Seg fault when initializing my queue's head/tail to NULL;

Question

I'm having issues solving this segmentation fault. I've been trying to fix this for a few hours, but I've run out of ideas+google searches, so I'm hoping to get some help.

What I have is a queue struct, which keeps track of head & tail nodes which are also structs.

The problem that I'm having is that any interaction with queue_1 is resulting in a segmentation fault.

typedef struct customer{
    int id;
    struct customer *next;
}customer_t;

typedef struct customer_queue{
    customer_t *head;
    customer_t *tail;
}queue;

queue *queue_1;



int main(){
    initialize_queue(queue_1); //segmentation fault here.
    customer_t *customer_1 = create_customer(1234); //this section works if we ignore the above line
}

void initialize_queue(queue *q){
    q->head = NULL;
    q->tail = NULL;
}

customer_t *create_customer(int id){
    customer_t *customer = malloc(sizeof(customer_t));
    customer->id = id;
    customer->next = NULL;
    return customer;
}

Answer 1

queue_1 is set to NULL when the program starts as it is an uninitialized variable at file scope. So when you pass it to initialize_queue and subsequently try to set the fields you're dereferencing a NULL pointer. This is undefined behavior which in this particular situation causes your code to crash.

You don't need to define queue_1 as a pointer to a queue . Just define it as a queue and initialize it:

queue queue_1 = { NULL, NULL};

Strictly speaking you don't need to initialize it as both members will get implicitly initialized to NULL, but better to be explicit about it.

Answer 2

void initialize_queue(queue *q){
    q = malloc(sizeof(queue));
    q->head = NULL;
    q->tail = NULL;
}

your queue is not allocated, to debug easily segmentation fault, use valgrind, it's a memory tracer, in complation add -g3 flag to get the line of the error on the execution, the execution : "valgring ./binary_name" and compilation: "gcc filename flags "-W -Wall .. -g""