c中处理scanf的分段错误

Question

I'm writing a program that requires me to do a union of two arrays. Here is my code so far. I get Segmentation fault as an error after I enter set A.

#include <stdio.h>

void Union(int a[], int b[], int set1, int set2)
{
    int u[20], i, j, unionIndex=0,trigger;

for(i=0; i<set1; i++)
{
    u[unionIndex] = a[i];
    unionIndex++;
}

for(i=0; i<set2; i++)
{
    trigger=0;
    for(j =0; j<set1; j++)
    {
        if(b[i] == u[j])
        {           
            trigger =1;
            break;
        }
    }
    if(trigger =0)
    {
        u[unionIndex]=b[i];
        unionIndex++;
    }
}

    for(i=0;i<unionIndex;unionIndex++)
    {
    printf(" %d",u[i]);
    }
}

   int main(void) {
   int N=0;
   int M=0;
   int i;
   int j;

   printf("Please enter the number of elements in set A: ");
   scanf("%d",N );
   int a[N];

   printf("Enter the numbers in set: ");
   for(i=0;i<N;i++)
   {
         scanf("%d",&a[i]);
   }

    printf("Please enter the number of elements in set B: ");
    scanf("%d",M );
    int b[M];

    printf("Enter the numbers in set: ");
    for(j=0;i<M;i++)
    {
         scanf("%d",&b[i]);
    }

    Union(a,b,N,M);
    return 0;
}

I'm pretty sure the issue has something to do with arrays because the program will compile but i get the error right after the user enters set A. I'm a beginner at C but I know a lot more about Java, so I'm thinking this has something to do with memory allocation. I'm not really sure how to solve the issue, so if you could point me in the right direction that would be helpful.

Answer 1

You need to pass the address of the variable to scanf()

Change

printf("Please enter the number of elements in set A: ");
scanf("%d",N );

to

printf("Please enter the number of elements in set A: ");
scanf("%d", &N);

---

Same goes for other place

printf("Please enter the number of elements in set B: ");
scanf("%d", &M);

---

There is another possible mistake

Its here

for(j =0; j<set1; j++)
{
    if(b[i] == u[j])

In this set1 is equal to N , so j will go from 0 to N-1 . And array u[] has only 20 elements. There is a possibility of array access out of bound if some user enter value more then 20 for N .

Answer 2

The problem, as I see it is in

scanf("%d",N );

and

scanf("%d",M );

It invokes undefined behavior as scanf() needs the argument to a format specifier to be a pointer to the type.

Just to clarify, you're essentially passing the address as 0 (value of the variable), which is not a valid addres, anyway.

You need to pass the address there, like

scanf("%d", &N );

and

scanf("%d", &M );

That said, in your Union() function, you're using a user-defined value to limit the for loop, against a constant value 20 . In case the user input is more than 20, you'll be overrunning the memory which invokes undefined behavior .

Answer 3

The reason you're getting the segmentation fault is because of how you're calling scanf when reading in N and M . The %d format specifier for scanf expects an int * , ie the address of an int, but you're passing in an int . This is undefined behavior.

So you can fix them like this:

 scanf("%d",&N );
 ....
 scanf("%d",&M );

Some addtional bugs:

When looping to read in the values for b :

for(j=0;i<M;i++)
{
     scanf("%d",&b[i]);
}

You have the wrong loop indexes:

for(j=0;j<M;j++)
{
     scanf("%d",&b[j]);
}

When checking trigger :

if(trigger =0)

This is an assignment, not a comparison:

if(trigger == 0)

When looping to print out u :

for(i=0;i<unionIndex;unionIndex++)

You're incrementing the wrong variable:

for(i=0;i<unionIndex;i++)

Finally, u need to have a length of at least set1 + set2 , otherwise you risk writing off the end of the array:

int u[set1+set2];

Fix those and you should get the desired results.