Can I use malloc when defining a structure? - c

Question

Is there any way to dynamically allocate memory for a structure in the definition of the structure, and if it can't be done, what is the best way to dynamically allocate memory for that structure ?
typedef struct user_t{
char user_name[30];
char email[50];
int movieswatched;
movie *movielist;
}user_t;
I want to allocate memory for movielist .

Answer 1

You can use a flexible array member:
typedef struct user_t{
char user_name[30];
char email[50];
int movieswatched;
movie movielist[]; // The flexible array member,
// must always be last and declared as an array without a size
}user_t;
A structure like this must be allocated dynamically (with e.g. malloc), for example as
user_t *my_user = malloc(sizeof *my_user + sizeof(movie) * 50); // Allocate space for 50 movies

Answer 2

You can't do it in the definition of the structure (which wouldn't make much sense anyway as you don't yet know the number of movies and because different user_t variables might have different number of movies).
You need to do it at runtime, for example like this:
user_t v;
...
v.movielist = malloc(sizeof(movie) * numberofmovies);

Answer 3

You could define a 'factory' function. That's a fancy name for a function that does both mallocs for you. Something like :
user_t *user_factory( const int movies_watched )
{
user_t *user = malloc(sizeof *my_user); // should check for NULL
user->movieswatched = movies_watched;
user->movielist = malloc(sizeof(movie) * movies_watched);
return user;
}
It's common to combine the mallocs, BTW, then you can use #someprogrammerdude's flexible array member approach (but still wrap it in the factory). The other advantage of their way is that it's harder to have an accidental memory leak.
If you really want an automatic variable as you have in your example, then you can have a half-way house of init (and deinit) functions:
void init_user_t( user_t *user, const int movies_watched )
{
user->movieswatched = movies_watched;
user->movielist = malloc(sizeof(movie) * movies_watched);
}
void deinit_user_t( user_t *user )
{
free( user->movielist );
user->movielist = NULL;
user->movieswatched = 0;
}
...
user_t my_user;
init_user_t( &my_user, 6 /*num_movies*/ );