ECE 103 Engineering Programming Chapter 35 C Pointers, Part 1 Herbert G. Mayer, PSU CS Status 8/4/2014 Initial content copied verbatim from ECE 103 material developed by Professor Phillip Wong @ PSU ECE

Syllabus Non-pointer Variables Pointer Variables Levels of Indirection

Non-pointer Variables A standard (non-pointer) variable stores values Declaration: datatype variable_name; Example: int x; x contains a value that is of type datatype The value stored in x is accessed directly & x is memory address where value of x is stored 2

What memory operations are performed? Example: int x, y; x = 3; y = x; What memory operations are performed? 3

int x, y; x = 3; y = x; x y x x y x y x y 3 3 3 3 Address Ax Address Ay copy 3 x Address Ax x Address Ax y Address Ay 3 copy x Address Ax y Address Ay 3 x Address Ax y Address Ay 3 4

Pointer Variables A pointer is a variable that can contain only the address of something else That something else may be another variable, function, or data object How are pointers used in C? To allow a function to permanently change arguments To access data or functions indirectly To track dynamically allocated memory at runtime To build advanced data structures such as linked lists 5

Declaration: datatype * p_name; The * symbol declares a variable as a pointer p_name contains an address p_name is said to “reference” or to “point to” the memory location at that address datatype is the type of value stored at the address The * symbol is also used to “dereference” a pointer: If p contains an address, then *p retrieves the value stored at that address (aka indirect access) & p_name is the address in memory where the pointer variable p_name itself is stored 6

Example: g is a pointer of type float * and references address 500. The memory at address 500 holds value 1.25., and g itself is stored at address 384: g contains the value 500 * g returns the value 1.25 & g returns the value 384 g 384 500 500 1.25 7

A pointer variable should be initialized to a valid address before being used: Example: float var = 3, count; /* Storage */ float *p1, *p2 = & var; /* Pointers */ p1 = &count; printf( "%p %p %f\n", &p2, p2, *p2 ); NULL is a predefined macro used for initializing a pointer so that it does not point to anything Example: double *p, *q = NULL; p = NULL; 8

Examples: int x, y, z; int *p, *q, *r; /* Pointers */ x = 3; p = & x; q = & y; r = p; y = x; z = *p; *p = 7; *q = *p; 9

int *p, *q, *r; /* Pointers */ int x, y, z; int *p, *q, *r; /* Pointers */ x = 3; (store a 3 in x) x Address Ax y Address Ay p Address Ap q Address Aq z Address Az r Address Ar 3 x Address Ax x Address Ax 3 10

p = &x; (store address of x in pointer p) q = &y; (store address of y in pointer q) Ax p Address Ap x Address Ax 3 p Address Ap x Address Ax Ax 3 Ay q Address Aq y Address Ay q Address Aq y Address Ay Ay 11

r = p; (copy contents of pointer p to pointer r) Address Ap r Address Ar Ax r Address Ar Ax p Address Ap x Address Ax 3 12

y = x; (copy contents of x to y) z = *p; (copy contents of x to z via *p) x Address Ax y Address Ay 3 y Address Ay 3 p Address Ap x Address Ax Ax z Address Az 3 z Address Az 3 p contains a memory address. If *p is on the right side of the equal sign, it retrieves the value stored at that memory address. 13

*p = 7; (store a 7 in x via *p) Address Ap x Address Ax Ax 7 3 x Address Ax 7 p contains a memory address. If *p is on the left side of the equal sign, the memory location that p points to will store the value on the right side. 14

*q = *p; (copy contents of x to y via pointers) Address Aq y Address Ay Ay p Address Ap x Address Ax Ax 7 3 y Address Ay 7 p contains a memory address. q contains a memory address. The memory location that q points to will get the value that is pointed to by p. 15

Assume addresses: x→100 y→104 z→108 p→120 q→124 r→128 Assume type int is 32 bit, and that pointers are also 32 bit. Start   End x = 3; p = &x; q = &y; r = p; y = x; z = *p; *p = 7; *q = *p; Addr Var Value 100 x 3 7 104 y ? 108 z 120 p 124 q 128 r 16

Levels of Indirection C supports multiple levels of indirection; e.g. Declarations: One level - datatype *pname; Pointer (e.g., int *p;) Two level - datatype **pname; Pointer to a pointer (e.g., float **q;) Three level - datatype ***pname; Pointer to a pointer to a pointer (e.g., double ***w;) 17

What does “multiple levels of indirection” mean? Example: int x; int *p; int **q; Standard variable → It stores a value of type integer. Pointer variable → It holds the address of a location in memory where data is stored. Pointer to a pointer variable → It holds the address of a location in memory that holds the address of another memory location where data is stored. 18

Example: int x; int *p; /* Pointer */ int **q; /* Pointer to another pointer */ x = 5; p = &x; /* p points to x */ q = &p; /* q points to p which points to x */ printf("%d %d %d\n", x, *p, **q); /* Displays 5 5 5 */ *p = 4; /* One level of indirect access */ printf("%d %d %d\n", x, *p, **q); /* Displays 4 4 4 */ **q = 3; /* Two levels of indirect access */ printf("%d %d %d\n", x, *p, **q); /* Displays 3 3 3 */ p = NULL; 19

Assume addresses: x→1150 p→1300 q→1500 Assume type int is 32 bit, and that pointers are also 32 bit. Start   End x = 5; p = &x; q = &p; *p = 4; **q = 3; p = NULL; Addr Var Value 1150 x 5 4 3 1300 p ? NULL 1500 q Conceptual diagram (just before executing *p = 4;) 1500 1300 q 1300 1150 p 1150 5 x 20

With great power comes great responsibility … Common error → Using a non-initialized pointer Pointer variable p contains random address x = *p; retrieves value from that random address → Program now mistakenly uses incorrect value *p = x; writes a value to that random address → Corrupted memory (may get memory fault message, or no warning at all) Always initialize a pointer to a valid address or NULL. If p is set to NULL, then *p generates a run-time error, which alerts you that a problem exists. 21