L04 dg c2

Information about L04 dg c2

Published on December 10, 2007

Author: Melinda

Source: authorstream.com

Content

Slide1:  Lecturer PSOE Dan Garcia www.cs.berkeley.edu/~ddgarcia inst.eecs.berkeley.edu/~cs61c CS61C : Machine Structures Lecture 4 – C Pointers 2005-01-26 Machine learns games!  A computer has learned to play “rock, paper, scissors” by observing and mimicking humans! This is one step toward the game theory holy grail - show it the rules and it plays well! www.comp.leeds.ac.uk/vision/cogvis/games.html Pointers & Allocation (1/2):  Pointers & Allocation (1/2) After declaring a pointer: int *ptr; ptr doesn’t actually point to anything yet. We can either: make it point to something that already exists, or allocate room in memory for something new that it will point to… (next time) Pointers & Allocation (2/2):  Pointers & Allocation (2/2) Pointing to something that already exists: int *ptr, var1, var2; var1 = 5; ptr = &var1; var2 = *ptr; var1 and var2 have room implicitly allocated for them. var1 ? var2 ? 5 5 ? More C Pointer Dangers:  More C Pointer Dangers Declaring a pointer just allocates space to hold the pointer – it does not allocate something to be pointed to! Local variables in C are not initialized, they may contain anything. What does the following code do? void f() { int *ptr; *ptr = 5; } Arrays (1/6):  Arrays (1/6) Declaration: int ar[2]; declares a 2-element integer array. int ar[] = {795, 635}; declares and fills a 2-elt integer array. Accessing elements: ar[num]; returns the numth element. Arrays (2/6):  Arrays (2/6) Arrays are (almost) identical to pointers char *string and char string[] are nearly identical declarations They differ in very subtle ways: incrementing, declaration of filled arrays Key Concept: An array variable is a “pointer” to the first element. Arrays (3/6):  Arrays (3/6) Consequences: ar is an array variable but looks like a pointer in many respects (though not all) ar[0] is the same as *ar ar[2] is the same as *(ar+2) We can use pointer arithmetic to access arrays more conveniently. Declared arrays are only allocated while the scope is valid char *foo() { char string[32]; ...; return string; } is incorrect Arrays (4/6):  Arrays (4/6) Array size n; want to access from 0 to n-1, but test for exit by comparing to address one element past the array int ar[10], *p, *q, sum = 0; ... p = &ar[0]; q = &ar[10]; while (p != q) /* sum = sum + *p; p = p + 1; */ sum += *p++; Is this legal? C defines that one element past end of array must be a valid address, i.e., not cause an bus error or address error Arrays (5/6):  Arrays (5/6) Array size n; want to access from 0 to n-1, so you should use counter AND utilize a constant for declaration & incr Wrong int i, ar[10]; for(i = 0; i < 10; i++){ ... } Right #define ARRAY_SIZE 10 int i, a[ARRAY_SIZE]; for(i = 0; i < ARRAY_SIZE; i++){ ... } Why? SINGLE SOURCE OF TRUTH You’re utilizing indirection and avoiding maintaining two copies of the number 10 Arrays (6/6):  Arrays (6/6) Pitfall: An array in C does not know its own length, & bounds not checked! Consequence: We can accidentally access off the end of an array. Consequence: We must pass the array and its size to a procedure which is going to traverse it. Segmentation faults and bus errors: These are VERY difficult to find; be careful! (You’ll learn how to debug these in lab…) Segmentation Fault vs Bus Error?:  Segmentation Fault vs Bus Error? http://www.hyperdictionary.com/ Segmentation Fault A fatal failure in the execution of a machine language instruction resulting from the processor detecting an anomalous condition on its bus. Such conditions include invalid address alignment (accessing a multi-byte number at an odd address), accessing a physical address that does not correspond to any device, or some other device-specific hardware error. A bus error triggers a processor-level exception which Unix translates into a “SIGBUS” signal which, if not caught, will terminate the current process. Bus Error An error in which a running Unix program attempts to access memory not allocated to it and terminates with a segmentation violation error and usually a core dump. Pointer Arithmetic (1/3):  Pointer Arithmetic (1/3) Since a pointer is just a mem address, we can add to it to traverse an array. p+1 returns a ptr to the next array elt. (*p)+1 vs *p++ vs *(p+1) vs *(p)++ ? x = *p++  x = *p ; p = p + 1; x = (*p)++  x = *p ; *p = *p + 1; What if we have an array of large structs (objects)? C takes care of it: In reality, p+1 doesn’t add 1 to the memory address, it adds the size of the array element. Pointer Arithmetic (2/3):  Pointer Arithmetic (2/3) So what’s valid pointer arithmetic? Add an integer to a pointer. Subtract 2 pointers (in the same array). Compare pointers (<, <=, ==, !=, >, >=) Compare pointer to NULL (indicates that the pointer points to nothing). Everything else is illegal since it makes no sense: adding two pointers multiplying pointers subtract pointer from integer Pointer Arithmetic (3/3):  int get(int array[], int n) { return (array[n]); /* OR */ return *(array + n); } Pointer Arithmetic (3/3) C knows the size of the thing a pointer points to – every addition or subtraction moves that many bytes. So the following are equivalent: Pointers in C:  Pointers in C Why use pointers? If we want to pass a huge struct or array, it’s easier to pass a pointer than the whole thing. In general, pointers allow cleaner, more compact code. So what are the drawbacks? Pointers are probably the single largest source of bugs in software, so be careful anytime you deal with them. Dangling reference (premature free) Memory leaks (tardy free) C Pointer Dangers:  C Pointer Dangers Unlike Java, C lets you cast a value of any type to any other type without performing any checking. int x = 1000; int *p = x; /* invalid */ int *q = (int *) x; /* valid */ The first pointer declaration is invalid since the types do not match. The second declaration is valid C but is almost certainly wrong Is it ever correct? Administrivia:  Administrivia Read K&R 6 for Friday There is a language called D! www.digitalmars.com/d/ Answers to the reading quizzes? Ask your TA in discussion Homework expectations Readers don’t have time to fix your programs which have to run on lab machines. Code that doesn’t compile or fails all of the autograder tests  0 Administrivia:  Administrivia Slip days You get 3 “slip days” per year to use for any homework assignment or project They are used at 1-day increments. Thus 1 minute late = 1 slip day used. They’re recorded automatically (by checking submission time) so you don’t need to tell us when you’re using them Once you’ve used all of your slip days, when a project/hw is late, it’s … 0 points. If you submit twice, we ALWAYS grade the latter, and deduct slip days appropriately You no longer need to tell anyone how your dog ate your computer. You should really save for a rainy day … we all get sick and/or have family emergencies! C Strings:  C Strings A string in C is just an array of characters. char string[] = "abc"; How do you tell how long a string is? Last character is followed by a 0 byte (null terminator) int strlen(char s[]) { int n = 0; while (s[n] != 0) n++; return n; } C Strings Headaches:  C Strings Headaches One common mistake is to forget to allocate an extra byte for the null terminator. More generally, C requires the programmer to manage memory manually (unlike Java or C++). When creating a long string by concatenating several smaller strings, the programmer must insure there is enough space to store the full string! What if you don’t know ahead of time how big your string will be? Buffer overrun security holes! Common C Errors:  Common C Errors There is a difference between assignment and equality a = b is assignment a == b is an equality test This is one of the most common errors for beginning C programmers! Pointer Arithmetic Peer Instruction Q:  Pointer Arithmetic Peer Instruction Q How many of the following are invalid? pointer + integer integer + pointer pointer + pointer pointer – integer integer – pointer pointer – pointer compare pointer to pointer compare pointer to integer compare pointer to 0 compare pointer to NULL #invalid 1 2 3 4 5 6 7 8 9 (1)0 Kilo, Mega, Giga, Tera, Peta, Exa, Zetta, Yotta:  Kilo, Mega, Giga, Tera, Peta, Exa, Zetta, Yotta KInd MEaty GIgolos TEach PEers EXotic ZEn Yoga [Albert Shau] Kiss me Gillard! Ten pecks expected, zen youngster! [Dimas Guardado] Kill! Mental girls' teen permits exhaust zealous youngsters. [Dimas Guardado] King Mental gilded ten pence, extorted zellions. Yowzah! [Dimas Guardado] Kirk met Gilligan's team peacefully except zealous Yoda. [Brendan Clark] Kill mean giant terrorists peacefully except zealot Yoda. [Brendan Clark] “Kiss me giant tepid peanut,” exalted zesty Yoda. [Brendan Clark] Kissing mean girls tests people except zealous youths. [Brendan Clark] Kiss me giant tepid peanut expelling zebra yolk. [Brendan Clark] KInd MEditerranean GIrls TEam PErforms EXtreme ZEn YOga [Timon Safaie] KIller MEdieval GIants TErribly PEtrify EXcited ZEbras Yonder [Amy Hwang] KIssing ME GIves TEn PEals (of) EXtra ZEalous Yodelling [Amy Hwang] Kilo, Mega, Giga, Tera, Peta, Exa, Zetta, Yotta:  Kilo, Mega, Giga, Tera, Peta, Exa, Zetta, Yotta Kim’s melodious giddiness terrifies people, excepting zealous yodelers Kirby Messed Gigglypuff Terribly, (then) Perfectly Exterminated Zelda and Yoshi Killed meat gives teeth peace except zebra yogurt Kind Men Give Tense People Extra Zeal (for) Yoga Killing melee gives terror; peace exhibits Zen yoga Killing messengers gives terrible people exactly zero, yo Kindergarten means giving teachers perfect examples (of) zeal (&) youth Kissing mediocre girls teaches people (to) expect zero (from) you Kinky Mean Girls Teach Penis-Extending Zen Yoga Kissing Mel Gibson, Teddy Pendergrass exclaimed, “Zesty, yo!” Pointer Arithmetic Peer Instruction A:  How many of the following are invalid? pointer + integer integer + pointer pointer + pointer pointer – integer integer – pointer pointer – pointer compare pointer to pointer compare pointer to integer compare pointer to 0 compare pointer to NULL Pointer Arithmetic Peer Instruction A ptr + 1 1 + ptr ptr + ptr ptr - 1 1 - ptr ptr - ptr ptr1 == ptr2 ptr == 1 ptr == NULL ptr == NULL #invalid 1 2 3 4 5 6 7 8 9 (1)0 “And in Conclusion…”:  “And in Conclusion…” Pointers and arrays are virtually same C knows how to increment pointers C is an efficient language, with little protection Array bounds not checked Variables not automatically initialized (Beware) The cost of efficiency is more overhead for the programmer. “C gives you a lot of extra rope but be careful not to hang yourself with it!” Kilo, Mega, Giga, Tera, Peta, Exa, Zetta, Yotta:  Kilo, Mega, Giga, Tera, Peta, Exa, Zetta, Yotta Kilted Meghans giggle terribly petting exalted zellous yodas [CL] Kissing me gives terrible peeps exactly zero, yo! [CL] Killer Megan gives Terrible Peter's excellent zebra yoghurt [YC] “Kiss me”, giant Terrible Peter exclaimed zealously, yo [YC] Kind Merchants Give Texan People Extra Zesty Yogurt [AW] Kittens' Meows Give to Terrific Peals of Extraordinarily Zealous Yowls [AW] Killer Mercenary Giants Temporarily Pester Exercising Zebras in Yorkshire [AW] Kiss me girl, terrible people examine zebras, yo. [JD] Kiss me, given ten pens extracted zen-like yo [AG] KIssing ME GIrl, TElls of my PEnchant for EXtra ZEsty Yoghurt [TM] Kissing me gingerly, Ted Peterson exclaimed, "Zesty, yo!” [DH] Kiss me girl teach petty exasperations zestful yodeling [AR] Kind Megan Gibson teaches people extremely zestful yoga [AC] Kissing mediocre girls/gimmicks teaches/tells people to expect zero/zest from you [MT] Kiss me, giant tease, people excuse zealous young [CR] Kicking mean girls and teasing pedestrians excite zealous youngsters [MH] Killin' me! Giant teacher's pet exaggerates zealously yo [KN] Kind Merlin gives tense people exceptional zebra yogurt [KL] Kinky metaphysics gibberish teaches people exquisite Zen yodeling [JC] Kingly men giving tedious penance exhibit zealous yowls [MH] Kinky mean girls terrorizing petty ex-boyfriends zeroing-on you [HC] Kind Merlin Gives Ten People Extremeley Zealous Yodas [RC] Kiss Me Goat Te Procure Extra Zloties, Yo [RG] Bonus Slide (near end): Arrays/Pointers:  Bonus Slide (near end): Arrays/Pointers An array name is a read-only pointer to the 0th element of the array. An array parameter can be declared as an array or a pointer; an array argument can be passed as a pointer. int strlen(char s[]) { int n = 0; while (s[n] != 0) n++; return n; } int strlen(char *s) { int n = 0; while (s[n] != 0) n++; return n; } Could be written: while (s[n]) Bonus Slide (near end): Pointer Arithmetic:  Bonus Slide (near end): Pointer Arithmetic We can use pointer arithmetic to “walk” through memory: C automatically adjusts the pointer by the right amount each time (i.e., 1 byte for a char, 4 bytes for an int, etc.) void copy(int *from, int *to, int n) { int i; for (i=0; i<n; i++) { *to++ = *from++; } }

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