Slide Set 9. for ENCM 335 in Fall Steve Norman, PhD, PEng

Transcription

1 Slide Set 9 for ENCM 335 in Fall 2018 Steve Norman, PhD, PEng Electrical & Computer Engineering Schulich School of Engineering University of Calgary October 2018

2 ENCM 335 Fall 2018 Slide Set 9 slide 2/32 Contents Pointers to pointers: Basic ideas Example application: strtod function Arrays of pointers Practical uses of arrays of pointers

3 ENCM 335 Fall 2018 Slide Set 9 slide 3/32 Outline of Slide Set 9 Pointers to pointers: Basic ideas Example application: strtod function Arrays of pointers Practical uses of arrays of pointers

4 ENCM 335 Fall 2018 Slide Set 9 slide 4/32 Pointers to pointers: Basic ideas Review: In C, a variable or function parameter of a pointer type is a container for a memory address. New idea: If such a container is located in memory, the container must have a memory address. Note that there are two different addresses involved: the address in the variable or parameter; the address of the variable or parameter. So, for example, if x is a variable of type pointer-to- whatever, then the expression &x is valid, and the type of &x is pointer-to-pointer-to- whatever.

5 ENCM 335 Fall 2018 Slide Set 9 slide 5/32 What to call these things? The term pointer to pointer is precise, helpful, and widely used in textbooks and online documents. The term double pointer (which seems to be reinvented annually by students) is confusing and unhelpful. Please don t use it!

6 ENCM 335 Fall 2018 Slide Set 9 slide 6/32 Pointer-to-pointer syntax and semantics int main(void) { int a, b, c; int *d, *e; int **f; d = &a; e = &b; f = &d; **f = 10; // (1) *f = e; e = &c; **f = 20; // (2) //... return 0; } Syntax refers to grammar; semantics refers to meaning. What does int **f; mean? Let s draw blob-and-arrow diagrams for points (1) and (2). Suppose that the addresses of a, b, and c are 3000, 3004, and 3008, and that the addresses of d and e are 3016 and Let s draw the diagrams again, showing addresses as numbers.

7 ENCM 335 Fall 2018 Slide Set 9 slide 7/32 * operators are processed one at a time ** is not an operator in C! Instead, the statement **f = 10; really means *(*f) = 10; To understand the statement... first think, What is *f? then think, Okay, then what is *(*f)? Let s make some brief notes about that. The same idea applies to multiple uses of [ ], and to mixed uses of * and [ ].

8 ENCM 335 Fall 2018 Slide Set 9 slide 8/32 Outline of Slide Set 9 Pointers to pointers: Basic ideas Example application: strtod function Arrays of pointers Practical uses of arrays of pointers

9 ENCM 335 Fall 2018 Slide Set 9 slide 9/32 Example application: strtod function A prototype in the C library <stdlib.h> header: double strtod(const char *nptr, char **endptr); strtod is short for string-to-double. x gets a value of 1.25, and y gets a value of 3.5. How does that happen? #include <stdlib.h> char s[ ] = " "; int main(void) { double x, y; char *p; x = strtod(s, &p); y = strtod(p, &p); return 0; }

10 ENCM 335 Fall 2018 Slide Set 9 slide 10/32 double strtod(const char *nptr, char **endptr) { double result; // (1) } complicated code! // (2) return result; Let s draw diagrams for the first time at point (1) and the first time at point (2). char s[ ] = " "; int main(void) { double x, y; char *p; x = strtod(s, &p); y = strtod(p, &p); return 0; } What items of information does strtod send to main, and how are they communicated?

11 ENCM 335 Fall 2018 Slide Set 9 slide 11/32 By the way, strtod can recognize many different ways of representing a double as a sequence of characters! Examples: string " e-19" "0x1.e4p3" meaning ( ) The second example represents a hexadecimal number multiplied by a power of two. Details of strtod conversions are not quiz or exam material in ENCM 335!

12 ENCM 335 Fall 2018 Slide Set 9 slide 12/32 Outline of Slide Set 9 Pointers to pointers: Basic ideas Example application: strtod function Arrays of pointers Practical uses of arrays of pointers

13 ENCM 335 Fall 2018 Slide Set 9 slide 13/32 Arrays of pointers Arrays of pointers are used to solve many practical problems in C. As you might suspect, arrays of pointers and pointers to pointers are closely related.

14 ENCM 335 Fall 2018 Slide Set 9 slide 14/32 Operator precedence: * and [ ] In discussion of operators in programming languages, unary means has one operand, and binary means has two operands. In C, what do the unary * and binary * operators mean? Important fact: In C, the [ ] operator has higher precedence than the unary * operator. What does that fact imply about how to read the following statement? y = *x[i]; Remember, apply operators one at a time.

15 ENCM 335 Fall 2018 Slide Set 9 slide 15/32 Variable declarations for arrays of pointers First, let s review declarations for variables foo and bar, neither of which will be an array of pointers: int foo[4]; int *bar; The identifiers here are foo and bar those will be the names of the variables. Here is how to determine the types of foo and bar... The type of foo[ index ] is int, so foo must be an array of ints. The type of *bar is int, so the type of bar must be pointer-to-int. (Of course, we re so used to the types of foo and bar that we don t need to consciously think through the above steps!)

16 ENCM 335 Fall 2018 Slide Set 9 slide 16/32 This declares the variable quux to have type array of 4 pointers to int... int *quux[4]; Why? What is the proper way to read the declaration? Hints: use ideas from the previous slide; remember to apply operators one at a time.

17 ENCM 335 Fall 2018 Slide Set 9 slide 17/32 Arrays of pointers: Drill problem #1 For the program on the next slide... Let s check that types are correct in all of the arguments sent to printf. Let s make a diagram for point (1). Let s determine the program output.

18 ENCM 335 Fall 2018 Slide Set 9 slide 18/32 #include <stdio.h> char ac[ ] = "ABCDwxyz"; int main(void) { char *ap[3]; int i; ap[0] = ac; ap[1] = &ac[6]; ap[2] = ac + 2; // (1) for (i = 0; i < 3; i++) printf("%s\n", ap[i]); printf("<%c><%c><%c>\n", ap[2][1], *ap[1], (*ap)[1]); return 0; }

19 ENCM 335 Fall 2018 Slide Set 9 slide 19/32 A note about initializers It s perfectly allowable and often convenient to use addresses in initializers for arrays of pointers. This is essentially the same program as the one on the previous slide... #include <stdio.h> char ac[ ] = "ABCDwxyz"; int main(void) { char *ap[3] = { ac, &ac[6], ac + 2 }; int i; for (i = 0; i < 3; i++) printf("%s\n", ap[i]); printf("<%c><%c><%c>\n", ap[2][1], *ap[1], (*ap)[1]); return 0; }

20 ENCM 335 Fall 2018 Slide Set 9 slide 20/32 Arrays of pointers: Drill problem #2 Assume that every call to malloc succeeeds in the program on the next slide. Make a diagram for point (1).

21 ENCM 335 Fall 2018 Slide Set 9 slide 21/32 #include <stdlib.h> #define N_EL 3 int main(void) { double *ap[4]; int i; for (i = 0; i < 4; i++) ap[i] = malloc(n_el * sizeof(double)); } **ap = 1.5; ap[1][2] = 2.5; ap[2][1] = 3.5; *ap[3] = 4.5; (*ap)[2] = 5.5; // (1) //... return 0;

22 ENCM 335 Fall 2018 Slide Set 9 slide 22/32 Outline of Slide Set 9 Pointers to pointers: Basic ideas Example application: strtod function Arrays of pointers Practical uses of arrays of pointers

23 ENCM 335 Fall 2018 Slide Set 9 slide 23/32 Practical uses of arrays of pointers Probably the most common practical use of an array of pointers is to manage a collection of strings or a collection of arrays of numbers. (The program for drill problem #2 (on slide 21) starts out as if it s going to usefully manage a collection of arrays of doubles in the heap, but then the program just makes a few random assignments to elements within the arrays of doubles.)

24 ENCM 335 Fall 2018 Slide Set 9 slide 24/32 A C program with a collection of strings For the program on the next slide... Let s explain why the type of the argument arr matches the type of the parameter strs. (This is an example of an important general idea.) Let s draw a diagram for point (1). Let s reverse engineer REQUIRES and PROMISES specifications for shortest.

25 #include <string.h> char *shortest(char **strs, int n) { char *result = strs[0]; int i, len_i, min_len = strlen(strs[0]); for (i = 1; i < n; i++) { len_i = strlen(strs[i]); if (len_i < min_len) { result = strs[i]; min_len = len_i; } } return result; } int main(void) { char *arr[ ] = {"apple", "banana", "kiwi", "mango"}; char *p; p = shortest(arr, 4); // (1) //... }

26 ENCM 335 Fall 2018 Slide Set 9 slide 26/32 main with no parameters In standard C, main must return an int and must have either zero or two parameters. (In non-standard C, lots of other possibilities exist!) We ve already seen main with no parameters many times in this course... int main(void) { //... }

27 ENCM 335 Fall 2018 Slide Set 9 slide 27/32 main with two parameters In standard C, a form of main that has two parameters is acceptable: int main(int argc, char **argv) { //... } The parameter types, int and pointer-to-pointer-to-char, are mandatory. The names, argc and argv, are not mandatory, but are so widely used that it is quite unhelpful to choose any other names for the parameters to main.

28 ENCM 335 Fall 2018 Slide Set 9 slide 28/32 You will often see this kind of definition of main: int main(int argc, char *argv[ ]) { //... } It s compatible with the C standard the type of argv here is really pointer-to-pointer-to-char.

29 ENCM 335 Fall 2018 Slide Set 9 slide 29/32 So, what are argc and argv used for? The most important (but not the only) use of argc and argv happens in dealing with command-line arguments, which might need to be dealt with when a command is entered... in a Linux terminal window (or a similar thing in some other Unix-like OS, such as OpenBSD) in the macos Terminal application in a Cygwin Terminal window in a Command Prompt window in Microsoft Windows There s a demonstration of this on the next slide...

30 ENCM 335 Fall 2018 Slide Set 9 slide 30/32 #include <stdio.h> int main(int argc, char **argv) { printf("command name was:\n %s\n", argv[0]); printf("argument strings from 1 to %d are:\n", argc - 1); for (int i = 1; i < argc; i++) printf(" %s\n", argv[i]); return 0; } If an executable called myprog is built using the above source code, what will the output be in response to the command below?./myprog --pleasedonotcrash foo.txt bar.txt

31 ENCM 335 Fall 2018 Slide Set 9 slide 31/32 int main(int argc, char **argv) { // (1) } //... more code... Suppose again that the executable is called myprog. Supposing that the commmand to run it is..../myprog hi bye... let s make a diagram for point (1).

32 ENCM 335 Fall 2018 Slide Set 9 slide 32/32 Remark: How exactly a terminal command line gets chopped into pieces to set up argc and argv depends on the shell. A shell is a program that can run in a terminal window and decide what to do in response to user commands. (Shells have other uses, too, such as interpreting programs called shell scripts.) bash is the name of the default shell on macos, most Linux systems and Cygwin. What will bash and myprog do with this command?./myprog "foo bar" qux\ quux