使用C中的链接列表实现Eratosthenes的筛选（分段错误）

Question

I'm attempting to implement the Sieve algorithm in C for a class. I'm not asking for this assignment to be done for me. I've got my functions written out already, but am currently getting a Segmentation Fault error. I'm not 100% sure what that is. Here is my code so far, can anyone see where this error comes from?

#define EXTERN

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

void clearAll() {
    int i, j;
    seg *p;
    p = head;
    for(i = 0; i < NSegs; i++) {
            p = p -> next;
            for(j = 0; j < 256; j++) {
                    p -> bits[j] = 0;
            }
    }
}
int setBit(int n) {
   int segment, index, hold, pos, i;
   seg *p;
   p = head;

   segment = n/256;
   hold = n;
   while(hold > 65) {
        hold = hold - 65;
        index++;
   }
   pos = (hold - 1)/2;

   for(i = 0; i < segment; i++) {
        p = p -> next;
        p->bits[index] = p->bits[index] | (1 << pos);
   }
}

    int testBitIs0(int n) {
   int segment, index, hold, pos, i, r;
   seg *p;
   p = head;
   segment = n/256;
   hold = n;
   while(hold > 65) {
        hold = hold - 65;
        index++;
   }
   pos = (hold - 1)/2;
   printf("%d, %d, %d ", segment, index, pos);
   for(i = 0; i < segment; i++) {
         p = p -> next;
        r = p->bits[index] & (1 << pos);
    }
}

void sieveOfE( int N )
{
   int i, j, k;


   k = 1;   // Start with 2 to find all primes

   while ( k <= N )
   {
        for ( i = k; i <= N; i++){
          if(i % 2 == 0) {
                break;
           }
          if ( testBitIs0(i) ){
              break;
          }
        }

        for ( j = 2*i; j <= N; j = j + i ){
          setBit(j);
         }
         k = i+1;
    } 
 }

int countPrimes(int n){
    int count, i;
    count = 0;
    for(i = 0; i <= n; i++) {
       if(testBitIs0(i) ){
            count++;
       }
    }
    return count;
}

int printPrimes(int n){
        int i;
        for(i = 0; i <= n; i++) {
            if(testBitIs0(i) ){
                printf("%d ", i);
           }
        }
        printf("\n\n");

} 

The Linked List has already been properly initialized in the main and header files. The initialization came from a skeleton file and should not be edited. But each linked list node contains a bit array and a pointer to the next node.

Answer 1

Give a programmer a fix for a segfault and you'll feed him for a day. Teach a programmer to use a debugger and he'll feed himself for a lifetime.

If you run your program under a debugger, it will trap the segfault crash at the line of code which caused it and you can examine the call stack. If you use the gdb debugger, the bt or backtrace command will display your stack.

Here is a GDB turorial .

As pointed out in the comments, segfault generally occurs when you try to dereference an invalid pointer, be it uninitialized, corrupted or wrong for any number of reasons.