![Pointers and Arrays
Since arrays are a contiguous set of variables
in memory, we can access them with pointers
int arr[5];
int *p = &arr[0];
*(p+0) = 1; // arr[0]
*(p+1) = 2; // arr[1]
*(p+2) = 4; // arr[2]
*(p+3) = 8; // arr[3]
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C Pointers
- 1. Pointers in C Omar Mukhtar
- 2. Outline Review of concepts in previous lectures Introduction to pointers Pointers as function arguments Pointers and arrays Pointer arithmetic Pointer-to-pointer
- 3. Review and Background Basic data types − Place-holders for numeric data Integer numbers (int, short, long) Real numbers (float, double) Characters / symbols codes (char) Arrays − A contiguous list of a particular data type Functions − Give “name” to a particular piece of code − Modularization & reuse of code
- 4. Pointers The most useful and tricky concept in C language − Other high level languages abstract-out this concept The most powerful construct too − Makes C very fast − Direct interaction with hardware − Solves very complicated programming problems
- 5. What are pointers? Just another kind of “placeholder” to hold “address” of memory location − Address is also a number − Itself resides on some memory location Memory Address Value 0x8004 ... 0x8008 variable A 129 0x800C ... 0x8010 0x8008 address of A 0x8014 ...
- 6. What are pointers? Declare variables a, b − int a, b; Declare a pointer − int* pa; Set value of a − a = 10; Point pa to address of a − pa = &a; Set value of a using pa − *pa = 12; pa = &b;
- 7. Pointer Operators “address-of” operator: & − Gets address of a variable De-referencing operator: * − Accesses the memory location this pointer holds address of
- 8. Pointers and Functions A function can be passed arguments using basic data types − int prod(int a, int b) { return a*b; } How to return multiple values from function? − void prod_and_sum(int a, int b, int*p, int* s) { *p = a*b; *s = a+b; }
- 9. In & Out Arguments of Function A function may like to pass values and get the result in the same variables. e.g. a Swap function. void swap(int* a, int* b) { int c; c = *b; *b = *a; *a = c; } int a = 5; b = 6; swap(&a, &b); // a hold 6 and b hold 5 now.
- 10. Pointers and Arrays Since arrays are a contiguous set of variables in memory, we can access them with pointers int arr[5]; int *p = &arr[0]; *(p+0) = 1; // arr[0] *(p+1) = 2; // arr[1] *(p+2) = 4; // arr[2] *(p+3) = 8; // arr[3] ...
- 11. Pointer Arithmetic Arithmetic operators work as usual on ordinary data types. − int a = 1; a++; // a == 2 It gets a bit complicated when arithmetic operators are used on pointers int* p = 0x8004; p++; What does p hold now? 0x8005??? Compiler knows that p is a pointer to integer type data, so an increment to it should point to next integer in memory. Hence 0x8008.
- 12. Pointer Arithmetic So an arithmetic operator increases or decreases its contents by the size of data type it points to int* pi = 0x8004; double* pd = 0x9004; char* pc = 0xa004; pi++; // pi == 0x8008 pd++; // pd == 0x900c pc++; // pc == 0xa005 Only '+' and '-' operator are allowed. '*' and '/' are meaningless.
- 13. Pointer-to-Pointer Pointer variable is just a place-holder of an address value, and itself is a variable. − Hence a pointer can hold address of other pointer variable. In that case it is called a “double pointer”. int*p; int **pp; pp = &p; e.g a function may like to return a pointer value void pp_example(int** p) { *p = 0x8004; } int *p; pp_example(&p);
- 14. Pointer Pitfalls Since pointer holds address of memory location, it must never be used without proper initialization. An uninitialized pointer may hold address of some memory location that is protected by Operating System. In such case, de- referencing a pointer may crash the program. An initialized pointer does not know the memory location, it is pointing to is, holds a valid value or some garbage. A pointer cannot track boundaries of an array.