c - 为什么会出现段错误(核心已转储)？

Question

我正在学习 C。为了完成一项学校作业，我编写了一些代码来打印特定范围内的素数。我使用 50000 作为位数组的最大字节。我的代码可以编译，但它会给我一个名为“segmentation fault:11”的错误(它在 46337 处停止)。有人可以告诉我我的代码中有什么问题吗？我该如何解决这个错误？谢谢。

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

//#define MAXBYTES 1000000
#define MAXBYTES 50000


void setBit(unsigned int A[], int k);
unsigned getBit(unsigned int A[], int k);
void print_prime (int prime_num);
void sieve_Prime(unsigned int bit_arr[]);

int main (int argc, char** argv)
{
    //int bit_arr[MAXBYTES];      //This is the bit array (32 X MAXBYTES)
    unsigned int bit_arr[MAXBYTES];      //or bit_arr[MAXBYTES]
    int i;

    for (i=0; i < MAXBYTES; i++)
    {
        bit_arr[i] = 0x00;            //initialize all bits to 0s
    }

    setBit(bit_arr, 0);             //0 is not prime, set it to be 1
    setBit(bit_arr, 1);             //1 is not prime, set it to be 1

    sieve_Prime(bit_arr);
    printf("\n");

    return 0;

}

//Set the bit at the k-th position to 1
void setBit(unsigned int A[], int k)
{
    int i = k/32;
    int pos = k % 32;

    unsigned int flag = 1;      //flag = 0000 ..... 00001
    flag = flag << pos;         //flag = 0000...010...000 (shifted k positions)

    A[i] = A[i] | flag;         //Set the bit at the k-th position in A[i];
}

//get the bit at the k-th position
unsigned getBit(unsigned int A[], int k)
{
    int i =k/32;
    int pos = k % 32;

    unsigned int flag = 1;

    flag = flag << pos;

    if (A[i] & flag)
        return 1;
    else
        return 0;
}


void print_prime (int prime_num)
{
    //print a prime number in next of 8 columns
    static int numfound=0;

    if (numfound % 8 == 0)
        printf("\n");
    if (prime_num+1 < MAXBYTES*8)                       
        printf("%d\t", prime_num);
    numfound++;
}

void sieve_Prime(unsigned int bit_arr[])
{
    int i;
    int k;

    int next_prime = 2;

    print_prime(2);

    while (next_prime+1 < MAXBYTES*8)                   
    {
        k = next_prime;

        //multiples of next_prime is not primpe
        while(next_prime*k < MAXBYTES*8)                
        {
            setBit(bit_arr, next_prime*k);      //set it to be 1
            k++;     
        }

        //find next_prime by skipping non-prime bits marked 1
        next_prime++;
        while (next_prime + 1 < MAXBYTES*8 && getBit(bit_arr, next_prime))
        {
            next_prime++;
        }
        print_prime(next_prime);

    }
}

Answer 1

在while(next_prime*k < MAXBYTES*8)

next_prime*k 的最大值是(MAXBYTES*8-1)*(MAXBYTES*8-1) . 这个大值不能保存在带符号的 int 中并可能变成负值导致段错误。

使用

while(unsigned(next_prime*k) < MAXBYTES*8) 

将消除段错误。