Pointers in C. Address vs. Value. Another Need for Addresses. Executing the Swap Function

Transcription

1 Address vs. Vlue Sometimes we wnt to del with the ddress of memory loction, rther thn the vlue it contins Pointers in C Bsed on slides McGrw-Hill Additionl mteril 200/2005 Lewis/Mrtin Modified y Din Plseti Adding column of numers in LC-: R2 contins ddress of loction Red vlue, dd to sum, nd increment R2 until ll numers hve een processed R2 is pointer It contins the ddress of dt R2 x100 ddress vlue x107 x2819 x0110 x010 x0100 x1110 x11b1 x0019 x100 x101 x102 x10 x10 x105 x106 x107 CIT 59 2 Another Need for Addresses Consider the following function tht's supposed to swp the vlues of its rguments. void swp(int, int ){ int temp = ; = ; = temp; int min(){ int = ; int = ; swp(,); CIT 59 Executing the Swp Function efore cll fter cll swp min temp These vlues chnged... Swp needs ddresses of vriles outside its own ctivtion record...ut these did not. CIT 59 1

2 Pointers in C C lets us mnipulte ddresses s vriles nd in use them in expressions. Declrtion int *p; /* p is pointer to n int */ A pointer in C is lwys pointer to prticulr dt type: int*, doule*, chr*, etc. Opertors *p -- returns the vlue pointed to y p.k. de-referencing pointer &z -- returns the ddress of vrile z int min(){ int i; int *ptr; Exmple store the vlue into the memory loction ssocited with i i = ; ptr = &i; *ptr = *ptr + 1; printf( %d\n, i); store the ddress of i into the memory loction ssocited with ptr xeffb xeffd xeffe 5 ptr i red the contents of memory t the ddress stored in ptr nd increment it y 1 Vlue of i will e 5, ecuse i ws modified indirectly vi ptr CIT 59 5 CIT 59 6 Exmple: LC- Code ; i is (offset 1), ptr (offset 0 i.e. the lst locl vrile dded to stck frme) ; i = ; AND R0, R0, #0 ; cler R0 ADD R0, R0, # ; put in R0 ADD R6, R6, #-1 ;djust stck pointer STR R0, R6, #0 ; store in i on top of stck ; int *ptr = &i; ADD R0, R6, #0 ; R0 = R6 + 0 (ddr of i) ADD R6, R6, -1 ;crete spce for ptr STR R0, R6, #0 ; store ddr of i in ptr AND R5, R5, #0 ADD R5, R6, #0 ;min s frme pointer ; *ptr = *ptr + 1; LDR R0, R5, #0 ; R0 = contents of ptr i.e. ddress of i LDR R1, R0, #0 ; lod contents (*ptr) ADD R1, R1, #1 ; dd one STR R1, R0, #0 ; store to ddress of i CIT 59 7 Pointers s Arguments Pssing pointer into function llows the function to red/chnge memory outside its ctivtion record void swp(int *, int *) { int temp = *; * = *; * = temp; Arguments re integer pointers. Cller psses ddresses of vriles tht t it wnts function to chnge CIT

3 Pssing Pointers to Function Code Using Pointers min() wnts to swp the vlues of nd psses the ddresses to swp(): swp(&, &); Code for pssing rguments: ADD R0, R5, #0 ; ddr of ADD R6, R6, #-1; R5 (Min s FP) STR R0, R6, #0 ADD R0, R5, #1 ; ddr of ADD R6, R6, #-1; STR R0, R6, #0 xeffb xeffd xeffe temp Inside the swp() routine ; int temp = *; LDR R0, R5, #7 ; R0= LDR R1, R0, #0 ; R1=M[]= STR R1, R5, #0 ; temp= ; * = *; LDR R1, R5, #8 ; R1= LDR R2, R1, #0 ; R2=M[]= STR R2, R0, #0 ; M[]= ; * = temp; LDR R2, R5, #0 ; R2=temp= STR R2, R1, #0 ; M[]= R5 xeffb xeffd xeffe R5 R2 R1 R0 R7 ret vl temp CIT 59 9 CIT Using Arguments for Results Pss ddress of vrile where you wnt result stored Exmple: scnf("%d %d", &dt1, &dt2); red deciml integers into dt1 nd dt2 1. Argument is not n ddress int n = 0; scnf("%d", n); Bd scnf Arguments Will use the vlue of the rgument s n ddress If you're lucky, progrm will crsh ecuse of trying to modify restricted memory loction (e.g., loction 0) Runtime error: Segmenttion Fult 2. Missing dt rgument scnf("%d"); Your progrm will just modify n ritrry memory loction, which cn cuse very unpredictle ehvior Becuse it will get ddress from stck, where it expects to find dt rgument CIT CIT 59 12

4 Null Pointer Sometimes we wnt pointer tht points to nothing. In other words, we declre pointer, ut we re not redy to ctully point to something yet. int *p; p = NULL; /* p is null pointer */ NULL is predefined mcro tht contins vlue tht non-null pointer should never hold. Often, NULL = 0, ecuse Address 0 is not legl ddress for most progrms on most pltforms Dereferencing NULL pointer: progrm crsh! int *p = NULL; printf( %d, *p); // CRASH! Output: Segmenttion fult CIT 59 1 Exmples: Pointer Prolems Wht does this do? int *x; *x = 10; Answer: writes 10 into rndom loction in memory Wht s wrong with: int* func(){ int x = 10; return &x; Answer: storge for x disppers on return, so the returned pointer is dngling A dngling pointer is pointer to storge element(int, chr, doule etc) tht is no longer llocted CIT 59 1 Declring Pointers The * opertor inds to the vrile nme, not the type All the sme: int* x, y; int *x, y; int *x; int y; Suggested solution: Declre only one vrile per line Avoids this prolem Esier to comment Clerer Don t worry out sving spce Arry s Locl Vrile Arry elements re llocted s prt of the ctivtion record int grid[10]; First element (grid[0]) is t lowest ddress of llocted spce grid[0] grid[1] grid[2] grid[] grid[] grid[5] grid[6] grid[7] grid[8] grid[9] CIT CIT 59 16

5 LC- Code for Arry References ; x = grid[] + 1 ADD R0, R5, #1 ; R0 = &grid[0] LDR R1, R0, # ; R1 = grid[] ADD R1, R1, #1 ; R1 = R1 + 1 STR R1, R5, #0 ; x = R1 ; grid[6] = 5; AND R0, R0, #0 ADD R0, R0, #5 ; R0 = 5 ADD R1, R5, #1 ; R1 = &grid[0] STR R0, R1, #6 ; grid[6] = R0 R5 x grid[0] grid[1] grid[2] grid[] grid[] grid[5] grid[6] grid[7] grid[8] grid[9] Reltionship etween Arrys nd Pointers An rry nme is essentilly pointer to the element in the rry chr dt[10]; i.e. dt = ddr where element is locted = &dt[0] Exmple: chr dt[10]; chr *cptr; cptr = dt; /* points to dt[0] */ CIT CIT Correspondence etween Ptr nd Arry Nottion Given the declrtions on the previous pge, ech line elow gives three equivlent expressions: cptr dt &dt[0] (cptr + n) (dt + n) &dt[n] *cptr *dt dt[0] *(cptr + n) *(dt + n) dt[n] Bewre Arrys re not the sme s pointers lthough they my look like wht is the difference etween rrys nd pointers? Arrys utomticlly llocte spce, ut cn't e relocted or resized. Pointers must e explicitly ssigned to point to llocted spce ut cn e ressigned (i.e. pointed t different ojects) t will, nd hve mny other uses esides serving s the se of locks of memory. CIT CIT

6 Pointer Arithmetic: Sutrction nd Equlity Nsty, ut C llows it: void function(int* strt, int* end) { int i; while (end - strt >= 0) { *strt t = 0; strt++; In function min(): int rry[10]; function(&rry[0], &rry[9]); Don t do this! Alterntive: while (end!= strt) { Significntly etter, ut still d Wht if strt is > end, or not prt of sme rry? CIT More on Pointer Arithmetic Address clcultions depend on size of elements In our LC- code, we've een ssuming one word per element e.g., to find th element, we dd to se ddress It's ok, ecuse we've only shown code for int, which h tkes up one word (equl to mchine width). If doule, we'd hve to dd 8 to find ddress of th element. C does size clcultions under the covers, depending on size of item eing pointed to: doule x[10]; doule *y = x; *(y + ) = 100; sme s x[] -- se ddress plus 6 lloctes 20 words (2 per element) CIT Pointer-to-Pointer The declrtion of pointer-to-pointer looks like int **ipp; Exmple: int i = 5 int *ip1 = &i, int ** ipp = &ip1; C vs. Jv C Pointers mkes it unsfe s there is no compile time checking on: Dereferencing null pointer Hving dngling pointer Cn esily right to memory spce eyond wht you declred No limit checking for rry length Jv removes unsfe fetures y not supporting the unry & ddress opertor supporting ddress rithmetic on references i.e. does not llow integer vlues to e dded/sutrcted to references CIT 59 2 CIT