Overview. CSC 2400: Computer Systems. Pointers in C. Pointers - Variables that hold memory addresses - Using pointers to do call-by-reference in C

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1 CSC 2400: Comuter Systems Ponters n C Overvew Ponters - Varables that hold memory addresses - Usng onters to do call-by-reference n C Ponters vs. Arrays - Array names are constant onters Ponters and Strngs 1

2 Exercse: Wrte a swa functon vod swa(nt x, nt y) { nt man(vod) { nt a = 11, b = 22; swa(a,b); rntf( a=%d, b=%d\n, a, b); Outut should be a=22, b=11 Memory Allocaton Each tme a varable s defned, memory sace s set asde for t. The amount of memory deends on the tye of the varable. Examles: char nt 1 byte 2 or 4 or 8 bytes The memory sace s accessed usng the varable name char c; nt ; 0xFFFFFFFF Memory Address RAM (man memory) 2

3 Functon Parameters C always asses arguments BY-VALUE Memory allocated for arameters when functon s called Argument VALUES coed nto newly allocated memory Memory automatcally de-allocated when functon ends vod swa(nt x, nt y) { nt man(vod) { nt a = 11, b = 22; swa(a,b); rntf( a=%d, b=%d\n ); Vsualzng Memory Sace vod swa(nt x, nt y) { nt man(vod) { nt a = 11, b = 22; swa(a,b); rntf( a=%d, b=%d\n ); y = 22 x = 11 b = 22 a = 11 Memory (stack) 3

4 What are Ponters? A onter s a varable that holds the ADDRESS of another varable If s an nteger varable then & s the address of. The oerator ADDRESS-OF or the REFERENCE oerator. & s called the You can store ths address nto a onter varable: nt *; /* onter defnton */ = &; /* onter ntalzaton */ 5 Vsualzng Ponters n Memory nt = 5; nt * ; = &; 100 = = 100 Memory 4

5 Declarng Ponters When declarng several onter varables n one statement - the astersk does not dstrbute throughout the statement: nt *, q; equvalent to nt * ; nt q; equvalent to nt *, * q; nt * ; nt * q; Intalzng Ponters We can also ntalze a onter at the ont of declaraton: nt ; nt * = &;??? Here s a common error: nt ; nt * = ; // ERROR: s not an address 5

6 Dereference * The value of the data beng onted at s obtaned by usng the dereference oerator *,* 77 If s a onter value, then * refers to the varable onted to by. Dereference Examle nt ; nt * = &; * = 101; (*)++;?????? Equvalent to = 101; Equvalent to ++; A dereferenced onter behaves exactly lke the varable t onts to. 6

7 swa Functon Revsted vod swa(nt *x, nt *y) { nt man(vod) { nt a = 11, b = 22; swa(, ); rntf( a=%d, b=%d\n ); Outut should be: a=22, b= y = 108 x = 112 b = 22 a = 11 Memory (stack) Note the Dfference Assume: j 5 7 After 1 = 2; 2 j After 2 j 7 *1 = *2; 7

8 Unntalzed Ponters Suose that we have the followng declaratons: nt ; nt * ; * = 100;??? What s the value of? Undefned! could hold an address that does not make sense at all, causng your rogram to crash f dereferenced. could ont to an address whch s accessble. Then the assgnment * = 100; would accdentally change some other data, whch could result n a crash at a later ont. Ths s a tough error to detect snce the cause and symtom may be wdely searated n tme. Puttng t all Together... nt, value; nt * ; // declares to be a onter to an nteger = 510; /* Ste 1 */ = &; /* Ste 2 */ value = *; /* Ste 3 */ After Ste1: After Ste2: value value After Ste3: value

9 The null Ponter The value of a onter can be: some garbage (onter unassgned) the address of some varable (for nstance, nt * = &; ) the constant 0 (the null onter, onts to absolutely nothng) someponter = 0; Ths statement does not cause someponter to ont to memory locaton zero; t guarantees that someponter does not ont to anythng. The null onter s a secal onter value that a rogram can test for: f (someponter == 0)... Arrays vs. Ponters 9

10 Arrays and Ponters An array name s bascally a constant onter Consder the declaraton: nt a[3]; a, &a[0] 1000 &a[1] 1004 &a[2] a[0] a[1] a[2]... The comler allocates three ntegers for the array object that occuy a contguous block of memory. The value of a s exactly &a[0], the address of the frst nteger n the array Arrays and Ponters - Examles Consder the followng declaratons: nt a[5] = {1, 2, 3; a 1 2 3?? nt * ; = &a[2]; a 1 2 3?? You can use the ndex [] oerator wth a onter: [0] = 17; [1] = 23; a ? Indexng can be used wth any onter, but t only makes sense when the onter onts to an array. 10

11 Arrays are NOT Ponters Declarng an array sets asde sace for ts elements char a[5]; a a[0] a[1] a[2] a[3] a[4] Declarng a onter varable sets asde only sace to hold the varable char * ; You can change a onter varable, but not the address of an array char b[6]; = b; // OK b = ; // ERROR! b b[0] b[1] b[2] b[3] b[4] b[5] Indexng Ponters nt a[5]; nt *, *q; = a; [1]= 44; q = + 2; q a q[-1] = 43; q[2] = 46; 11

12 Ponter Arthmetc nt a[5]; 108 a Subscrt: a[] means *(a+) 4 bytes nt *; = a + 2; /* f a s 100, a+2 s 108 = * szeof(nt) */ Note: arthmetc scales by data sze (e.g., nt of 4 bytes) Quant usage of onter arthmetc Add u the elements of an array: nt a[100]; nt sum, *;... for (=a; <a+100; ++) sum += *; More straghtforwardly: nt a[100]; nt sum, ;... for (=0; <100; ++) sum += a[]; 12

13 Array Parameters to Functons vod rntarray(nt *, nt n) { nt ; for (=0; <n; ++) rntf( %d\n,[]); nt fb[5] = {1, 1, 2, 3, 5; nt man(vod) { rntarray(fb, 5); Array Params Ponter Params vod rntarray(nt *, nt n) {... vod rntarray(nt [5], nt n) {... vod rntarray(nt [ ], nt n) {... vod rntarray(nt [1000], nt n) {... nt man(vod) { All these declaratons are equvalent! Try them out. rntarray(fb, 5); 13

14 Exercse: Reverse Array Reverse the values n an array Inuts: nteger array a, and number of elements n Outut: values of a stored n reverse order Algorthm Swa the frst and last elements n the array Swa the second and second-to-last elements Imlement usng onter arthmetc Revew: Strngs A strng s just an array of characters (onter to character), termnated by a \0 char (a null, ASCII code 0). char mystrng[6] = { H, e, l, l, o, \0 ; char mystrng[6] = Hello ; char mystrng[] = Hello ; Equvalent mystrng H e l l o \0 char *yourstrng = Hello ; Dfferent yourstrng H e l l o \0 14

15 Strng and Ponter Manulaton char mystrng[] = Hello ; char *yourstrng = Hello ; mystrng H J e l l y o \0 yourstrng H C e l l o \0 mystrng[0] = J ; yourstrng[0] = C ; yourstrng = mystrng; yourstrng[4] = y ; mystrng = yourstrng; Try out ths code. Comutng the Length of a Strng nt length(char* s) { char* = s; whle (*) ++; return ( s); s Try out ths code. mystrng H e l l o \0 15

16 Ponters and Strngs vod f(char *s) { char * = s; whle (*s) s++; b a r \0 for (s--; s > ; s--,++) { char c = *s; *s = *; * = c; What does ths functon do? Boxes and Arrows DRAW dagrams! They really hel. Examle: you want an array of strngs: NULL z e r o \0 o n e \0 t w o \0 char *query[4] = { zero, one, two,null; how to arse t: *(query[4]) ostfx oerators bnd tghter than refx; whenever you re not sure, just ut the arentheses n 16

17 Summary Ponters - Varables that hold memory addresses - Use wth oerators & (reference) and *(dereference) - Should ALWAYS be ntalzed before use - The NULL onter onts to nothng - Can be ndexed just lke arrays - Ponter arthmetc scales by data sze 17

Data Types. Tour of C. Like Java, Like C. Array Initialization. Array Declaration. Data Types, Arrays, Strings and Pointers in C.

Data Types. Tour of C. Like Java, Like C. Array Initialization. Array Declaration. Data Types, Arrays, Strings and Pointers in C. Dt Tyes Tour of Dt Tyes, Arrys, Strngs nd Ponters n chr short nt long flot double long double sgned/ unsgned chr, short, nt, long 2 Lke Jv, Lke Oertors sme s n Jv - Arthmetc nt = +; ++; --; *=2; +, -,