c - 複数のプロセスで同時に乱数を生成するにはどうすればよいですか？

Question

まったく同時に実行されている複数のプロセスの乱数をCで生成するにはどうすればよいですか？

srand使用したかったrandのですが、方法がわかりません（プロセスIDを使用しているのでしょうか？）。

Answer 1

たとえば、プロセス ID に基づいて、プロセスごとに異なるシードを使用できます。

srand(getpid());

そして、ただ使用しますrand()。

Answer 2

After years and years of weird errors from poor random seeds, I eventually wrote a code to get random numbers from /dev/urandom, which are the same random numbers used for SSL. In my code I used these "good" random numbers to seed the C stdlib rand() function, which was sufficent for my task as long as it got a good seed:

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

//HOW MANY BITS IN A CHAR? THIS MACRO IS USUALLY DEFINED
#ifndef CHAR_BIT
#define CHAR_BIT 8
#endif

//PATH TO RANDOM NUMBER GENERATOR
const char inPath[]="/dev/urandom";

//EXTRACTS A PSEUDORANDOM unsigned long FROM THE OPERATING SYSTEM VIA /dev/urandom
unsigned long get_urandom(){

  //OPEN INPUT STREAM
  FILE *inFile;
  inFile=fopen(inPath,"r");

  //FAILED TO OPEN STREAM
  if(inFile==NULL){
    fprintf(stderr, "# Failed to open random device %s\n", inPath);
    return 0;
  }

  //HOW MANY BITS IN A CHAR? HOW MANY CHARS IN AN UNSIGNED LONG?
  const int bitsInChar  = CHAR_BIT;
  const int charsInLong = sizeof(unsigned long) / sizeof(char);

  //GET RANDOM UNSIGNED LONG ONE CHARACTER AT A TIME
  unsigned long randomSeed=0;
  for(int i=0; i<charsInLong; i++)
    randomSeed |= fgetc(inFile) << i*bitsInChar;

  //CLOSE STREAM AND RETURN
  fclose(inFile);
  return randomSeed;

}

int main(){

  // We can use the above function directly to get our random numbers, but this is slow.
  printf("We can get random numbers from the OS via get_urandom: %u\n", get_urandom());  

  // Or we can just call get_urandom once, to use as a seed for the stdlib rand() function.
  unsigned int randomSeed=get_urandom();
  srand(randomSeed);
  printf("Or, using a random seed from get_urandom: %u\n", randomSeed);
  printf("We can get random numbers from stdlib, e.g.: %u\n", rand());
}

Here's the same code again with the function rewritten in C++:

#include <fstream>
using namespace std;

//HOW MANY BITS IN A CHAR? THIS MACRO IS USUALLY DEFINED
#ifndef CHAR_BIT
#define CHAR_BIT 8
#endif

//EXTRACTS A PSEUDORANDOM unsigned long FROM THE OPERATING SYSTEM VIA /dev/urandom
unsigned long get_urandom(const char inPath[]="/dev/urandom"){

  //OPEN INPUT STREAM
  ifstream inFile(inPath);

  //FAILED TO OPEN STREAM
  if(inFile.fail()){
    fprintf(stderr, "# Failed to open random device %s\n", inPath);
    return 0;
  }

  //HOW MANY BITS IN A CHAR? HOW MANY CHARS IN AN UNSIGNED LONG?
  const int bitsInChar  = CHAR_BIT;
  const int charsInLong = sizeof(unsigned long) / sizeof(char);

  //GET RANDOM UNSIGNED LONG ONE CHARACTER AT A TIME
  unsigned long randomSeed=0;
  for(int i=0; i<charsInLong; i++)
    randomSeed |= inFile.get() << i*bitsInChar;

  //CLOSE STREAM AND RETURN
  inFile.close();
  return randomSeed;

}

int main(){

  // We can use the above function directly to get our random numbers, but this is slow.
  printf("We can get random numbers from the OS via get_urandom: %u\n", get_urandom());  

  // Or we can just call get_urandom once, to use as a seed for the stdlib rand() function.
  unsigned int randomSeed=get_urandom();
  srand(randomSeed);
  printf("Or, using a random seed from get_urandom: %u\n", randomSeed);
  printf("We can get random numbers from stdlib, e.g.: %u\n", rand());
}