Segmentation fault error in structure code-CodePudding

#include <stdio.h>

#define x 10

struct data
{
    int num;
    int state;
};

struct queue
{
    struct data *q1[x];
    int num;
};

struct queue *q;

void insert(struct queue *q, struct data *d)
{
    d->state = 0;
    q->q1[0] = d;
    q->num = 1;
}

int main()
{
    struct data *d;
    // struct queue *q = (queue*)malloc(sizeof(struct queue));
    insert(q, d);
    printf("%d\n %d\n", q->q1[0]->state, q->num);
}

I wrote the code like this. And it caused segmentation fault (core dumped). I wonder why it is happened and how should I edit it.

Thank you.

CodePudding user response：

q and d are pointers. You can't access structure members, because No memory has been allocated for this structure.

#include <stdlib.h>
<...>
struct data *d = (struct data *)malloc(sizeof(struct data));
struct queue *q = (struct queue *)malloc(sizeof(struct queue));
<...>
free(d);
d = NULL;
free(q);
q = NULL;

CodePudding user response：

You should make sure that all the pointers you use are initialized. defining a pointer d, without pointing it to a valid structure, and then trying to use the memory that it points to, will probably generate a segmentation fault.