Modules, Make�les, Editors, Git Modules, Make�les, Editors, Git Creating large programs Creating large programs K08 Δομές Δεδομένων και Τεχνικές Προγραμματισμού • A large program might contain hundreds of thousands lines of code Κώστας Χατζηκοκολάκης • Having such a program is a single .c �le is not practical - Hard to write - Hard to read and understand - Hard to maintain - Slow to compile • We need to split it in semantically related units / / 1 2 Modules Modules Information Hiding Information Hiding • A module (ενότητα) is a collection of related data and operations • A notion closely related to abstraction • • Since the user does not need to know how the module is implemented, They allow to achieve abstraction (αφαίρεση), a notion of fundamental importance in programming anything not necessary for using the module should be hidden - internal data, auxiliary functions, data types, etc • The user of the module only needs to know what the module does • This allows to modify parts of the program independently • Only the author of the module needs to know how it is implemented - a function visible only within the module cannot a�ect other parts of - This is useful even when the author and the user are the same person the program • They will be used to implement Abstract Data Types later in this course - think of changing a car's tires, it should not a�ect its engine! / / 3 4
Modules in C Modules in C Modules in C Modules in C • A module in C is represented by a header �le module.h • Eg. A stats.h module with two functions - we already know several modules: stdio.h , string.h , … // stats.h - Απλά στατιστικά στοιχεία για πίνακες • It simply declares a list of functions #include <limits.h> // INT_MIN, INT_MAX - also constants and typedefs // Επιστρέφει το μικρότερο στοιχείο του array (ΙΝΤ_ΜΑΧ αν size == 0) int stats_find_min(int array[], int size); • Describes what the module does // Επιστρέφει το μεγαλύτερο στοιχείο του array (ΙΝΤ_MIN αν size == 0) - often with documentation for these functions int stats_find_max(int array[], int size); • Pre�xing all functions with stats_ is a good practice (why?) / / 5 6 Using a C module Using a C module Implementing a C module Implementing a C module • • The module's implementation is provided in a �le module.c #include "module.h" • Use the provided functions • module.c contains the de�nitions of all functions declared in module.h • As users , we don't need to know how the module is implemented! // stats.c - Υλοποίηση του stats module #include "stats.h" // minmax.c - Το βασικό αρχείο του προγράμματος int stats_find_min(int array[], int size) { #include <stdio.h> int min = INT_MAX; // "default" τιμή, μεγαλύτερη από όλε #include "stats.h" for (int i = 0; i < size; i++) int main() { if (array[i] < min) int array[] = { 4, 35, -2, 1 }; min = array[i]; // βρέθηκε νέο ελάχιστο printf("min: %d \n ", stats_find_min(array, 4)); return min; printf("max: %d \n ", stats_find_max(array, 4)); } } / / 7 8
Compiling a program with modules Compiling a program with modules Separate compilation Separate compilation • Simply compiling minmax.c together with module.c works • We can compile each .c �le separately to create an .o �le • Then link all .o �les together to create the executable gcc minmax.c stats.c -o minmax • But this compiles both �les every time gcc -c minmax.c -o minmax.o gcc -c stats.c -o stats.o • What if we change a single �le in a program with 1000 .c �les? gcc minmax.o stats.o -o minmax • If we change minmax.c , we only need to recompile that �le and relink - Makefile s make this very easy / / 9 10 Multiple implementations of a module Multiple implementations of a module Compiling with multiple implementations Compiling with multiple implementations • The same module.h can be implemented in di�erent ways • minmax.c is compiled without knowing how stats.h is implemented - this is abstraction ! // stats_alt.c - Εναλλακτική υλοποίηση του stats module • We can then link with any implementation we want #include "stats.h" // Επιστρέφει true αν value <= array[i] για κάθε i gcc -c minmax.c -o minmax.o int smaller_than_all(int value, int array[], int size) { for (int i = 0; i < size; i++) # use the first implementation if (value > array[i]) gcc -c stats.c -o stats.o return 0; gcc minmax.o stats.o -o minmax int stats_find_min(int array[], int size) { # OR the second for (int i = 0; i < size; i++) gcc -c stats_alt.c -o stats_alt.o if (smaller_than_all(array[i], array, size)) gcc minmax.o stats_alt.o -o minmax return array[i]; return INT_MAX; // εδώ φτάνουμε μόνο σε περίπτωση κενού array } / / 11 12
Multiple implementations of a module Multiple implementations of a module Make�les Make�les • All implementations should provide the same high-level behavior • Good programmers are lazy - So the program will work with any of them - they want to spend their time programming, not compiling • But one implementation might be more e�cient than some other • Nobody likes typing the same gcc ... commands 100 times - This often depends on the speci�c application • We can automate compilation with a Makefile • Which implementation of stats.h would you choose? / / 13 14 A simple Make�le A simple Make�le A simple Make�le - �rst problem A simple Make�le - �rst problem • We modify minmax.c , but make refuses to rebuild minmax # Ενα απλό Makefile (με αρκετά προβλήματα) # Προσοχή στα tabs! minmax: $ make minmax gcc -c minmax.c -o minmax.o make: 'minmax' is up to date. gcc -c stats.c -o stats.o gcc minmax.o stats.o -o minmax • solution: dependencies • This means: to create the �le minmax run these commands minmax: minmax.c stats.c • To compile we run make minmax gcc -c minmax.c -o minmax.o gcc -c stats.c -o stats.o - or simply make to compile the �rst target in the Makefile gcc minmax.o stats.o -o minmax • this means: minmax depends on minmax.c , stats.c - if any of these �les is newer (last modi�cation time) than minmax itself, the commands are run again! / / 15 16
A simple Make�le - second problem A simple Make�le - second problem Implicit rules Implicit rules • We modify minmax.c , but make recompiles everything • make knows how to make foo.o if a �le foo.c exists, by running • Solution: separate rules for each �le we create gcc -c foo.c -o foo.o • This is called an implicit rule minmax.o: minmax.c gcc -c minmax.c -o minmax.o • So we don't need rules for .o �les! stats.o: stats.c gcc -c stats.c -o stats.o minmax: minmax.o stats.o minmax: minmax.o stats.o gcc minmax.o stats.o -o minmax gcc minmax.o stats.o -o minmax • To build minmax we need to build minmax.o , stats.o - minmax.o depends on minmax.c which is newer, so make recompiles - stats.o depends on stats.c which is older, so no need to recompile / / 17 18 Variables Variables CFLAGS variable CFLAGS variable • We can use variables to further simplify the Makefile • A special variable - To create a variable: VAR = ... • Passed as arguments to the compiler when compiling a .o �le using an - To use a variable we write $(VAR) anywhere in the Makefile implicit rule • This allows to easily reuse the Makefile • Eg. enable all warnings, treat them as errors, and allow debugging # Αρχεία .o (αλλάζουμε απλά σε stats_alt.o για τη δεύτερη υλοποίηση!) CFLAGS = -Wall -Werror -g OBJS = minmax.o stats.o # Το εκτελέσιμο πρόγραμμα EXEC = minmax $(EXEC): $( OBJS ) gcc $( OBJS ) -o $( EXEC ) / / 19 20
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