Quickly generate a stream of random data

pcg-random.c

#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <time.h>
#include <pthread.h>

// see https://github.com/imneme/pcg-c
#include "pcg_variants.h"

// INEXACT output; if we are interrupted the buffer if not flushed.
static char out_buf[2][1024*1024];
static int out_cur = 0;
static volatile int wbuf = 0;

pthread_barrier_t barrier;

static void *writer(void *x) {
   (void) x;

   while (1) {
      pthread_barrier_wait(&barrier); // wait 1, sync for buffer ready
      pthread_barrier_wait(&barrier); // wait 2, wbuf is being updated
      write(STDOUT_FILENO, out_buf[wbuf^1], sizeof(out_buf[0]));
   }

   return NULL;
}

static inline void out8(void *b) {
   if (__builtin_expect(out_cur == sizeof(out_buf[0]), 0)) {
      pthread_barrier_wait(&barrier); // wait 1
      wbuf ^= 1;
      out_cur = 0;
      pthread_barrier_wait(&barrier); // wait 2
   }
   memcpy(out_buf[wbuf]+out_cur, b, 8);
   out_cur += 8;
}

int main(int argc, char **argv) {
   (void) argc;

   pthread_t th;

   pthread_barrier_init(&barrier, NULL, 2);
   pthread_create(&th, NULL, writer, NULL);
   pthread_detach(th);

   char *end = NULL;
   uint64_t seed = 0x853c49e6748fea9bULL;
   
   if (argv[1]) {
      seed = strtoull(argv[1], &end, 10);

      if (end)
         seed = 0x853c49e6748fea9bULL;
   }

   pcg64_random_t r[1];
   pcg64_srandom_r(r, seed, 1);

   while (1) {
      uint64_t i = pcg64_random_r(r);
      out8(&i);
   }

   return 0;
}