是random_shuffle线程安全吗？如果不是，使用rand_r [英] Is random_shuffle threadsafe? and using rand_r if it is not

问题描述

是std :: random_shuffle线程安全吗？我认为不是因为正常的rand（）不是线程安全的。如果是这样，我如何使用rand_r与random_shuffle，以便我可以给每个线程一个唯一的种子。我已经看过使用random_shuffle自定义随机生成器的例子，但是对我来说还不清楚。

谢谢。

rand_r 与 std :: random_shuffle ，您可以使用<需要写一个（相当微不足道的）包装器。传递给 random_shuffle 的随机数生成器需要接受指定要生成的数字范围的参数， rand_r 不会。

您的包装器看起来像这样：

  class rand_x {
 unsigned int seed; 
 public：
 rand_x（int init）：seed（ini​​t）{} 
 
 int operator（）（int limit）{
 int divisor = RAND_MAX / +1）; 
 int retval; 
 
 do {
 retval = rand_r（& seed）/ divisor; 
} while（retval> limit）; 
 
 return retval; 
} 
}; 
  

您可以使用 random_shuffle 例如：

  std :: random_shuffle（whatever.begin（），whatever.end（），rand_x（some_seed））; 
  

Is std::random_shuffle threadsafe? I presume not since the regular rand() is not threadsafe. If that is the case, how would I use rand_r with random_shuffle so that I can give each thread a unique seed. I've seen examples of using custom random generators with random_shuffle, but it is still unclear to me.

Thanks.

解决方案

To use rand_r with std::random_shuffle, you'll need to write a (fairly trivial) wrapper. The random number generator you pass to random_shuffle needs to accept a parameter that specifies the range of numbers to be produced, which rand_r does not.

Your wrapper would look something like this:

class rand_x { 
    unsigned int seed;
public:
    rand_x(int init) : seed(init) {}

    int operator()(int limit) {
        int divisor = RAND_MAX/(limit+1);
        int retval;

        do { 
            retval = rand_r(&seed) / divisor;
        } while (retval > limit);

        return retval;
    }        
};

You'd use it with random_shuffle something like:

std::random_shuffle(whatever.begin(), whatever.end(), rand_x(some_seed));

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