Question

我尝试使用pthreads库在C for Linux中创建一个多线程应用程序，该库使用带有N + 1个项的无限级数来近似pi。从命令行传递可变N和T.我正在使用Pi的Nilakantha近似公式。 N是求和的数字序列的上限，T是计算该总和的子线程的数量。例如，如果我运行命令＆＃34; ./ pie 100 4＆＃34;。父线程将创建4个索引为0到3的子线程。我有一个名为vsum的全局变量，它是一个使用malloc动态分配的双数组来保存值。所以用4个线程和100作为上限。我的程序应该计算：

 Thread 0 computes the partial sum for i going from 0 to 24 stored to an element vsum[0]
 Thread 1 computes the partial sum for i going from 25 to 49 stored to an element vsum[1] 
 Thread 2 computes the partial sum for i going from 50 to 74 stored to an element vsum[2] 
 Thread 3 computes the partial sum for i going from 75 to 99 stored to an element vsum[3]

每个线程进行计算后。主线程将通过将所有数字从vsum [0]加到vsum [T-1]来计算总和。

我刚刚开始了解线程和进程。任何帮助或建议将不胜感激。谢谢。

到目前为止我编写的代码：

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


double *vsum;
int N, T;

void *PI(void *sum) //takes param sum and gets close to pi
{
  int upper = (int)sum;
  double pi = 0;
  int k = 1;

  for (int i = (N/T)*upper; i <= (N/T)*(upper+1)-1; i++)
  {

    pi += k*4/((2*i)*(2*i+1)*(2*i+2));

    if(i = (N/T)*(upper+1)-1)
    { 
      vsum[upper] = pi;
    }
   k++;
  }
 pthread_exit(0);
}





int main(int argc, char*argv[])
{
     T = atoi(argv[2]);
     N = atoi(argv[1]);
  if (N<T)
   {
     fprintf(stderr, "Upper bound(N) < # of threads(T)\n");
     return -1;
   }
     int pie = 0;
     pthread_t tid[T]; //thread identifier
     pthread_attr_t attr; //thread attributes
     vsum = (double *)malloc(sizeof(double));//creates dyn arr
//Initialize vsum to [0,0...0]

  for (int i = 0; i < T; i++){
  {
   vsum[i] = 0;
  }




  if(argc!=2) //command line does not give proper # of values
  {
     fprintf(stderr, "usage: commandline error <integer values>\n");
     return -1;
  }
  if (atoi(argv[1]) <0) //if its is negative/sum error
  {
      fprintf(stderr, "%d must be >=0\n", atoi(argv[1]));
      return -1;
  }

//CREATE A LOOP THAT MAKES PARAM N #OF THREADS 

      pthread_attr_init(&attr);

   for(int j =0; j < T;j++)
   {
     int from = (N/T)*j;
     int to = (N/T)*(j+1)-1;
   //CREATE ARRAY VSUM TO HOLD VALUES FOR PI APPROX. 
      pthread_create(&tid[j],&attr,PI,(void *)j);
 printf("Thread %d computes the partial sum for i going from %d to %d stored to an element vsum[%d]\n", j, from, to, j);

   }
  //WAITS FOR THREADS TO FINISH
   for(int j =0; j <T; i++)
   {
     pthread_join(tid[j], NULL);
   }

  //LOOP TO ADD ALL THE  vsum array values to get pi approximation
   for(int i = 0; i < T; i++)
   {
     pie += vsum[i];
   }

    pie = pie +3;

   printf("pi computed with %d terms in %d threads is %d\n",N,T,pie);


   vsum = realloc(vsum, 0);
   pthread_exit(NULL);

 return 0;
}

这是错误我没看到我参加我的计划：我在这里缺少什么？

            ^
pie.c:102:1: error: expected declaration or statement at end of input
 }

当我尝试运行我的程序时，我得到以下内容：

./pie.c: line 6: double: command not found
./pie.c: line 7: int: command not found
./pie.c: line 8: int: command not found
./pie.c: line 10: syntax error near unexpected token `('
./pie.c: line 10: `void *PI(void *sum) //takes param sum and gets close to pi'

Answer 1

我没有查看代码的逻辑，但我看到了以下编程错误。

更改

pthread_create(&tid[j],&attr,PI,j);

到

pthread_create(&tid[j],&attr,PI,(void *)j);

pthread_create()将第4个参数作为void *传递给线程函数。

同时修复线程函数PI以将传递的参数用作int，如

void *PI(void *sum) //takes param sum and gets close to pi
{
  int upper = (int)sum; //don't use `atoi` as passed param is int.
  ... 
  //your existing code
}

第3个错误是行

realloc(vsum, 0);

通过传递0重新分配，您实际上只是释放vsum，因此您可以使用free(vsum)。如果你确实想重新分配，你应该使用函数返回的新分配的内存，如vsum = realloc(vsum, 0);

Answer 2

pthread的语法是

pthread_create（threadId，threadAttribute，callingMethodName，调用方法的参数）;

例如：

void printLetter( void *p)
{
int i=0;
char c=(char *)p;
while (i<10000)
{
 printf("%c",c);
}
}

int main()
{

pthread_t thread_id;
char c='x';
pthread_create (&thread_id, NULL, &printLetter, &c);
pthread_join (thread_id, NULL);
return 0;
}
}

多线程pthread错误

2 个答案: