为什么会出现分段错误 11？

Question

Why do I get segmentation fault 11? I get it quite often, and I know this time it is about the function. If anyone can help, please do, the code is down below, I am trying to make a program, that WITH A FUNCTION. can rearrange an array in ascending order and then print it in main in reverse order.

#include "stdio.h"

void changxr(int *counter, int *arrsize, int *j, int *arr[]);

int main()
{
    int a, i, j, counter;
    int arrsize;
    int arr[100];
    printf("pick an arraysize: \n");
    scanf("%d", &arrsize);
    printf("type %d numbers \n", arrsize);
    for (counter = 0; counter < arrsize; counter++)
    {
        scanf("%d", &arr[counter]);
    }

    for (int c = arrsize - 1; c >= 0; c--)
    {
        printf("%d ", arr[c]);
    }

    changxr(&counter, &arrsize, &j, &arr[&counter]);

    for (counter = arrsize - 1; counter >= 0; counter--)
    {
        printf("%d ", arr[counter]);
    }
}

void changxr(int *counter, int *arrsize, int *j, int *arr[])
{
    int a;
    for (*counter = 0; *counter < *arrsize; *counter++)
    {
        for (*j = *counter + 1; *j < *arrsize; *j++)
        {
            if (*arr[*counter] > *arr[*j])
            {
                a = *arr[*counter];
                *arr[*counter] = *arr[*j];
                *arr[*j] = a;
            }
        }
    }
}

New code:

#include "stdio.h"

void changxr(int arrsize, int *arr[]);

int main()
{
    int a, i, j, counter;
    int arrsize;
    int arr[100];
    printf("pick an arraysize: \n");
    scanf("%d", &arrsize);

    printf("type %d numbers \n", arrsize);
    for (counter = 0; counter < arrsize; counter++)
    {
        scanf("%d", &arr[counter]);
    }

    for (int c = arrsize - 1; c >= 0; c--)
    {
        printf("%d ", arr[c]);
    }

    changxr(arrsize, &arr[counter]);

    for (counter = arrsize - 1; counter >= 0; counter--)
    {
        printf("%d ", arr[counter]);
    }
}

void changxr(int arrsize, int *arr[])
{
    int a, counter, j;
    for (counter = 0; counter < arrsize; counter++)
    {
        for (j = counter + 1; j < arrsize; j++)
        {
            if (*arr[counter] > *arr[j])
            {
                a = *arr[counter];
                *arr[counter] = *arr[j];
                *arr[j] = a;
            }
        }
    }
}

This is what I got from debugging:

"Program received signal SIGSEGV, Segmentation fault.
0x0000555555555355 in changxr (arrsize=3, arr=0x0) at main.c:33
33              for(j=counter+1; j<arrsize; j++) { if(*arr[counter]>*arr[j]){
(gdb) continue"

Answer 1

You do not need two levels of indirection ( int *arr[] , *arr[counter] , *arr[j] ). When arr is passed to a function, it will decay to a pointer-to-its-first-element, or more simply, an int * .

&arr[counter] is also an int * , but it is the address of the array element one past the elements you've initialized. This would start your sorting function in the incorrect place.

Your program segfaults because it attempts to use this value as an int ** ( int *[] ).

gcc -Wall highlights this clearly:

prog.c: In function ‘main’:
prog.c:24:22: warning: passing argument 2 of ‘changxr’ from incompatible pointer type [-Wincompatible-pointer-types]
   24 |     changxr(arrsize, &arr[counter]);
      |                      ^~~~~~~~~~~~~
      |                      |
      |                      int *
prog.c:3:32: note: expected ‘int **’ but argument is of type ‘int *’
    3 | void changxr(int arrsize, int *arr[]);
      |                           ~~~~~^~~~~

Things to do:

• Simply pass the array and the length of the array to your function.
• Use a single level of indirection in your function definition, and when accessing the array.
• Use a variable-length array .
• Use an auxiliary function for printing.
• Declare variables when you need them, in the correct scope.

You should also consider checking the return value of scanf is the expected number of successful conversions .

The refactored code:

#include <stdio.h>

void changxr(int *a, size_t length)
{
    for (size_t i = 0; i < length; i++) {
        for (size_t j = i + 1; j < length; j++) {
            if (a[i] > a[j]) {
                int temp = a[i];
                a[i] = a[j];
                a[j] = temp;
            }
        }
    }
}

void print_reverse(int *a, size_t length)
{
    printf("[ ");
    while (length--)
        printf("%d ", a[length]);
    printf("]\n");
}

int main(void)
{
    size_t size;

    printf("Pick an array length: ");

    if (1 != scanf("%zu", &size) || size == 0) {
        fprintf(stderr, "Invalid length input.\n");
        return 1;
    }

    int array[size];

    printf("Enter %zu numbers:\n", size);

    for (size_t i = 0; i < size; i++) {
        if (1 != scanf("%d", array + i)) {
            fprintf(stderr, "Invalid integer input.\n");
            return 1;
        }
    }

    printf("Array (in reverse): ");
    print_reverse(array, size);

    changxr(array, size);

    printf("Array, Sorted (in reverse): ");
    print_reverse(array, size);
}