1

Exam / Homework

• Exam 1– Solutions Posted– Questions?

• Starting K&R chapter 4 tonight

• Read Glass and Ables Section on MAKE

• Start on HW4 - Due Class 13

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Functions and C Program Structure

• C makes functions efficient and easy to use

• Program != a few big functions

• Program == many small functions

• Functions may reside in separate source files

• Source files compiled and loaded separately

• An Introduction to MAKEFILE is coming (A process that K&R does not go into)

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C Program File Structure• In file main.c: /* function prototypes */#include “function.h” /* indirect */int foobar(int, float); /* directly */int main() { int n = foobar(function(), 1.0);}int foobar(int i, float j){ /* statements using i and j */return (some integer expression);

}

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C Program File Structure

• In file function.h:/* function.h: function prototype */

int function(void);

• In file function.c:/* function.c: function code */

#include “function.h”

int function()

{

/* function code */

}

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Function Prototypes

• Special case for null argument lists• Always code function prototype as:

int foo (void);

• Keeps compiler parameter checking enabled• Demonstration of bad example:

int foo ();

x = foo (i, j, k); /* compiler won’t catch! */

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Post-fix Notation

• For hw4, you need to know how to convert an algebraic (in-fix) expression to post-fix

• Some calculators work this way, e.g H-P

• Algebraic (123 + 21) * (567 – 432)

• Post-fix 123 21 + 567 432 - *

• No parentheses are needed!

• How’s that work?

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Post-fix Implementation with a Stack

Enter: 123 21 + 567 432 - *

Stack

States:123

123

21 144

144

567

144

567

432

144

135 19440Empty

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Post-fix Calculator Program• Pseudo-code for In-fix calculator (See K&R, pg 75)

while (next the character is not an EOF)if (number)

push it on the stackelse if (operator)

pop operand(s) /* may be one or two */do operationpush result back on the stack

else if (newline)pop and print value from top of stack

else error

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Post-fix Calculator Program

• See K&R, page 76 and then page 78• Operand order is not important for some ops

+ , *, &, |

• Operand order is important for some ops!-, /, %

• There is only one operand for some ops~, !,

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Post-fix Calculator Program

• Break down the pseudo-code into functions– push / pop– get next number or operator, i.e. get op ( )– getch / ungetch

• Both push and pop access a set of variables for the stack static external

• Both getch and ungetch access a buffer for characters from stdin static external

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Communication Between Functions

• Sometimes closely related functions need to share access to variables in situations where they never call each other and therefore can’t pass arguments to the shared variables

• Ex: push and pop share the stack variables• Declare “static” variables inside source file for the

functions (before / outside the braces for any of the functions) “Static External” per K&R

• Makes them “globally” accessible to all functions in the source file (similar to Java class variables)

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Multi-Module ProgramsGlass & Ables, pg 397(3 ed.)

• To compile all .c files and create executablegcc main.c getop.c stack.c getch.c –o calcit

• To compile only one and create executable with objects of previously compiled filesgcc main.c getop.o stack.o getch.o –o calcit

• To build an executable from a complex set of separate files skipping any unneeded stepsmake –f makefile (makefile documents build rules)

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Dependency Tree for a Buildcalcit

main.o getop.o stack.o getch.o

main.c getop.c stack.c getch.c

getop.h stack.h getch.h

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Make (Glass, pg 347)

• makefile rule format

target: dependency list for needed files

command list to create target

• Example:

calcit: main.o getop.o stack.o getch.o

gcc main.o … getch.o -o calcit

• Note: Must use tabs to indent – not spaces

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Make

• CC is defined early in the makefile with the line:CC = gcc

• Then using $(CC) causes substitutioncalcit: main.o getop.o stack.o getch.o

$(CC) main.o getop.o stack.o getch.o -o calcit

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Make

• The user can type the command "make", or make with a named target, for example:

“make getop.o” to cause the rules leading up to the target getop.o to be executed, if needed.

• When user types "make" without a target name, the FIRST rule listed will be the default target constructed. Thus, the "calcit" rule should be the first rule in the makefile

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Make• At the end of the makefile, there is a rule with a target

name "clean".

clean:

rm *.o • Since "clean" is not a file:

– There is no dependency list (ages of files don't matter)

– It is called when you give the command "make clean".

– It deletes .o files to reduce the clutter in the directory.

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Header Files• Principles for contents of .h files

– Each .h file includes all of the function prototypes and symbolic constants needed to invoke those functions

int function (int ); /* no code for function */

#define VALUE0_FOR_ARGUMENT 0

– No statements that allocate memory in .h files!!

• Don’t use a single “calc.h” file (K&R, pg 82)• A reusable solution shown in “Dependency Tree”

– An xxx.h file for each xxx.c source file of functions

– #include getop.h in getop.c and all source files that call functions provided in the getop.c source file (i.e. main)

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Header Files – Math Library

• When you want to use math functions

Example: SIN( ) or SQRT( )

• Write

#include <math.h>

• But also invoke compiler with special flag:

gcc -lm . . . (lm means library, math)

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Back to Post-fix Calculator Program

• Look at getop, getch, ungetch functions

• In getop( ), we try to input a char string for a number. But if the string ends with a '+', we want to put that back in stdin, somehow, for a successive invocation of getch

• We do this with ungetch( )

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getch.c

#define BUFSIZE 100static char buf[BUFSIZE]; /* buffer for ungetch */static int bufp = 0; /* next free position in buf */

/* get a possibly pushed back char from stdin */int getch(void){

return (bufp > 0) ? buf[--bufp] : getchar( );}

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getch.c

/* push char back for getch later */void ungetch(int c){if (bufp >= BUFSIZE)

printf("ungetch: too many characters in buffer.\n");else

buf[bufp++] = c;}