c - 段错误而未更改我的程序

Question

我试图编写一个使用线程计算素数的程序，该程序正在运行，但未给出期望的结果(它告诉所有数字都是素数)。今天，我尝试再次运行该程序，即使我没有更改程序，也遇到了段错误。我尝试使用gdb查找发生的时间，并且我认为它在pthread_join函数中发生。

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

int n_threads; // number of threads
int* numbers; // array of numbers
int elements_per_thread;

int isPrime(int n){
    int i = 0;
    for(i=0; i < n; i++)
        if(n%i == 0)
            return 0;
    return 1;
}

void * threadPrime(void * arg){
    int* idxp = (int*)arg;
    int idx = *idxp;
    int start = idx* elements_per_thread;
    int finish = (idx+1)*elements_per_thread-1;
    int i;
        for(i=start; i<finish; i++)
            if(!isPrime(i))
                numbers[i]=0;
    pthread_exit(NULL);
}

int main(int argc, char* argv[]) {

  if (argc != 3) {
    printf("usage: %s largest_number number_threads\n", argv[0]);
    return 1;
  }

  int largest_number = atoi(argv[1]); // value of the largest number to test for primality
  int n_numbers = largest_number-1; // number of numbers to test
  n_threads = atoi(argv[2]);

  // create and fill vector of numbers to test  
  numbers = (int *)malloc(n_numbers*sizeof(int)) ;  // allocate a vector for n_numbers integers

  int i;
  for (i = 2; i <= largest_number; i++) 
    numbers[i-2] = i;

  int* id = (int *)malloc(n_threads*sizeof(int));
  // compute primes

 pthread_t* thid = (pthread_t *)malloc(n_threads*sizeof(int));
 for(i=0;i<n_threads;i++){
    id[i] = i;
    if(pthread_create(&thid[i],NULL,threadPrime,(void*)(id+i)) != 0){
       printf("Erro\n");
        exit(0);
    }
 }

 for(i=0;i<n_threads;i++){
    if(pthread_join(thid[i],NULL) != 0){
        printf("Erro\n");
        exit(0);
    }
 }

  // print result 
   printf("Primes:");
  int n_primes = 0;
  for (i = 0; i < n_numbers; i++) 
     if (numbers[i]) {
        n_primes++;
        printf (" %d", numbers[i]);
     }
  printf("\nTotal: %d primes\n", n_primes);

  return 0;         
}

问题解决了。更正下面的代码。

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

int n_threads; // number of threads
int* numbers; // array of numbers
int elements_per_thread;

int isPrime(int n){
    int i = 0;
    for(i=2; i < n; i++)
        if(n%i == 0)
            return 0;
    return 1;
}

void * threadPrime(void * arg){
    int* idxp = (int*) arg;
    int idx = *idxp;
    int start = idx*elements_per_thread;
    int finish = (idx+1)*elements_per_thread;
    int i;
        for(i=start; i<=finish; i++)
            if(!isPrime(numbers[i]))
                numbers[i]=0;
    pthread_exit(NULL);
}

int main(int argc, char* argv[]) {

 if (argc != 3) {
    printf("usage: %s largest_number number_threads\n", argv[0]);
    return 1;
  }

  int largest_number = atoi(argv[1]); // value of the largest number to test for primality
  int n_numbers = largest_number-1; // number of numbers to test
  n_threads = atoi(argv[2]);

  // create and fill vector of numbers to test  
  numbers = (int *)malloc(n_numbers*sizeof(int)) ;  // allocate a vector for n_numbers integers

  int i;
  for (i = 2; i <= largest_number; i++) 
    numbers[i-2] = i;

  int* id;
  id = (int *)malloc(n_threads*sizeof(int));

  // compute primeselements_per_thread = n_numbers/n_threads;

  elements_per_thread = (n_numbers/n_threads)+1;

 pthread_t* thid = malloc(n_threads*sizeof(*thid));

 for(i=0;i<n_threads;i++){
    id[i] = i;
    if(pthread_create(&thid[i],NULL,threadPrime,(void*)(id+i)) != 0){
        printf("Erro\n");
        exit(0);
    }
 }

 for(i=0;i<n_threads;i++){  
    if(pthread_join(thid[i],NULL) != 0){
        printf("Erro\n");
        exit(0);
    }
 }

  // print result 
  printf("Primes:");
  int n_primes = 0;
  for (i = 0; i < n_numbers; i++) 
     if (numbers[i]) {
        n_primes++;
        printf (" %d", numbers[i]);
     }
  printf("\nTotal: %d primes\n", n_primes);

  return 0;         
}

Answer 1

存在许多问题:

1)循环的起始索引应为2。否则，您将在这里被零除错误:

for(i=0; i < n; i++) // should be i=2
   if(n%i == 0)
        return 0;

2)根本没有设置 elements_per_thread。因此它将是0(因为它是一个全局变量)，并且永远不会调用线程函数中的循环。在main()中设置它:

elements_per_thread = n_numbers/n_threads;

3)当您调用 isPrime()时，您正在传递 i。但是您确实想传递 numbers[i]。您还希望在素数测试中包括 finish。所以应该是

    for(i=start; i<=finish; i++)
        if(!isPrime(numbers[i]))
            numbers[i]=0;

4)线程数组分配错误。它应该是

pthread_t* thid = malloc(n_threads * sizeof *thid);

有更多有效的测试素数的方法(例如，您只需要检查最多n/2即可查看素数)。但是，解决上述问题后，您将拥有一个有效的代码，并考虑在以后进行改进。