how to allocate space for contained char pointer embedded in parent struct with malloc. I only have access to containing struct pointer

Question

I have following struct

struct connection
{
    int *new_socket;
    int type;
    struct sockaddr_in address;
    char *request_line; 

};

And I created a pointer of connection struct like struct connection *con_obj=malloc(sizeof(struct connection)) now I like to allocate space for

 con_obj->request_line

Can I do this

*(con_obj->request_line)= malloc(sizeof(char )*val);

or do I need to do this

con_obj->request_line= malloc(sizeof(char )*val);//I don't think so

can anyone please tell this

@kiner_shah Many StackOverflow users disagree with "you need to cast it to appropriate type" in case of C, indeed they recommend against it. https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc/605858#605858 — Yunnosch, Oct 01 '21 at 07:13
You need: `con_obj->request_line = malloc(sizeof(char) * val)`. Why didn't you try it? Your compiler gives you valuable information. — Jabberwocky, Oct 01 '21 at 07:19
@kiner_shah you don't _need_ to cast, but you _can_ cast it to the appropriate type, but such a cast is useless. — Jabberwocky, Oct 01 '21 at 07:21
@Yunnosch, you're right. I just read this post: https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — kiner_shah, Oct 01 '21 at 07:22
And do not forget to check `if (con_obj != NULL)` before trying to access `con_obj>request_line`. — kiner_shah, Oct 01 '21 at 07:23
`*(con_obj->request_line)` this accesses a single `char` and your compiler should warn about "conversion making integer from pointer with different size" or similar. `con_obj->request_line` is a pointer and assigning an address is perfectly valid. If you don't get a warning for the first version, you need to increase warning level. — Gerhardh, Oct 01 '21 at 07:29
Have a look at: [Member access operators](https://en.cppreference.com/w/c/language/operator_member_access) — Erdal Küçük, Oct 01 '21 at 07:38

0___________ · Answer 1 · 2021-10-01T07:43:08.280

The correct way is :

con_obj->request_line= malloc(val * sizeof(*con_obj->request_line));

You should test if the pointer con_obj is not NULL

Your fist version:

*(con_obj->request_line)= malloc(sizeof(char )*val);

invokes undefined befaviour as you dereference the unassigned poiter. The assignement itself is also wrong because you assign the char value with void *. You could discover it yourself if you have not ignored the compiler warnings. ALWAYS READ COMPILER WARNINGS. NEVER IGNORE THEM.

how to allocate space for contained char pointer embedded in parent struct with malloc. I only have access to containing struct pointer

1 Answers1