Introduction to C: Pointers

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1 Introduction to C: Pointers <Pointer> <Objects> <Pointer> Nils Moschüring PhD Student (LMU) Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 1

2 1 Introduction 2 Pointers Basics Useful: Function arguments Useful: Arrays Double Pointers void Pointers 3 Function pointers 4 Riddle & End Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 2

3 Introduction Pointers are a specific type of variable available in some programming languages, most notably C and C++. The history of pointers is comprised of opposites: they enable greatness they promote readability they are the best thing about C they enable great fubarness they exist only to brag they are the most evil thing about C Of course, both sides are totally right. That s why they are interesting to understand. Many modern programming languages have abolished them. The PSC makes heavy use of them. Important term: ud=undefined behaviour Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 4

4 Memory and Values This is our physical memory. Each memory is referred to by a unique address. Value Address #1 #2 #3 #4 #5 #6 #7 #8 After 1 i n t i = 3; we get: Value 3 Address #1 #2 #3 #4 #5 #6 #7 #8 The address of object i is #2. The OS decides that! Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 5

5 So what do pointers do? Let s define and initialize a pointer: 1 i n t i p = NULL; Result: Value 3 Null Address #1 #2 #3 #4 #5 #6 #7 #8 Now we let it point to i by setting the value of ip to &i: 2 i p = & i ; The ampersand (&) is called address of or reference operator. Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 7

6 So what do pointers do? Now we get this: Value 3 #2 Address #1 #2 #3 #4 #5 #6 #7 #8 And the following is true: 1 i == 3; 2 i p == 3; 3 i p == #2 == & i ; 4 &i p == #5; The asterisk ( ) is called value of or dereference operator. Note: The asterisk in the definition of ip is not an operator, but a part of the variable type (ip is of type pointer to int). Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 8

7 What s the use? Why do we need them? Call by Reference Call by Value Less copying Return more values New array manipulation techniques. C Arrays are always pointers! Messing things up really hard. What do other languages do? They all utilize pointers, but you can not directly control them. Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 9

8 Function arguments and pointers Our code: 1 struct b i g S t r u c t { 2 i n t s t u f f [ ] ; 3 } ; 4 i n t main ( ) { 5 struct b i g S t r u c t mybigstruct ; 6 func ( mybigstruct ) ; 7 return 1; 8 } Now we want to do stuff in this function. One way: 9 i n t func ( struct b i g S t r u c t a ) { 10 p r i n t f ( %d\n, a. s t u f f [ 5 0 ] ) ; 11 } THIS IS BAD! Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 10

9 Function arguments and pointers Better way to do it: 1 i n t func ( struct b i g S t r u c t * a ) { 2 p r i n t f ( %d\n, a >s t u f f [ 5 0 ] ) ; 3 } 4 i n t main ( ) { 5 struct b i g S t r u c t mybigstruct ; 6 func (& mybigstruct ) ; 7 return 0; 8 } This won t copy anything but ONE address (4-8 byte)! And you can write, not only read, all the values! Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 11

10 Pointers to struct What s the arrow? 1 p r i n t f ( %d\n, a >s t u f f [ 5 0 ] ) ; It must be equal to: 2 p r i n t f ( %d\n, ( a ). s t u f f [ 5 0 ] ) ; So it s a shortcut which makes the code much easier to read. The arrow operator, -> (that s a minus sign followed immediately by a greater than), dereferences a pointer to select a field. Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 12

11 Arrays in C and C++ What happens when creating an array? 1 i n t array [ 5 ] ; It looks strange, but this: Value #3 Address #1 #2 #3 #4 #5 #6 #7 #8 With the following true: 2 array == #3; 3 &array == #3; The first expression shows the automatic decay mechanism of C: The array automatically decays into the pointer pointing to it when only referenced by its name (in most cases). How is that an array?! Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 13

12 Arrays in C and C++ Let s find out! You could do, for example: 1 array [ 0 ] = 9; now, in the physical memory, you would like to have: Value #3 9 Address #1 #2 #3 #4 #5 #6 #7 #8 And this is what you get! So what is this [0] operator doing? It must be equal to the asterisk! Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 14

13 Arrays in C and C++ But what about: 1 array [ 3 ] = 10; You will get: Value # Address #1 #2 #3 #4 #5 #6 #7 #8 So the above equals to: 2 ( array + 3) = 10; The jumping increment is determined by the pointer type (no void pointers UD!). The [] is called offset operator. Note: The brackets in the definition of array are not operators, they are part of the type array. Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 15

14 Summary of Arrays in C and C++ 1 i n t array [ 5 ] ; 2 i n t const array = ( i n t ) malloc ( sizeof ( array ) 5 ) ; What does this do: 3 char p = NULL; 4 for ( p = N i l s ; p!= \0 ; p++) { 5 p r i n t f ( %c, p ) ; 6 } CORRECT! Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 16

15 Constant strings in depth What is Nils equal to? C will create a constant string in a buffer by internally calling something like: 1 const char b u f f e r [ 5 ] ; But what does this mean? Inserting the approximation given above, it should approximately be: 2 const char const b u f f e r =( char ) malloc ( sizeof ( b u f f e r ) 5 ) ; After that, C will insert the characters and add a trailing \0. Nils is a pointer type object with two specialties: Its value (i.e. the address it points to) can not be written (the second const). 3 / N i l s = / b u f f e r =& i ; / / Compiler e r r o r The memory to which it points can not be written (the first const). 4 / N i l s [ 1 ] = / b u f f e r [ 1 ] = o ; / / Compiler / runtime e r r o r Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 17

16 Higher Dimensional Arrays Think about this statement: 1 i n t array2d [ 1 0 ] [ 1 0 ] ; According to what we ve seen so far, the following 2 array2d [ 2 ] [ 3 ] = 4; means: 3 ( ( array2d + 2) + 3) = 4; An array of pointers, pointed to by our array variable. Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 18

17 Obvious follow-up: Pointers to Pointers We declare and assign a double pointer: 1 i n t ipp = &i p ; Now we ll get this: Value 3 #2 #5 Address #1 #2 #3 #4 #5 #6 #7 #8 If the address of ipp is #7. Things that are true in this case: 2 i == i p == ipp == 3; 3 ipp == i p == & i == #2; 4 ipp == &i p == #5; 5 &ipp == #7; Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 19

18 What s the use of this mess? (except multidimensional arrays) You can thus change where the pointers are pointing to! 1 i n t j ; 2 ipp = & j ; Now, if the address of j is #4, you ll get this: Value 3 #4 #5 Address Variable #1 #2 #3 #4 #5 #6 #7 #8 i j ip ipp This enables you to change the targets of pointers via functions. In certain situations this can make the code far better. (and really difficult to understand) Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 20

19 Traps and problems Suppose you need a function which creates a list of objects for you: 1 struct m y s t r u c t l i s t ; 2 generate my structs (& l i s t ) ; Double pointers are needed! Lets look at this function: 3 void generate my structs ( struct m y s t r u c t l i s t ) { 4 l i s t = malloc (5 sizeof ( l i s t ) ) ; How can write a data field of the second object? 5 ( l i s t ) [ 2 ]. a t t r = 5; 6 } wrong (operator precedence): 7 l i s t [2] > a t t r = 5; 8 * l i s t [ 2 ]. a t t r = 5; First one will not produce a compilation error! In most cases not even a runtime error! Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 21

20 Solution to this Avoid double pointers. Seriously. How to do it without them? Use more levels of structures No double pointers needed Extra level of names which always helps to clarify the code 1 struct m y l i s t s t r u c t { 2 struct m y s t r u c t l i s t ; 3 } ; 4 void generate ( struct m y l i s t s t r u c t l i s t s t r u c t ) { 5 l i s t s t r u c t > l i s t = 6 malloc (5 sizeof ( l i s t s t r u c t > l i s t ) ) ; 7 l i s t s t r u c t > l i s t [ 2 ]. a t t r = 5; 8 } 9 struct m y l i s t s t r u c t l i s t s t r u c t ; 10 generate (& l i s t s t r u c t ) ; Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 22

21 void Pointers It s a special type of Pointer Pointers pointing to a value that has no type They can point to any datataype (and even functions) They are essentially just bare pointers, just addresses They can t be directly dereferenced, as the result wouldn t have a type You need to always typecast them to a different pointer type before dereferencing. Example: 1 i n t main ( ) { 2 void vp = malloc ( sizeof ( i n t ) ) ; 3 p r i n t f ( %d\n, ( ( i n t ) vp ) ) ; 4 f r e e ( vp ) ; 5 return 0; 6 } Incrementing and decrementing (or using the offset operator) will use 1 byte as the step size (even though sizeof(void) does not work). Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 23

22 void Pointers, two examples The malloc function Definition: 1 void malloc ( s i z e t size ) ; The OS never knows about the type of your memory But it does remember its length. void function arguments 2 void increase ( void data, i n t psize ) { 3 i f ( psize == sizeof ( char ) ) 4 { char pchar ; pchar = ( char ) data ; ++( pchar ) ; } 5 else i f ( psize == sizeof ( i n t ) ) 6 { i n t p i n t ; p i n t = ( i n t ) data ; ++( p i n t ) ; } 7 } Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 24

23 Function pointers You can create pointers to functions. This enables, among other things, Late-Binding (PSC!). Declaration: 1 i n t ( fp ) ( int, i n t ) ; This is a function pointer to a function of the following type: return value: int two int arguments Functions with prototype: int function( int, int ); Assignement & Function call: 2 fp = &f u n c t i o n ; 3 ( fp ) ( 1, 1 ) ; / / Employing the ( ) f u n c t i o n c a l l operator The Ampersand can also be left out, since function pointers are also subject to automatic decay. Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 26

24 Function pointers, example Obligatory senseless example: 1 i n t s u b s t r a c t i o n ( i n t i, i n t j ){ return i j ; } 2 i n t plus ( i n t i, i n t j ){ return i + j ; } 3 i n t operation ( i n t i, i n t j, i n t ( fp ) ( int, i n t ) ) { 4 return ( fp ) ( i, j ) ; 5 } 6 i n t main ( ) { 7 i n t ( minus ) ( int, i n t ) = &s u b s t r a c t i o n ; 8 p r i n t f ( %d\n, operation ( 1, 1, minus ) ) ; 9 p r i n t f ( %d\n, operation ( 1, 1, &plus ) ) ; 10 return 0; 11 } Returns: Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 27

25 Riddle & End 1 i n t number ( i n t i ){ return i 3;} 2 i n t main ( i n t arg ) { 3 s t a t i c i n t count = 0; 4 i n t ( f p a r r a y [ 1 0 ] ) ( i n t ) ; 5 for ( i n t i =0; i <10; i ++) f p a r r a y [ i ] = &number ; 6 ( f p a r r a y + 4) = &main ; 7 for ( i n t i = ( f p a r r a y[ arg ] ) ( arg ) ; i <10; i ++) { 8 p r i n t f ( %d, ( ( f p a r r a y + i ) ) ( i ) ) ; 9 count ++; i f ( count >5) break ; 10 } 11 return 666; 12 } Take-Home-Message Now you re thinking in pointers... Nils Moschüring PhD Student (LMU), Introduction to C: Pointers 29

Reference operator (&)

Reference operator (&) Pointers Each cell can be easily located in the memory because it has a unique address and all the memory cells follow a successive pattern. For example, if we are looking for cell 1776 we know that it