What is the most correct way to generate random numbers in C with pthread

Question

I have several threads running concurrently and each of them must generate random numbers. I want to understand if there is a pattern to follow, to understand if it is corre

Answer 1

rand_r is thread-safe but also is reentrant.

Code below generates uint128_t pseuso-random numbers using xorshift algorithm.

Additional properties:

• shared-reentrant
• lock-free
• thread-safe
• ultrafast
• seeded from two variant sources of enthropy

uintx_types.h:

#ifndef UINTX_TYPES_H_INCLUDED
#define UINTX_TYPES_H_INCLUDED

#include 
#include 

typedef __uint128_t     uint128_t;
typedef __uint64_t      uint64_t;

#define UINT128_C(hi, lo)   (((uint128_t)(hi) << 64) | (uint128_t)(lo))
#define UINT128_MIN         UINT128_C( 0x0000000000000000, 0x0000000000000000 )
#define UINT128_0           UINT128_MIN
#define UINT128_MAX         (~(UINT128_0) - 1) 

#endif // UINTX_TYPES_H_INCLUDED

lf.h:

#ifndef LF_H_INCLUDED
#define LF_H_INCLUDED

#define AAF(ADDR, VAL)          __sync_add_and_fetch((ADDR), (VAL))

#endif // LF_H_INCLUDED

rand.h:

#ifndef RAND_H_INCLUDED
#define RAND_H_INCLUDED

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

#include "lf.h"
#include "uintx_types.h"


#define URANDOM     "/dev/random"   

void        srand_init(void);
uint128_t   rand_range_128(uint128_t min, uint128_t max);

#endif // RAND_H_INCLUDED

rand.c:

#include "rand.h"

uint64_t    r[2];

uint64_t xorshift64star(int index) 
{   
    uint64_t    x;

    x = r[index];
    x ^= x >> 12; // a
    x ^= x << 25; // b
    x ^= x >> 27; // c
    x = x * UINT64_C(2685821657736338717);
    return AAF(&r[index], x);
}

void srand_init(void)
{
    struct timespec ts;
    size_t          nbytes;
    ssize_t         bytes_read;
    int             fd;

    clock_gettime(CLOCK_REALTIME, &ts);
    r[0] = (uint64_t)(ts.tv_sec * 1.0e9 + ts.tv_nsec);
    xorshift64star(0);

    if ((fd = open(URANDOM, O_RDONLY, S_IRUSR | S_IRGRP | S_IROTH)) == -1)
    {
        r[1] = r[0] + 1;
        xorshift64star(1);
    }
    else
    {
        nbytes = sizeof(r[1]);
        bytes_read = read(fd, &r[1], nbytes);
        if ((bytes_read == 0) || (r[1] == 0ull))
        {
            r[1] = r[0] + 1;
            xorshift64star(1);
        }
        close(fd);
    }
}

uint64_t rand_64(void)
{
    return xorshift64star(0);
}

uint128_t rand_128(void) 
{
    uint128_t       r;

    r = xorshift64star(0);
    r = (r << 64) | xorshift64star(1);
    return r;
}


uint128_t rand_range_128(uint128_t min, uint128_t max)
{
    return (rand_128() % (max+1-min))+min;
}

test.c:

#define KEYS 1000

int main(int argc, char **argv)
{
    int             i;
    uint128_t       key;

    srand_init();

    for(i = 0; i <= KEYS; i++)
    {
        key = rand_range_128(UINT128_MIN, UINT128_MAX);
        printf("%016"PRIx64"%016"PRIx64"\n", (uint64_t)(key >> 64), (uint64_t)key);

    }
    return 0;
}

Compile with gcc(4.9.2) under Linux.