Why do I get a segfault when trying to access members of struct?

Question

I am getting a segmentation fault whenever I try to print the value stored within a struct. Using Valgrind, I was able to narrow it down a bit to one block of code, but I am unsure what I am doing wrong. Am I initializing my struct incorrectly?

Whenever I run valgrind, it tells me that there is an invalid write of size 8 where I say newList->data = d.

typedef struct List {
  struct List *np[Ends];        // next/prev neighbors
  Data data;
} *List;

typedef void *Data;

static void put(Data d) {
  List newList = malloc(sizeof(List));
  newList->data = d;
}

Answer 1

This is an example of why it's bad practice to typedef a pointer.

Because List is defined as an alias for struct List *, sizeof(List) gives you the size of a pointer, not the size of the struct. As a result, you're not allocating enough memory.

You instead want either malloc(sizeof(struct List)) or (preferably) malloc(sizeof *newlist)