Is it possible to create a struct whose size is not known at compile time?

Question

As the question states I am looking to create a struct in C whose total size I do not know at compile time.

For example, I would like to create a struct that contains a count value and an array with count elements. I know this could be implemented as:

typedef struct myStruct{
    int count;
    int *myArray;
} myStruct;

However, I want this struct to take up one solid block of memory so I could use memcpy() on it at a later point in time. Like this:

typedef struct myStruct{
    int count;
    int myArray[count];
} myStruct;

Answer 1

It sounds like you're looking for flexible array members:

typedef struct myStruct
{
    int count;
    int myArray[];
} myStruct;

Then, when you allocate it later:

myStruct *x = malloc(sizeof(myStruct) + n * sizeof(int));
x->count = n;