(in);
    prng_seed[0] = seed[0] & PRNG_MAX;
    prng_seed[1] = seed[1] & PRNG_MAX;
    prng_seed[2] = seed[2] & PRNG_MAX;
    prng_seed[3] = seed[3] & PRNG_MAX;
    prng_seed[4] = seed[4] & PRNG_MAX;

    /* Note: We might wish to "churn" the pseudorandom
             number generator state, to call prng()
             a few hundred or thousand times. For example:
       for (n = 0; n < 1000; n++) prng();
             This way, even if the seed has clear structure,
             for example only some low bits set, we start
             with a PRNG state with set and clear bits well
             distributed.
    */

    return 0;
}

int prng_randomize(void)
{
    if (!prng_randomize_from(fopen("/dev/urandom", "r")))
        return 0;
    if (!prng_randomize_from(fopen("/dev/arandom", "r")))
        return 0;
    if (!prng_randomize_from(fopen("/dev/random", "r")))
        return 0;
    /* Other sources? */
    /* No randomness sources found. */
    return -1;
}

The corresponding main() to above would be

int main(void)
{
    int  i;

    if (prng_randomize())
        fprintf(stderr, "Warning: No randomness sources found!\n");

    for (i = 0; i < 10; i++)
        printf("%u\n", prng());

    return EXIT_SUCCESS;
}

Note that PRNG_MAX has a dual purpose. On one hand, it tells the maximum value prng() can return -- which is an unsigned int, not int like rand() . On the other hand, because it must be 2 ³² -1 = 4294967295, we also use it to ensure the temporary results when generating the next pseudorandom number in the sequence remain 32-bit. If the uint32_t type, declared in stdint.h or inttypes.h were available, we could use that and drop the masks ( & PRNG_MAX ).

Note that the prng_randomize_from() function is written so that it still works, even if the randomness source cannot provide all requested bytes at once, and returns a "short count" . Whether this occurs in practice is up to debate, but I prefer to be certain. Also note that it does not accept the state if it is all zeros, as that is the one single prohibited initial seed state for the Xorwow PRNG.

You can obviously use both srand() / rand() and prng() / prng_randomize() in the same program. I wrote them so that the Xorwow generator functions all start with prng.

Usually, I do put the PRNG implementation into a header file, so that I can easily test it (to verify it works) by writing a tiny test program; but also so that I can switch the PRNG implementation simply by switching to another header file. (In some cases, I put the PRNG state into a structure, and have the caller provide a pointer to the state, so that any number of PRNGs can be used concurrently, independently of each other.)

Answer 2

however that doesn't seem to work for me getpid hasn't been declared.

That's because you need to include the headers for getpid() :

   #include <sys/types.h>
   #include <unistd.h>

Another option is to use time() to seed (instead of getpid() ):

   srand((unsigned int)time(NULL));

Answer 3

As other answer pointed, you need to include the unistd.h header. If you don't want to do that then put the declaration of getpid() above main() . Read the manual page of getpid() here http://man7.org/linux/man-pages/man2/getpid.2.html

One approach may be

#include <stdio.h>
#include <stdlib.h>
pid_t getpid(void); /* put the declrataion of getpid(), if don't want to include the header */
int main(void) {

   /* .. some code .. */
   return 0;
} 

Or you can use time() like

srand((unsigned int)time(NULL));