c++ - int 到 unsigned int 的无效转换

Question

此代码是为作业提供的。但是，马上就出现了计算 pi 蒙特卡洛函数中的错误。从 int 到 unsigned int 的转换错误统计信息无效。我不确定这是否有意为之，但我无法更正此问题。如果您想知道，此函数是线程示例的一部分。非常感谢任何建议

void *compute_pi( void *s )   
{
int seed;
int ii;
int *hit_pointer;
int local_hits;
double rand_no_x;
double rand_no_y;

hit_pointer = (int *) s;
seed = *hit_pointer;
local_hits = 0;

for( ii=0; ii < sample_points_per_thread; ii++ )
{
  rand_no_x = (double) (rand_r( &seed ))/(double)RAND_MAX;//*error is these lines
  rand_no_y = (double) (rand_r( &seed ))/(double)RAND_MAX;//*error is these lines
  if(((rand_no_x - 0.5) * (rand_no_x - 0.5) +
  (rand_no_y - 0.5) * (rand_no_y - 0.5)) < 0.25)//*error 
local_hits++;
  seed *= ii;
 }

*hit_pointer = local_hits;
 pthread_exit(0); 
 }

我做了一些调试，现在看看错误可能在哪里。我现在发布调用 compute pi 的主要函数。我相信这是将参数解析为样本点和线程数的地方。当我运行该程序时，它要求输入，但由于段错误而失败。有什么建议吗？

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

  #define MAX_THREADS 512

  void *compute_pi( void * );


  int sample_points;
  int total_hits;
  int total_misses;
  int hits[ MAX_THREADS ];
  int sample_points_per_thread;
  int num_threads;



  int main( int argc, char *argv[] )
  {
  /* local variables */
  int ii;
  int retval;
  pthread_t p_threads[MAX_THREADS];
  pthread_attr_t attr;
  double computed_pi;

  /* initialize local variables */
  retval = 0;


  pthread_attr_init( &attr );
  pthread_attr_setscope( &attr, PTHREAD_SCOPE_SYSTEM );

  printf( "Enter number of sample points: " );
  scanf( "%d", &sample_points );
  printf( "Enter number of threads: " );
  scanf( "%d%", &num_threads );

  /* parse command line arguments into sample points and number of threads */
  /* there is no error checking here!!!!! */
  sample_points = atoi(argv[1]);
  num_threads = atoi(argv[2]);


  total_hits = 0;
  sample_points_per_thread = sample_points / num_threads;

  for( ii=0; ii<num_threads; ii++ )
  {
  hits[ii] = ii;
  pthread_create( &p_threads[ ii ], &attr, compute_pi, (void *) &hits[ii] );
  }

  for( ii=0; ii<num_threads; ii++ )
  {
   pthread_join( p_threads[ ii ], NULL );
   total_hits += hits[ ii ];
    }

   computed_pi = 4.0 * (double) total_hits / ((double) (sample_points));


   printf( "Computed PI = %lf\n", computed_pi );


   /* return to calling environment */
   return( retval );
   }

Answer 1

来自rand_r(3) man page :

int
rand_r(unsigned *ctx);

如您所见，它需要一个 unsigned 指针 - 如果您将一个 int 传递给它并且您在编译器中打开了适当的警告/错误(并且它似乎你这样做了)，你会得到那个错误。更改 seed 的类型，您将被设置。