======Exercise 16: Structs And Pointers To Them====== In this exercise you'll learn how to make a struct, point a pointer at them, and use them to make sense of internal memory structures. I'll also apply the knowledge of pointers from the last exercise and get you constructing these structures from raw memory using malloc. As usual, here's the program we'll talk about, so type it in and make it work: #include <stdio.h> #include <assert.h> #include <stdlib.h> #include <string.h> struct Person { char *name; int age; int height; int weight; }; struct Person *Person_create(char *name, int age, int height, int weight) { struct Person *who = malloc(sizeof(struct Person)); assert(who != NULL); who->name = strdup(name); who->age = age; who->height = height; who->weight = weight; return who; } void Person_destroy(struct Person *who) { assert(who != NULL); free(who->name); free(who); } void Person_print(struct Person *who) { printf("Name: %s\n", who->name); printf("\tAge: %d\n", who->age); printf("\tHeight: %d\n", who->height); printf("\tWeight: %d\n", who->weight); } int main(int argc, char *argv[]) { // make two people structures struct Person *joe = Person_create( "Joe Alex", 32, 64, 140); struct Person *frank = Person_create( "Frank Blank", 20, 72, 180); // print them out and where they are in memory printf("Joe is at memory location %p:\n", joe); Person_print(joe); printf("Frank is at memory location %p:\n", frank); Person_print(frank); // make everyone age 20 years and print them again joe->age += 20; joe->height -= 2; joe->weight += 40; Person_print(joe); frank->age += 20; frank->weight += 20; Person_print(frank); // destroy them both so we clean up Person_destroy(joe); Person_destroy(frank); return 0; } To describe this program, I'm going to use a different approach than before. I'm not going to give you a line-by-line breakdown of the program, but I'm going to make you write it. I'm going to give you a guide through the program based on the parts it contains, and your job is to write out what each line does. includes I include some new header files here to gain access to some new functions. What does each give you? struct Person This is where I'm creating a structure that has 4 elements to describe a person. The final result is a new compound type that lets me reference these elements all as one, or each piece by name. It's similar to a row of a database table or a class in an OOP language. function Person_create I need a way to create these structures so I've made a function to do that. Here's the important things this function is doing: + I use malloc for "memory allocate" to ask the OS to give me a piece of raw memory. + I pass to malloc the sizeof(struct Person) which calculates the total size of the struct, given all the fields inside it. + I use assert to make sure that I have a valid piece of memory back from malloc. There's a special constant called NULL that you use to mean "unset or invalid pointer". This assert is basically checking that malloc didn't return a NULL invalid pointer. + I initialize each field of struct Person using the x->y syntax, to say what part of the struct I want to set. + I use the strdup function to duplicate the string for the name, just to make sure that this structure actually owns it. The strdup actually is like malloc and it also copies the original string into the memory it creates. function Person_destroy If I have a create, then I always need a destroy function, and this is what destroys Person structs. I again use assert to make sure I'm not getting bad input. Then I use the function free to return the memory I got with malloc and strdup. If you don't do this you get a "memory leak". function Person_print I then need a way to print out people, which is all this function does. It uses the same x->y syntax to get the field from the struct to print it. function main In the main function I use all the previous functions and the struct Person to do the following: + Create two people, joe and frank. + Print them out, but notice I'm using the %p format so you can see where the program has actually put your struct in memory. + Age both of them by 20 years, with changes to their body too. + Print each one after aging them. + Finally destroy the structures so we can clean up correctly. Go through this description carefully, and do the following: * Look up every function and header file you don't know about. Remember that you can usually do man 2 function or man 3 function and it'll tell you about it. You can also search online for the information. * Write a comment above each and every single line saying what the line does in English. * Trace through each function call and variable so you know where it comes from in the program. * Look up any symbols you don't know as well. ======What You Should See====== After you augment the program with your description comments, make sure it really runs and produces this output: $ make ex16 cc -Wall -g ex16.c -o ex16 $ ./ex16 ======Joe is at memory location 0xeba010:====== ======Name: Joe Alex====== Age: 32 Height: 64 Weight: 140 ======Frank is at memory location 0xeba050:====== ======Name: Frank Blank====== Age: 20 Height: 72 Weight: 180 ======Name: Joe Alex====== Age: 52 Height: 62 Weight: 180 ======Name: Frank Blank====== Age: 40 Height: 72 Weight: 200 ======Explaining Structures====== If you've done the work I asked you then structures should be making sense, but let me explain them explicitly just to make sure you've understood it. A structure in C is a collection of other data types (variables) that are stored in one block of memory but let you access each variable independently by name. They are similar to a record in a database table, or a very simplistic class in an object oriented language. We can break one down this way: * In the above code, you make a struct that has the fields you'd expect for a person: name, age, weight, height. * Each of those fields has a type, like int. * C then packs those together so they can all be contained in one single struct. * The struct Person is now a compound data type, which means you can now refer to struct Person in the same kinds of expressions you would other data types. * This lets you pass the whole cohesive grouping to other functions, as you did with Person_print. * You can then access the individual parts of a struct by their names using x->y if you're dealing with a pointer. * There's also a way to make a struct that doesn't need a pointer, and you use the x.y (period) syntax to work with it. You'll do this in the Extra Credit. If you didn't have struct you'd need to figure out the size, packing, and location of pieces of memory with contents like this. In fact, in most early assembler code (and even some now) this is what you do. With C you can let C handle the memory structuring of these compound data types and then focus on what you do with them. ======How To Break It====== With this program the ways to break it involve how you use the pointers and the malloc system: * Try passing NULL to Person_destroy to see what it does. If it doesn't abort then you must not have the -g option in your Makefile's CFLAGS. * Forget to call Person_destroy at the end, then run it under Valgrind to see it report that you forgot to free the memory. Figure out the options you need to pass to Valgrind to get it to print how you leaked this memory. * Forget to free who->name in Person_destroy and compare the output. Again, use the right options to see how Valgrind tells you exactly where you messed up. * This time, pass NULL to Person_print and see what Valgrind thinks of that. * You should be figuring out that NULL is a quick way to crash your program. ======Extra Credit====== In this exercise I want you to attempt something difficult for the extra credit: Convert this program to not use pointers and malloc. This will be hard, so you'll want to research the following: * How to create a struct on the stack, which means just like you've been making any other variable. * How to initialize it using the x.y (period) character instead of the x->y syntax. * How to pass a structure to other functions without using a pointer. Copyright (C) 2010 Zed. A. Shaw Credits