How to access union inside structure? - c

Question

I have the following code:
/* sample.c */
#include<stdio.h>
#include<malloc.h>
#include<stdlib.h>
#include"hermes.h"
#include<string.h>
int main (){
struct hermes *h ;
h = ( struct hermes *) malloc ( sizeof ( struct hermes *));
strcpy ( h->api->search_response->result_code , "123" );
printf("VALue : %s\n" , h->api->search_response->result_code );
return 0;
}
/* hermes.h */
struct hermes {
union {
/* search response */
struct {
int error_code;
char *result_code;
char *user_track_id;
struct bus_details bd;
}*search_response;
}*api;
};
I get a segmentation fault when I try to access the elements. Could anyone tell me what is the right way to access these elements?

Answer 1

Your malloc() line isn't correct:
h = ( struct hermes *) malloc ( sizeof ( struct hermes *));
should be:
h = ( struct hermes *) malloc ( sizeof ( struct hermes));
Remove the * in the sizeof(). Otherwise, you're only allocating enough for a pointer rather than the struct itself.
Also, the cast isn't necessary in C.

Answer 2

Use this struct:
#define MAX 512 /* any number you want*/
struct hermes {
union {
/* search response */
struct {
int error_code;
char result_code[MAX];
char user_track_id[MAX];/* can use different sizes too*/
struct bus_details bd;
}search_response[MAX];/* can use different sizes too*/
}*api;
};
Or if you want to use your current struct, malloc the pointer element like:
h->api = malloc((sizeof(int)+sizeof(char)*MAX*2+sizeof(struct bus_details))*MAX)

Answer 3

It's not a problem of accessing the elements. That's about all that you are doing correctly.
Here are some of the things that are wrong. First, you aren't allocating enough space for a hermes struct, just enough for a pointer. Then, even if you malloc( sizeof ( struct hermes ) );, the one element (api) is an uninitialized pointer. You can't just follow uninitialized pointers down deep into the data structure, because they will be pointing to who knows where in memory. You first need to allocate something for h->api to point to. Then you need to allocate space for h->api->search_response. If you correct all that, then you are copying a string to ... who knows where? You should use strdup, not strcpy to create a new string, then you should assign the return value to result_code. Also, your union has only one element, so it's kind of pointless (unless there's more to it that you haven't posted).
EDIT Here's one way of initializing h:
h = malloc( sizeof( struct hermes ) );
h->api = malloc( sizeof( *h->api ) );
h->api->search_response = malloc( sizeof( h->api->search_response ) );
h->api->search_response->result_code = strdup( "123" );
Note that in a well-behaved program that cleans up after itself, each of these allocations will have to be freed individually as well, in reverse order of the calls to malloc. Since you immediately call exit(0), no harm is done in this case if you don't.