C pointers. (Reek, Ch. 6) 1 CS 3090: Safety Critical Programming in C

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Stewart Wells
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1 C pinters (Reek, Ch. 6) 1

2 Review f pinters A pinter is just a memry lcatin. A memry lcatin is simply an integer value, that we interpret as an address in memry. The cntents at a particular memry lcatin are just a cllectin f bits there s nthing special abut them that makes them ints, chars, etc. Hw yu want t interpret the bits is up t yu. Is this... an int value?... a pinter t a memry address?... a series f char values? 0xfe4a10c5 2

3 Review f pinter variables A pinter variable is just a variable, that cntains a value that we interpret as a memry address. Just like an uninitialized int variable hlds sme arbitrary garbage value, an uninitialized pinter variable pints t sme arbitrary garbage address char *m; (char *) m 3

4 Fllwing a garbage pinter What will happen? Depends n what the arbitrary memry address is: If it s an address t memry that the OS has nt allcated t ur prgram, we get a segmentatin fault If it s a nnexistent address, we get a bus errr Sme systems require multibyte data items, like ints, t be aligned: fr instance, an int may have t start at an even-numbered address, r an address that s a multiple f 4. If ur access vilates a restrictin like this, we get a bus errr If we re really unlucky, we ll access memry that is allcated fr ur prgram We can then prceed t destry ur wn data! 4

5 Hw can yu test whether a pinter pints t smething meaningful? There is a special pinter value NULL, that signifies pinting t nthing. Yu can als use the value 0. char *m = NULL;... if (m) {... safe t fllw the pinter... } Here, m is used as a Blean value If m is false, aka 0, aka NULL, it is nt pinting t anything Otherwise, it is (presumably) pinting t smething gd Nte: It is up t the prgrammer t assign NULL values when necessary 5

6 Indirectin peratr * Mves frm address t cntents char *m = dg ; char result = *m; d g NUL (char *) m gives an address f a char m *m instructs us t take the cntents f that address result gets the value d d result 6

7 Address peratr & Instead f cntents, returns the address char *m = dg, **pm = &m; d g NUL pm needs a value f type char ** m Can we give it *m? N type is char Can we give it m? N type is char * (char *) &m gives it the right value the address f a char * value (char **) pm 7

8 Pinter arithmetic C allws pinter values t be incremented by integer values char *m = dg ; d g NUL char result = *(m + 1); (char *) m gives an address f a char m (m + 1) gives the char ne byte higher *(m + 1) instructs us t take the cntents f that address result gets the value result 8

9 Pinter arithmetic A slightly mre cmplex example: char *m = dg ; char result = *++m; d g NUL m gives an address f a char (char *) ++m changes m, t the address ne byte higher, m and returns the new address *++m instructs us t take the cntents f that lcatin result gets the value result 9

10 Pinter arithmetic Hw abut multibyte values? Q: Each char value ccupies exactly ne byte, s bviusly incrementing the pinter by ne takes yu t a new char value... But what abut types like int that span mre than ne byte? A: C des the right thing : increments the pinter by the size f ne int value 17 (int) 42 (int) int a[2] = {17, 42}; int m = a; int result = *++m; (int *) m 42 (int) result 10

11 Example: initializing an array #define N_VALUES 5 flat values[n_values]; (flat []) values &values[0] &values [N_VALUES] (flat) (flat) (flat) (flat) (flat) 0 0 (dne!) (flat *) flat *vp; vp fr ( vp = &values[0]; vp < &values[n_values]; ) *vp++ = 0; 11

12 A nte n assignment: Rvalues vs. Lvalues What s really ging n in an assignment? Different things happen n either side f the = int a = 17, b = 42; b = a; a is the rvalue (right value) We g t the address given by a... and get the cntents (17) 17 (int) a 42 (int) b b is the lvalue (left value) We g t the address given by b...and get the cntents? N! We dn t care abut 42! We just want the address f b t stre 17 int 12

13 A nte n assignment: Rvalues vs. Lvalues This explains a certain asymmetry in assignments invlving pinters: d g NUL char *m = NULL, **pm = NULL; m = dg ; pm = &m; (char *) Here, m is an lvalue It s understd that the address f m is what s needed m (char **) Once again, we need the address f m but since it s an rvalue, just plain m will give the cntents f m use & t get the address instead pm 13

14 Example: strcpy string cpy char *strcpy(char *dest, cnst char *src) (assume that) src pints t a sequence f char values that we wish t cpy, terminated by NUL (assume that) dest pints t an accessible prtin f memry large enugh t hld the cpied chars strcpy cpies the char values f src t the memry pinted t by dest strcpy als gives dest as a return value 14

15 Example: strcpy string cpy char *strcpy(char *dest, cnst char *src) { } cnst char *p; char *q; fr(p = src, q = dest; *p!= '\0'; p++, q++) *q = *p; *q = '\0'; return dest; (char *) src d g NUL p (char *) (char *) q (char *) dest d g NUL 15

16 Pinter subtractin and relatinal peratins Only meaningful in special cntext: where yu have tw pinters referencing different elements f the same array q p gives the difference (in number f array elements, nt number f bytes between p and q (in this example, 2) p < q returns 1 if p has a lwer address than q; else 0 (in this example, it returns 1) (flat *) p (flat *) q (flat) (flat) (flat) (flat) (flat) (flat) (flat) (flat) (flat) (flat) 16

LAB 7 (June 29/July 4) Structures, Stream I/O, Self-referential structures (Linked list) in C

LAB 7 (June 29/July 4) Structures, Stream I/O, Self-referential structures (Linked list) in C Due: July 9 (Sun) 11:59 pm 1. Prblem A Subject: Structure declaratin, initializatin and assignment. Structure