0 Chap. 5 Pointers and Arrays 5.3Pointers and Arrays 5.4Address Arithmetic 5.5Character Pointers and Functions 5.6Pointer Arrays; Pointers to Pointers 5.7Multidimensional Arrays 5.8Initialization of Pointer Arrays 5.9Pointers vs Multi-dimensional Arrays 5.10Command-line Arguments: main(int argc, *argv[]) 5.11Pointers to Functions 5.12Complicated Declarations + Summary of Basic Declarations 6.7 typedef – first visit System-oriented Programming, B. Hirsbrunner, diuf.unifr.ch/pai/sp, Lecture 6 – 24 March 2015 kr5: Pointers and Arrays (~90') + exercises: ~90'

1 *** 5.3 Pointers and Arrays, 5.4 Address Arithmetic *** a The name of this array is “a” and its value is equivalent to the address of its first element: &a[0]. a+2 p+3 a+i points to the i-th element beyond a. p+i points to the i-th element beyond p. As p is a variable, p=a or ++p are legal; but as an array name is not a variable, expression like a=p or ++a are illegal ! p int *p=a; a[3] a[1] a[0] a[2] int a[4]; The declaration int a[4]; defines an array of size 4, that is, a block of 4 consecutive variables of type int, named a[0], a[1], a[2], a[3]. Tricky: p-a is legal, but not p+a ! Reminder: if p is a generic pointer, i.e. void *p, the only defined operation is the back and forth casting to any other type of pointers. come back to this slide if you are lost with pointers and arrays

2 5.5 Character Pointers and Functions Within the called function, this array name becomes a local pointer variable! When an array name a is passed to a function, what is passed is the address of a's first element, i.e. &a[0]. That means: as a formal parameter in a function definition, char a[] and char *p are equivalent : int f(char a[]); int f(char *p); and in both cases, the following calls will work: f(a); // char a[] = "hello"; f(p); // char *p = a; //same result as previous call f("hello"); // constant string ! //-–- crash if f's body write to a cell of the //––– constant string Let be the prototype: int f(char a[]); "Proof": next slide

3 in (1) 'a' is an immutable pointer to a mutable string in (2) 'p' is a mutable pointer to the same object as 'a' in (3) 'q' is a mutable pointer to a possibly immutable string in (7) "hello" is an immutable string A "Proof" : a test program and its function frames /* Return length of string s */ int strlen(char s[]) { int n; for (n = 0; *s != '\0'; s++) n++; return n; } void main() { (0) int i1, i2, i3, i4; (1) char a[] = "hello"; (2) char *p = a; (3) char *q = "hello"; (4) i1 = strlen(a); (5) i2 = strlen(p); (6) i3 = strlen(q); (7) i4 = strlen("hello"); } ‘o’‘o’ ‘l’‘l’ ‘l’‘l’ ’e' '\0' 'h' constant string (sdh segment) (7) strlen's frame (stack segment) (4), (5) s n strlen's frame (stack segment) (7) s n ‘o’‘o’ ‘l’‘l’ ‘l’‘l’ '\0' mutable string (sdh segment) (1), (2) ‘e’‘e’ 'h''h' ‘o’‘o’ ‘l’‘l’ ‘l’‘l’ ’e' '\0' 'h' possibly constant string (sdh segment) (3) strlen's frame (stack segment) (6) s n a: q p q i1 i3 i2 main's frame (stack segment) (0) - (3) i4 p sdh stands for stack, data or heap (it is up to the compiler or the runtime to decide)

4 5.6 Pointer Arrays; Pointers to Pointers Since pointers are variables themselves, they can be stored in arrays just as other variables can. Example: Design a program that will sort a set of text lines into alphabetic order. First Idea de\0 jk\0 abc\0 de\0 jk\0 abc\0 The sorting process has three steps Read all the lines of input Sort them Print them in order Complete solution See pp (will not be discussed in this course) Second Idea d e \0 j k \0 a b c \0 d e \0 j k \0 a b c \0

5 5.7 Multidimensional Arrays Initialization of Pointer Arrays Array definition char a[2]; // a is an array[2] of char, // i.e. a constant pointer to an array[2] of char char a[2][3]; // a is an array[2] of 'array[3] of char', // i.e. a matrix of 2 rows and 3 columns Array definition with initialization char a[2] = {'a','b'}; char a[2][3] = {{'a','b','c'}, {'d','e','f'}}; char a[2][3];

6 5.9 Pointers vs. Multi-dimensional Arrays Tricky char *ap[]; // ap is an array[] of pointer to char; char (*pa)[]; // pa is a pointer to an array[] of char; char aa[][8]; // aa is an array[] of array[8] of char; Two Examples

7 #include /* echo command-line arguments; 1st version */ int main(int argc, char *argv[]) { int i; for (i = 1; i < argc; i++) printf("%s%s", argv[i], (i < argc-1) ? " " : ""); printf("\n"); return 0; } *** 5.10 Command-Line Arguments: %echo hello *** stack and data segments just after the initialization of main(c,*v[]), called by %echo hello NUL L i c main's frame (stack segment) $main 2 kr115_1 Assume the binary code is saved in the file 'echo' (A) (data segment) *(v+2) *v *(v+1) v[0] v[1] v[2] v v (B) (data segment) v[0][0] v[0][1] ‘h’‘h’ ‘o’‘o’ ‘e’‘e’ ‘c’‘c’ '\0' v[0] v[1][0] v[1][1] ’\0' ‘l’‘l’ ‘l’‘l’ ‘h’‘h’ ‘e’‘e’ ’o' (C) (data segment) v[1] argv is an array of pointer to char. By convention argv is NULL-terminated, and the objects pointed by argv[i] are strings

8 Reminder : Arithmetic, Precedence and Associativity Pointer arithmetic is well defined: v+1, ++v If v is a pointer then the item pointed by v can be named by *v and v[0]; i.e. the value of v is equal to *v's address; and the value of *v is equal to v[0][0]'s address [] has higher precedence than * i.e. *v[] is equivalent to *(v[]) Associativity of [] is left to right i.e. v[][] is equivalent to (v[])[] * and ++ have same precedence, but are evaluated from right to left: *++v is equivalent to *(++v) ++*v is equivalent to ++(*v)

9 Navigation through (A) Typical Usage for (i=0; v[i] != '\0', ++i) {…} while ((++v)[0] != '\0') {…} 1. v[0] // dereferenced with the index [0] a. v[0], v[1], v[2], … or v[++i] b. v[0], (v+1)[0], (v+2)[0], … or (++v)[0] Remark With expression like *++v, be aware that the original value of v will be lost 2. *v // dereferenced with the operator * a. *v, *(v+1), *(v+2), … or *++v

10 Navigation through (B) 1_2. *v[0] // double dereferenced, first with the index [0], then with the operator * a. *v[0], *(v[0]+1), *(v[0]+2),… or *++v[0] 1_1. v[0][0] // double dereferenced with the indexes [0] and [0] a. v[0][0], v[0][1], v[0][2],… or v[0][++i] b. v[0][0], (v[0]+1)[0], (v[0]+2)[0],… or (++v[0])[0] 2_1. (*v)[0] // double dereferenced, first with the operator *, then with the index [0] a. (*v)[0], (*v)[1], (*v)[2],… or (*v)[++i] b. (*v)[0], (*v+1)[0], (*v+2)[0],… or (++*v)[0] 2_2. **v // double dereferenced with the operator * a. **v, *(*v+1), *(*v+2),… or *++*v Usage ( see kr117, line command 'find'): Navigation through B 0, C 0, … and X 1, X 2, … while ( -- argc > 0 && (*++argv)[0] == '-') while (c = *++argv[0]) // same as (c=*++argv[0]) != '\0' If you are lost, don't panic: draw a picture!

Command-Line Arguments: find [-nx] pattern Synopsis find [-nx] Description The command 'find' prints all lines that match pattern, with the options: -x: for "except", signaling the inversion (print all lines which don't contain the pattern) -n: for "number", to request line numbering main(int argc, char *argv[]) {...} Remark The complete program, which can be found in kr117, is very interesting as it contains a standard navigation through the argument 'char *argv[]' to find the options –x, -n, -xn, -nx: while (--argc>0 && (*++argv)[0] == '-') while (c = *++argv[0]) switch (c) { case 'x':... case –n':... default:... } We will come back to this program in the Series of exercises !

Pointers to Functions Prototype char *f(); // f is a constant pointer to a fct with no arg. // and returning a pointer to a variable of type char // in short: f is a function returning a pointer to char char (*f)(); // f is a variable of type pointer to a fct with no arg. // and returning a value of type char // in short: f is a pointer to a function returning a char Remark This is similar to the expressions *a[] and (*a)[] Example #include int f(int p(int), int n) {return p(n);} int g(int (*p)(int), int n) {return p(n);} int h1(int n) {return n+1;} int h2(int n) {return n+2;} main() { printf("%d %d\n", f(h1,10), g(h2,10)); }

13 *** Summary of Basic Declarations *** Basic types char c; // c is a variable of type char // in short: c is a char char *c; // c is a variable of type pointer to char // in short: c is a pointer to char Array and function char a[5]; // a is a constant pointer to an array[5] of char // in short: a is an array[5] of char char f(); // f is a constant pointer to a fct with no arg. and // returning a char // in short: f is a function returning a char Pointers char *a[5]; // a is an array[5] of pointer to char; char (*a)[5]; // a is pointer to an array[5] of char, char *f(); // f is a function returning a pointer to char char (*f)(); // f is a pointer to a function returning a char void f(char a[2]); // a is a pointer to an array[2] of char void f(char *a); // a is a pointer to a char void f(char a[]); // dito !!! // hint: in f's body, 'a' is a variable pointer come back to this slide if you are lost with pointers, arrays or functions

Complicated Declarations

typedef – first visit Examples of declarations typedef int length_t; // types int (old) and length_t (new) are synonyms typedef char *string_t; // string_t is a new type, synonym to type char * typedef int (*fct_t)(int); // fct_t is a new type, synonym to a 'pointer to a function // with one argument of type int and returning an int' Examples of usage length_t size; string_t str, a[], f(string_t); fct_t g;