C程序中的分段错误

Question

I am getting segmentation error when trying to do this simple c sorting program I am a novice in C language . And can you please explain me why i am getting segmentation error

#include<stdio.h>

int main(void)
{
    int prev,next,result,total_number;
    int i,j=1,b;
    int a[i];

    printf("Number of values to be entered");
    scanf("%d",total_number);
    printf(" enter the values \n");
    for(i=0;i<=total_number-1;i++)
    {
        printf(" enter the values \n");
        scanf("%d",a[i]);
    }
    for(i=0;i<=total_number-2;i++)
    {
        for(j=1;j<=total_number-1;j++)
        {
            if(a[i]>a[j])
            {
                b=a[i];
                a[i]=a[j];
                a[j]=b;
            }
            else
            {
                break;
            }
        }
    }
    for(i=0;i<total_number-1;i++)
    {
        printf("The numbers are %d",a[i]);
    }

}

Answer 1

The segmentation fault is caused by this line:

scanf("%d",total_number);

You're missing the address of (&) operator, it should be like this:

scanf("%d",&total_number);

The operator is also missing in this line:

    scanf("%d",a[i]);

You can find more details on scanf in the glibc manual :

Another area of difference between scanf and printf is that you must remember to supply pointers rather than immediate values as the optional arguments to scanf; the values that are read are stored in the objects that the pointers point to. Even experienced programmers tend to forget this occasionally, so if your program is getting strange errors that seem to be related to scanf, you might want to double-check this.

But there are other subtle errors in your code, too: the fact the int i in array[i] is not initialized leads to undefined behaviour ie, anything could happen. You can use malloc to allocate space for the array, but a simple reordering could be enough:

scanf("%d",&total_number);
int a[total_number];

Uses user input to allocate the array.

Lastly, it seems like you're trying to implement an insertion sort algorithm, but the logic is slightly flawed: even correcting the bugs in the code the input

1 3 5 0

gets "ordered" to

1 5 3 0

But I don't know what you were trying to implement. In case you were actually trying to implement insertion sort, you could use the pseucode from the insertion sort wiki article to get an idea of what's missing in your implementation.

Answer 2

int a[i]; is creating an array of arbitrary size since i is not initialized.

Instead, you can dynamically create the equivalent of an array of int once you know total_number.

int* a;

. . . snip . . .

/* once you know total_number */

a = (int*) malloc(total_number, sizeof(int));

/* you can use a with array notation as long as you stay in bounds */

a[i] = something;

/* don't forget to free a when you are done */

free(a);