Question

I am working on a project for school and I have managed to figure out a work around by doing something really clunky with my code. I have a structure that holds multiple fields, and I am trying to access the following field (as it is declared in the struct named current_event).

int *number_of_couples;

and later down into the program I call this value so I can dynamically allocate an array based on this *number_of_couples field. Essentially I am trying to use this as a “length” operator as C does not have (good) solution.

Before I was trying to implement the following code:

int *permutable_array;
permutable_array = malloc((current_event->number_of_couples) * sizeof(int)); //Line 91
if(permutable_array == NULL){
    panic("permutable_array"); //Ensures that Malloc was successful. 
}

which would in turn throw the following error:
Line 91: error: invalid operands to binary * (have ‘int *’ and ‘unsigned int’)

I have made a very sloppy work around because at this point I just want to get the program working (hour three of pure programming! rock on!). So I implemented:

int *permutable_array;
int avoid_my_bug = (int) current_event->number_of_couples;
permutable_array = malloc(avoid_my_bug * sizeof(int));
if(permutable_array == NULL){
    panic("permutable_array");
}

Which works. Now I somewhat understand the error that it is telling me. I suspect it has something to do with the fact that I stored the value of number_of_couples as a pointer within current_event, which is also a pointer. So really if the value of number_of_couples is 4, the path the program makes to get to that value is:

ptr_to_current_event -> ptr_to_number_of_couples -> 4

I can use my sloppy work-around, but it is obvious that I am doing it to avoid a bug. I would rather learn why the code will not compile. I also tried doing:

permutable_array = malloc((*current_event->number_of_couples) * sizeof(int)); //Line 91

which should have dereferenced the pointer returned by current_event->number_of_couples, however it crashes. Any solutions?

*EDIT*
It is initialized by the following line of code:

fscanf(input_file, "%i", &current_event->number_of_couples);

and is referenced at least three times (there is a valid int value stored in it) by the program before reaching my code. Remember, the second bit works, therefore it is definitely initialized.

Answer 1

You shouldn’t be storing integer values inside pointers. Pointers are made to point at things, you should rarely need to convert a pointer to an integer type. If you do you should use either the uintptr_t or intptr_t types.

You should either dereference the pointer (assuming it is pointing at a valid int) or change the type of number_of_couples so that it is not a pointer. If you leave it as a pointer, you must ensure it is pointing to a valid int object before trying to dereference it, but based on your usage and context above, it doesn’t need to be a pointer.