CS-1030 Dr. Mark L. Hornick 1 Pointers are fun!

CS-1030 Dr. Mark L. Hornick 2 Pointers are… …used to store the address of an object An address is legitimate data by itself in C Every memory location has an address …a data type Ex: int is a datatype “pointer to int” is also a (separate) datatype Since they hold memory addresses, all pointers are the same size in bytes (unlike the data they point to)

CS-1030 Dr. Mark L. Hornick 3 Pointer declaration confusion Declaration of a pointer uses the * prefix int *pValue; // pointer to an integer int* pValue; // another way of declaring the same Be careful with multiple declarations on one line: int *pValue1, *pValue2; int* pValue1, pValue2; // No!!! int* pValue1; // OK int* pValue2; // OK

CS-1030 Dr. Mark L. Hornick 4 Pointers – unary operators New unary operators: & prefix – “address of” (can use on pointers or regular variables) int aValue = 3; int *pValue = &aValue; // sets pValue to the addr of aValue * prefix - dereference (can only use on pointers) int someOtherValue; someOtherValue = *pValue; // sets someOtherValue = aValue

CS-1030 Dr. Mark L. Hornick 5 Pointers – unary operators New unary operators: ++ prefix or postfix – increment the address stored in the pointer int aValue = 3; int *pValue = &aValue; // addr might be 0x1234 pValue++; // new addr is 0x1236; why not 0x1235???? // what does (*pValue)++ do??? -- prefix or postfix – decrement the address stored in the pointer

CS-1030 Dr. Mark L. Hornick 6 Confusing syntax explained int aValue = 1; // an int int anotherValue = 2; int* pValue = &aValue; // ptr to aValue // above line is same as: // int* pValue; // ptr, but not init’d // pValue = &aValue; // now init’d pValue = &anotherValue // points to anotherValue // what does pValue = anotherValue mean??? // whatever pValue points to now equals aValue: *pValue = aValue;// same as anotherValue=aValue; A common C convention: pointer variable names are prefixed with “p”

CS-1030 Dr. Mark L. Hornick 7 More Confusing syntax int aValue = 1; // an int int anotherValue = 2; int* p1 = &aValue; // ptr to aValue int* p2 = &anotherValue; *p1 = *p2; // same as aValue=anotherValue p1 = p2; // now both point to anotherValue

CS-1030 Dr. Mark L. Hornick 8 The NULL pointer Pointers should only be made to point at valid addresses The exception is the NULL pointer, where int* pValue = NULL; // points to address 0 // or int* pValue = 0; This is a convention that allows you to check to see if a pointer is valid

CS-1030 Dr. Mark L. Hornick 9 Pointing a pointer at a valid address Since a pointer holds an address, the pointer can be made to point anywhere in (data) memory uint8_t* pValue; // 16-bit addr of 8-bit value pValue = 0x0038; // what is at 0x38??? Now explain what this does: *pValue = 0xFF;

CS-1030 Dr. Mark L. Hornick 10 Pointers and arrays Say you define an array of characters: char s[]=“ABCD”; The string variable is actually a pointer to the beginning of the array, so we can write: char* ps = s; // or ps = &s[0]; Dereferencing the pointer gives the value of the character at that address: char c = *ps; // same as c = s[0]; Incrementing the pointer advances the pointer address to the next character in the array: ps++; // same as ps = &s[1];

Pointer Arithmetic Incrementing a character pointer advances the pointer address by one byte, to the next character in the array: ps++; // value of ps increases by 1; Say you define an array of longs: long larray[]= {1L, 2L, 3L, 4L}; long* pl = larray; Incrementing a long pointer advances the pointer address by four bytes, to the next long in the array: pl++; // value of pl increases by 4; CS-1030 Dr. Mark L. Hornick 11 Pointer arithmetic is dependent on the size of the element the pointer is declared to reference.

CE-2810 Dr. Mark L. Hornick 12 Pointers as function arguments Consider a function: int16_t add( uint8_t* px, uint16_t* py); uint8_t* means “pointer to unsigned 8-bit int” uint16_t* means “pointer to unsigned 16-bit int” Since Atmega32 uses 2-byte addressing, all pointers are 2 byte variables Since Atmega32 uses 2-byte addressing, all pointers are 2 byte variables GNU GCC passes arguments left to right using registers r25 to r8 Above, value of px is placed in r24:r25 (low, high bytes) Px’s value is the address of some 8-bit uint8_t value somewhere in memory Value of py (2 bytes) is placed in r22:r23 Calling such a function: uint8_t x=3; uint16_t y=4; uint8_t* px = &x; uint16_t* py = &y; int16_t sum = add(&x, &y); sum = add(px, py); px(high) px(low) x=3...

CE-2810 Dr. Mark L. Hornick 13 Within the function, the arguments must be dereferenced to manipulate the values Implementing such a function: uint16_t add(uint8_t* px,uint16_t* py ) { uint8_t tempx = *px; uint16_t tempy = *py; uint16_t sum = tempx + tempy; //or uint16_t sum = *px + *py; return sum; }

CS-1030 Dr. Mark L. Hornick 14 The const specifier Prevents objects passed by address from being changed within a function Here’s the modified declaration of the method that passes by address: void printValue(const int* pValue); Usage: printName( &value );

CS-1030 Dr. Mark L. Hornick 15 Notation for const and pointers is tricky int x; const int* ptr1 = &x; // x is constant // Can’t use ptr1 to change x, but can change ptr1 to point to another object int* const ptr2 = &x; // ptr2 constant // Can’t change ptr2 to another object, but can change value of x const int* const ptr2 = &x; // Can’t do either