C Programming

Separate CompilationSeparate Compilation

make and makefilesmake and makefiles

Topics

• Program organization in multiple filesProgram organization in multiple files• Function and variable scope and lifetimeFunction and variable scope and lifetime• The “make” utility and makefilesThe “make” utility and makefiles

circleUtils.c/* circleUtils.c */

// for function prototypes#include “circleUtils.h”

/*** contains #defines, typedefs, etc used only** in this .c file*/

/*** contains code for each callable** function defined in circleUtils.h*/

circleUtils.h

• A header (.h) file contains everything necessary to A header (.h) file contains everything necessary to compile a .c file that compile a .c file that #includes#includes it it

• #includes#includes any any .h.h files necessary to compile files necessary to compile prototypesprototypes

• Contains Contains #defines#defines and and typedeftypedefs necessary to s necessary to compile the prototypescompile the prototypes

• Contains prototypes for circle function available to Contains prototypes for circle function available to calling code in other .c filescalling code in other .c files

sample.c/* sample.c */

#include <stdio.h>#include “circleUtils.h” // for prototypes, #defines, etc

int main( ){

// code that calls functions whose prototype// are found circleUtils.h

return 0;}

1/20/10

Compiling and linking• When a program’s code is separated into multiple When a program’s code is separated into multiple .c files, we must compile each files, we must compile each .c file and then file and then combine the resulting combine the resulting .o files to create an files to create an executable program.executable program.

• The files may be compiled separately and then linked The files may be compiled separately and then linked together. The together. The -c flag in the first two command tells flag in the first two command tells gcc to to “compile only” which results in the creation of “compile only” which results in the creation of .o (object) (object) files. In the 3files. In the 3rdrd command, the presence of the command, the presence of the.o extension extension tells tells gcc to link the files into an executable to link the files into an executable

gcc -c -Wall circleUtils.cgcc -c -Wall sample.cgcc -Wall -o sample sample.o circleutils.o

• Or if there only a few files, compiling and linking can be Or if there only a few files, compiling and linking can be done all in one stepdone all in one step– gcc -Wall -o sample sample.c circleUtils.c

Compiler vs linker

• The compiler translates one .c file into a .o fileThe compiler translates one .c file into a .o file– Verifies that all functions are being called correctly– Verifies that all variables exist– Verifies language syntax

• The linker combines multiple .o files (and C libraries) The linker combines multiple .o files (and C libraries) to create an executableto create an executable– “Finds” functions called by one .c/.o file, but defined in another

• E.g. printf( ), scanf( )

– “Finds” global variables used by one .c/.o file, but defined in another (more on this soon)

• Both can be invoked with gcc (see previous lecture)Both can be invoked with gcc (see previous lecture)

Program organization

• main( ) is generally defined in its own .c file and main( ) is generally defined in its own .c file and generally just calls helper functions generally just calls helper functions – E.g. project1.c

• Program-specific helper functions in another .c fileProgram-specific helper functions in another .c file– E.g. proj1Utils.c– If there are very few helpers, they can be in the same file as

main( )

• Reusable functions in their own .c fileReusable functions in their own .c file– Group related functions in the same file– E.g. circleUtils.c

• Prototypes, typedefs, #defines, etc. for reusable Prototypes, typedefs, #defines, etc. for reusable function in a .h filefunction in a .h file– Same file root name as the .c file. E.g. circleUtils.h

The make Utility

Typing out the gcc commands for a project gets less appealing as the project gets bigger.

The "make" utility automates the process of compiling and linking.

With make, the programmer specifies what the files are in the project and how they fit together. make takes care of the appropriate compile and link steps.

make can speed up your compiles since it is smart enough to know that if you have 10 .c files but you have only changed one, then only that one file needs to be compiled before the link step.

make has some complex features, but using it for simple things is pretty easy.

This slide and those that follow are adapted from Section 2 of “UnixProgrammingTools.pdf” by Nick Parlante, et al at Stanford. Used with permission.

How make works

• You tell You tell makemake what files are in your project, how they what files are in your project, how they fit together, how to compile, and how to link them by fit together, how to compile, and how to link them by creating a file that creating a file that makemake reads and interprets reads and interprets– By default, make looks for a file named either “makefile” or

“Makefile”

• At the Unix console, simply type the command At the Unix console, simply type the command makemake to compile all necessary files and create a new to compile all necessary files and create a new executableexecutableunix> make

• makemake can perform other tasks defined in the can perform other tasks defined in the makefilemakefile as well (more on this later) as well (more on this later)

The makefile

• A makefile consists of variable definitions, A makefile consists of variable definitions, dependency/build rules and commentsdependency/build rules and comments

• Comments begin with the ‘#’ character and extend to Comments begin with the ‘#’ character and extend to the end of the linethe end of the line# makefile for project 1

# name, date, etc

The makefile (2)• Variable definitionsVariable definitions

– A variable name is defined to represent a string of text, similar to #define in C. They are most often used to represent names of files, directories, and the compiler.

– Variable are defined simply by setting them to some value (string)– The most common variables are typically

• CC -- the name of your C compiler• CFLAGS -- the set of flags that you wish to pass to the compiler• LDFLAGS -- the set of flags that you wish to pass to the linker• OBJS -- the set of object (.o) files that are linked together to create your

executable

• To use a variable, use the dollar sign ($) followed by the name of To use a variable, use the dollar sign ($) followed by the name of the variable in parenthesis or curly bracesthe variable in parenthesis or curly bracesCC = /usr/local/bin/gccCFLAGS = -g -Wall$(CC) $(CFLAGS) -c main.c

• Variables that are not initialized are set to the empty stringVariables that are not initialized are set to the empty string

The make file (3)

• A dependency/build rule defines how to make a A dependency/build rule defines how to make a target based on changes to the files on which the target based on changes to the files on which the target depends.target depends.

• The order of the rules is irrelevant, except that the The order of the rules is irrelevant, except that the first rule is the default rule -- the rule that will be first rule is the default rule -- the rule that will be used when used when make is executed with no arguments is executed with no arguments

• A dependency/build rule is made up of two partsA dependency/build rule is made up of two parts– A dependency line followed by one or more command lines

The make file (4)

• Example dependency/build ruleExample dependency/build rulesample.o : sample.c circleUtils.h

<TAB> $(CC) $(CFLAGS) -c sample.c

• The first (dependency) line of this rule says that The first (dependency) line of this rule says that sample.o must must be rebuilt whenever be rebuilt whenever sample.c or or circleUtils.h changes. changes. Generally a .o file depends on its own .c file and any non-library Generally a .o file depends on its own .c file and any non-library .h files it .h files it #includes. In this example, we would expect that . In this example, we would expect that ssample.c #includes circleUtils.h

• The second (command) line tells make how to rebuild sample.oThe second (command) line tells make how to rebuild sample.o– Gotcha -- the command line MUST be indented with a TAB character.

Spaces won’t do. This can be a problem when cutting/pasting the contents of a makefile from a terminal screen. Some editors will automatically change a TAB into spaces.

Sample makefile

See See /afs/umbc.edu/users/c/m/cmsc313/pub/code/makefile/afs/umbc.edu/users/c/m/cmsc313/pub/code/makefile

make features

• The The make utility has some built-in default rules. utility has some built-in default rules.• In particular, the default rule for C files isIn particular, the default rule for C files is

$(CC) $(CFLAGS) -c source-file.c$(CC) $(CFLAGS) -c source-file.c

• Special syntax canSpecial syntax can be used to create the list of be used to create the list of the .o filesthe .o filesOBJS = $(SRCS: .c=.o)OBJS = $(SRCS: .c=.o)

Variable Scope and Lifetime

• The The scopescope of a variable refers to that of a variable refers to that part of a program that may refer to the part of a program that may refer to the variable.variable.

• The The lifetimelifetime of a variable refers to the of a variable refers to the time in which a variable occupies a time in which a variable occupies a place in memoryplace in memory

• The scope and lifetime of a variable are The scope and lifetime of a variable are determined by how and where the determined by how and where the variable is definedvariable is defined

static and extern

• The keyword The keyword staticstatic is used to is used to– Limit the scope of a function or global variable– Extend the lifetime of a local variable

• The keyword The keyword externextern is used to is used to– Inform the compiler that a (global) variable is defined in a

different .c file

Function Scope

• All functions are external because standard C does All functions are external because standard C does not allow nesting of function definitions.not allow nesting of function definitions.– So no “extern” declaration is needed– All functions may be called from any .c file in your

program unless they are also declared as static.

• static functions may only be used within the .c file in functions may only be used within the .c file in which they are definedwhich they are defined• Their scope is limited

Local variables• Local variables are defined within the opening and variables are defined within the opening and

closing braces of a function, loop, if-statement, etc. closing braces of a function, loop, if-statement, etc. (a code “block”) Function parameters are local to the (a code “block”) Function parameters are local to the function.function.– Are usable only within the block in which they are

defined– Exist only during the execution of the block unless

also defined as static– Initialized variables are reinitialized each time the

block is executed if not defined as static– static local variables retain their values for the

duration of your program. When used in functions, they retain their values between calls to the function.

Global Variables

• GlobalGlobal ( (externalexternal) variables are defined outside of any ) variables are defined outside of any function, typically near the top of a .c file.function, typically near the top of a .c file.– May be used anywhere in the .c file in which they are

defined.– Exist for the duration of your program– May be used by any other .c file in your application

that declares them as “extern” unless also defined as static (see below)

– Static global variables may only be used in the .c file that declares them

– “extern” declarations for global variables should be placed into a header file

randomInt.c/* a global variable to be used by code in other .c files.** This variable exists until the program ends** Other .c files must declare this variable as "extern“ */long randomInt;

/* a function that can be called from any other function** sets randomInt to a value from 1 to max, inclusive */void getRandomInt( int max ){

randomInt = getNext( );}

/* a function that can only be called from functions within this file */

static long getNext( ){ /* lastRandom is a local variable that can only be used

inside this function, but persists between calls to this function */

static long lastRandom = 100001;

lastRandom = (lastRandom * 125) % 2796203; return (lastRandom % max) + 1;}

randomInt.h

#ifndef RANDOMINT_H

#define RANDOMINT_H

// global variable in randomint.c

extern int randomInt;

// prototypes for non-static

// functions in randomInt.c

void getRandomInt( int max );

#endif

variableScope.c - part 1#include <stdio.h>

// extern definition of randomInt and prototype for getRandomInt#include “randomInt.h”

/* a global variable that can only be used by functions in this .c file */static int inputValue;

/* a function that can only be called by other functions in this .c file */static void inputPositiveInt( char prompt[ ] ){ /* init to invalid value to enter while loop */ inputValue = -1; while (inputValue <= 0) { printf( "%s", prompt); scanf( "%d", &inputValue); }}

variableScope.c - part 2/* main is the entry point for all programs */int main( ){ /* local/automatic variables that can only be used in this

function and that are destroyed when the function ends */ int i, maxValue, nrValues;

inputPositiveInt("Input max random value to generate: "); maxValue = inputValue;

inputPositiveInt("Input number of random ints to generate: "); nrValues = inputValue;

for (i = 0; i < nrValues; i++) { getRandomInt( maxValue ); printf( “%d: %d\n", i + 1, randomInt ); }

return 0;}