Linked-list node eliminating function

Question

I am just trying to eliminate nodes that are containing even numbers x % 2 == 0.

I wrote an extremely simple function but after I use it, the complier won't show the modified list and I don't get why.

If the head contains an even element it works properly (I did this in main), but when I try to delete something from the interior, it literally does not want to work...

I really don't know what I did wrong.

#include <stdio.h>
#include <stdlib.h>
struct test{
    int val;
    struct test *next;
};

void erase (struct test *x)
{ 
    struct test *t;
    t=x->next;
    x->next=t->next;
    free(t);
    return ;
}

int main ()
{
    
    struct test *head,*p,*q, *aux;
    int n,i;
    
    printf("Number of elements:");
    scanf("%d",&n);
    p=(struct test * )malloc(sizeof(struct test));
    
    printf("First value:");
    scanf("%d",&p->val);
    p->next=NULL;
    head=p;
    for(i=2;i<=n;i++)
    {
        q=(struct test * )malloc(sizeof(struct test));
        printf("Value %d:",i);
        scanf("%d",&q->val);
        q->next=NULL;
        p->next=q;
        p=q;
    }
    
    for(p=head;p!=NULL;p=p->next) {
        printf("%d ",p->val);
    }
    while((head->val)%2==0) {
        aux=head;
        head=head->next;
        free(aux);
    }
    
    for( p=head ; p!=NULL; p = p->next) {
        if((p->next->val)%2==0) {
            erase (p);
        }
    }
    printf("\n");
    for(p=head ;p!=NULL;p=p->next) {
        printf("%d ",p->val);
    }
}

Answer 1

After the while loop

while((head->val)%2==0) {
    aux=head;
    head=head->next;
    free(aux);
}

the pointer head can be equal to NULL. So the next for loop can invoke undefined behavior. Moreover you are redundantly advancing the pointer p after erasing a node with an even value.

Substitute this for loop

for( p=head ; p!=NULL; p = p->next) {
    if((p->next->val)%2==0) {
        erase (p);
    }
}

for the following code

if ( head != NULL ) {
    for( p=head ; p->next!=NULL; ) {
       if((p->next->val)%2==0) {
           erase (p);
       }
       else {
           p = p->next;
       } 
   }
}