getting C segmentation fault 11

Question

I was given the assignment of writing a C code which contains a function, which when passed through an array of length n returns a pointer to the array’s largest element. In order to make it easy for us, the length n was set to 10.

I wrote the code below and compiled it and it didn't show any error. Then, I ran this code and it asked for the 10 inputs for the array, but instead of returning the pointer with the largest element, it read segmentation fault 11.

I am new to pointers, and so could not figure out what exactly went wrong. Any help will be greatly appreciated.

#include<stdio.h>
#define N 10

int *find_largest(int a[], int n)
{
    int i;
    int *max;
    *max = a[0];

    for(i = 1; i < n; i++)
    {
        if(a[i] > *max)
            *max = a[i];
    }
    return max;
    // this function returns the pointer with the max. value                       
}

int main(void)
{
    int b[N], i;
    int *p;

    printf("Enter %d numbers: ", N);
    for(i = 0; i < N; i++)
        scanf("%d", &b[i]);
        //scans all the inputs                                                         

    p = find_largest(b, N);
    //p gets assigned the value of the max valued pointer                          

    printf("largest element: %d", *p);

    return 0;
}

Edit- After looking at the wonderful advices given, I modified my code a bit. I changed *max=a[0] to max= &a[0], and *max=a[i] to max=&a[i]. Now, the code runs without any error, but instead of returning the pointer with the largest value, it returns the largest input instead.

Answer 1

In this line, you write to the place the uninitialised ponter is pointing to.

*max = a[0];

This causes undefined behaviour (or "UB", reading up on it is worth the time) and to be strictly avoided; among other reasons because it is a very likely reason for segfaults.

What you probably want to do is to write the address of the first element into the pointer.

max = &a[0];

Later you write the place the pointer is pointing to, updating it to the max value you have found.

*max = a[i];

Guessing from the rest of your code, you probably want to just keep the address of the max element, so that you can return its address as it is in the input array, without changing the values in the input array (which your currently do, and it seems unlikely that you intend to...).

max = &a[i];

Referring to your comment (that the value is returned, not the pointer):
For returning a pointer (not a value), make sure that you

return max; // the pointer, not the value it is pointing to

Which is what you actually do in the currently shown code version (possibly before an edit...).
Do not return *max; that would return the value.

If by "return" you mean "print", then you need to change the

printf("largest element: %d", *p);

Use p instead of *p and change the format specifier accordingly.
You probably want "largest element at address: %p". (Compare http://en.cppreference.com/w/c/io/fprintf )

printf("largest element at address: %p", p);

Answer 2

 int *max;
 *max = a[0];

This is UB (undefined behavior) since max is not pointing to valid memory and you try to dereference it.

max = &a[0];

Moreover your logic to find the max is wrong. Why do you reset it every time to 0th element. It should be something like.

for(i = 1; i < n; i++)
{
  if(a[i] > *max)
    max = &a[i];
}