ECE 353: Lab C

1. ECE 353: Lab C Pointers and Structs

2. Basics • A pointer holds an address to some variable • Notation: • Dereferencing operator: * • int *x is a declaration that x is a pointer to a variable of type int • x contains the location that the pointer is pointing to • Referencing operator: & • & means “address of.” • If we declare int y, then &y is the address of y

3. Given a definition of the form int *i • &i is the address of the pointer itself • i is the address this pointer points to • *i is the content of the address the pointer points to • Declaring a pointer does NOT automatically initialize it: be careful to avoid dangling pointers

4. Pointer Arithmetic • Valid operations: • Assignment of pointers of the same type. • Addition or subtraction by an integer: all results are in units of the type to which the pointer points • Subtracting or comparing two pointers to members of the same array (the compiler is not obliged to warn you if they are not to the same array) • Zero assignment or comparison with zero • Illegal operations: • Adding two pointers • Multiplication, division, shifting, masking • Adding by a non-integer quantity • Assignment of pointers of different types without a cast (except for void *)

5. Precedence Table • The precedence table specifies the order in which operations are carried out • Is the value of 3+4*4 equal to 28 or to 19? • Is the condition (a&b==1) different from ((a&b)==1)? • If we had declared i as a pointer to int, • Is *i+1 different from *(i+1)? • Is *i++ different from *i+1? • Is *i++ different from (*i)++? • Is *i++ different from ++*i? • Is *i() different from (*i)()? • What does 10**i mean? • If j is another pointer to int, what is *i-*j? • Is the declaration int *k[100] an array of pointers or a pointer to an int array? • What does (*(void(*)())0)() mean? (From Koenig: C Traps and Pitfalls, 1989)

6. Pointers and Arrays • The name of the array is a pointer to the start of that array. • If a[10] is declared as an array of ints, a is a pointer to the first element of that array. • However, an array name is NOT a variable: you cannot assign a pointer to an array name or increment it with ++. • Because of the way array accesses are implemented, 3[a] means the same thing as a[3] (since *(a+3) is the same as *(3+a)).

7. Strings • There is no string type built into the language • Strings are processed as character arrays • String literals are specified between double-quotes • These arrays are stored with a end-delimiter of \0. • Pointers are often used with strings • string.h should be included for access to the relevant library functions • Example: char *roomNumber; roomNumber = “ELAB 303”;

8. Pointers to Pointers • int **p means that p is a pointer to a pointer to data of type int • There is a similarity between pointers to pointers and two-dimensional arrays • However, a pointer is NOT an array. • Storage: • Global & static variables: Data segment • Dynamic memory: Heap • Local variables: Stack

9. const • const indicates to the compiler that you will not change the item so qualified. • Examples: • const int x; • int *const y; • const int *a; • const int *const b;

10. Examples of Declarations • int **a; // ptr to ptr • int *b[10]; // array of ptrs • int (*c)[10]; // ptr to array • int *d(); // d returns ptr to int • int (*e)(); // e is ptr to int function • char (*(*x())[])(); // ? • char (*(*y[3])())[5]; // ?? Kernighan & Ritchie, The C Programming Language, 1988

11. structs • Keyword struct followed by an optional structure tag (or name) followed by structmembers within braces. • A struct member can be the same as the tag: the context disambiguates them. • A struct member may itself be a struct • Reference to a member is via structure_name.member_name

12. Legal Operations on structs • Copying • Assignment • Taking its address • Accessing its members • Passing to, and returning it from, a function call • Structure parameters are passed by value

13. Pointers to structs • We can declare pointers to structs in the usual way. • If q is a struct type, then the declaration struct q *ptr; defines a pointer to this type • Question: If x is a field of struct type q, is (*ptr).x different from *ptr.x?

14. -> notation • -> is shorthand: if ptr is a pointer to a structure of which x is a member, then ptr -> x refers to member x of the struct pointed to by ptr • The usual precedence conditions apply: • Is ++ptr->x the same as ++(ptr->x)? • Is ++ptr->x the same as (++ptr)->x?