Avoiding multiple includes

Hi all

I met a 'little' problem concerning multiple includes of the same header file:
Let's say I have the four files
- user.c
- user.h
- capture.c
- capture.h

The file 'user.h' holds the struct definition:

Code:

struct user
{
    char *name;
    int *ports;
    AES_KEY dkey;        //Encryption key of the user
    AES_KEY ekey;        //Decryption key of the user
    struct user *right;
    struct user *left;
};

The file 'capture.h' holds the struct definition:

Code:

struct knock_request
{
    char *username;
    char *s_ip;
    time_t timestamp;
    AES_KEY dkey;        //Key to decrypt the data
    char *s_address;
    char *knock_data;
};

Now as I use 'AES_KEY' within the two structs, I have to include <openssl/aes.h> in the two header files, otherwise I get warnings when compiling?...So far that would be OK, there are no multiple includes of the same file till here.

But let's assume that the file "capture.c" needs both "user.h" and "capture.h" included? I would have the file <openssl/aes.h> included two times. How should I avoid that? Is there some 'standard' way? Like using a void pointer to point to the actual encryption/decryption keys?

Regards
Rafael

The standard "Include Guard" method is like this:

FileA.h:

Code:

#ifndef FILE_A_H_INCLUDED
#define FILE_A_H_INCLUDED

...

#endif /* FILE_A_H_INCLUDED */

FileB.h:

Code:

#ifndef FILE_B_H_INCLUDED
#define FILE_B_H_INCLUDED

...

#endif /* FILE_B_H_INCLUDED */

Sure thanks, I know that.
But do all header files make use of that guard? If not it wouldn't be of use here.
I just looked in the <openssl/aes.h> header file and saw that they're using the guard, didn't know that. But what if they didn't? What would I do then? As I don't want to rely on "everyone does it"...

Well everyone usually does do this. It's a convention.

If someone was writting a library or something that was made to be redistributed, then it'd be stupid for them not to.

Ok then, that sounds good...
thanks!