Question

How do I access elements in this stuct if I initialize a list as follows:

group **list = (group **) malloc(sizeof(group)); 

typedef struct
{
    // ID of the group, 'A' to 'D'
    char id;

    // a list of members in the group 
    char **members;
} group;

I tried using (*list)->id = 'A' and it compiles but then gets a segmentation fault error when running the program.

Answer 1

Although it compiles fine, you didn’t allocate memory correctly.

You allocated memory for group **list, which is eventually an array of pointers to struct group. What I think you intended to do doing is:

group** list = malloc(sizeof(group*) * 5);  // e.g. 5 pointers

Now for each pointer in the array, allocate its own memory:

int i;
for (i = 0; i < 5; i++) {
    list[i] = malloc(sizeof(group));
}

For instance, to access id in the 2nd struct, you do:

list[1]->id = 'A';

Note that *list access the first struct, and is equivalent to list[0].

Sidenote:
Two levels of indirection allow you to store the structs in a non-continuous way in the memory. Alternatively, you can use one level of indirection and store them continuously:

group list* = malloc(sizeof(group) * 5);  // Again, 5 structs

and then access the members with:

list[0].id = 'A';