What is a pointer in C?

A pointer in C is a variable that stores the memory address of another variable. Using pointers, we can directly manipulate memory. For example, 'int *p' declares a pointer 'p' to an integer. Pointers are useful for dynamic memory allocation and efficient array handling. 
 
int a = 10;
int *p = &a;
printf("Value: %d", *p);

Both malloc() and calloc() are used for dynamic memory allocation in C. malloc() allocates a block of memory without initializing it, whereas calloc() allocates and initializes memory to zero. calloc() also takes two arguments (number of blocks, size of each), while malloc() takes one (total memory size). 
 
int *arr = malloc(5 * sizeof(int));
int *arr2 = calloc(5, sizeof(int));

In C, ++i is the pre-increment operator, meaning the value of 'i' is incremented first, then used. i++ is the post-increment operator, which means the current value of 'i' is used first, and only after that, 'i' is incremented. This distinction is important when used within expressions. 
 
int i = 5;
printf("%d", ++i); // Output: 6
printf("%d", i++); // Output: 6, but i becomes 7

Function pointers in C store the address of a function. They are used to pass functions as arguments, return functions from other functions, or call functions dynamically. A function pointer is declared like this: 'void (*fptr)()', where fptr is a pointer to a function that takes no arguments and returns void. 
 
void func() { printf("Hello"); }
void (*fptr)() = func;
fptr();

A structure in C is a user-defined data type that allows grouping of variables of different data types under a single name. It is useful when handling complex data types like student records or employee data. Structures can contain members like int, char arrays, and other data types. 
 
struct Person { char name[20]; int age; };
struct Person p1 = { "John", 25 };