C - pointer to array of pointers - c

Question

i have to store the pointers of a data structure.
So, i need an array of pointer.
I created a simple example, but i receive a segmentation fault.
Why i cannot do
buf->data[i] = pkt ??
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define MAX_SIZE 500000
#define MAX_CHAR 1024
char *string = "HelloHelloHello";
struct pkt {
char *buf;
int seq;
};
struct buffer {
int a;
int b;
struct pkt **data;
};
struct buffer *new_buffer() {
struct buffer *new;
new = malloc(sizeof(struct buffer) * MAX_SIZE);
if (new == NULL) {
perror("malloc(buffer)");
exit(EXIT_FAILURE);
}
struct pkt *data;
data = malloc(sizeof(struct pkt) * MAX_SIZE);
if (data == NULL) {
perror("malloc(data)");
exit(EXIT_FAILURE);
}
new->data = &data;
new->a = 0;
new->b = 0;
return new;
}
struct pkt *new_pkt(char *data, int seq) {
struct pkt *pkt;
pkt = malloc(sizeof(struct pkt));
if (pkt == NULL) {
perror("malloc(pkt)");
exit(EXIT_FAILURE);
}
pkt->buf = malloc(sizeof(char) * MAX_CHAR);
if (pkt->buf == NULL) {
perror("malloc(pkt->buf)");
exit(EXIT_FAILURE);
}
strncpy(pkt->buf, data, MAX_CHAR);
pkt->seq = seq;
return pkt;
}
void add_pkt(struct pkt *pkt, struct buffer *buf, int i) {
buf->data[i] = pkt; //segmentation fault
}
int main() {
struct buffer *my_buffer = new_buffer();
struct pkt *pkt;
int i;
for(i=0; i<MAX_SIZE; ++i) {
pkt = new_pkt(string, i);
add_pkt(pkt, my_buffer, i);
printf("[%d]", i);
}
return 0;
}

Answer 1

change
struct pkt *data;
data = malloc(sizeof(struct pkt) * MAX_SIZE);
if (data == NULL) {
perror("malloc(data)");
exit(EXIT_FAILURE);
}
new->data = &data;
to
new->data = malloc(sizeof(struct pkt*) * MAX_SIZE);
if (new->data == NULL) {
perror("malloc(new->data)");
exit(EXIT_FAILURE);
}
struct pkt *data; is a local variable, saving its address and using it after the scope ends, is undefined behaviour. Also you want a memory area where you can save some struct pkt*s and not struct pkts.
Since you are not freeing anything, you have a big memory leak.