functions and interfacescs-2303, c-term 20101 functions and interfaces in c cs-2303, system...

Functions and InterfacesCS-2303, C-Term 2010 1

Functions and Interfaces in C

CS-2303, System Programming Concepts

(Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie and from C: How to Program, 5th and 6th editions, by Deitel and Deitel)


Your First C Program

#include <stdio.h>

int main (void) {

printf(″Hello, World!\n″);

return 0;

}


Fundamental Rule in C

• Every identifier must be declared before it can be used in a program

• Definition:– “identifier”• A sequence of letters, digits, and ‘_’• Must begin with a letter or ‘_’• Case is significant

– Upper and lower case letters are different

• Must not be a “reserved word” — see p. 192 of K&R

• Definition:– “declare”• Introduce an identifier and the kind of entity it refers to• Optionally, define associated memory or program


So where is printf declared?

#include <stdio.h>

int main (void) {


return 0;

}

Answer: in this file!


#include <file.h>

• Logically:–• Equivalent to an interface in Java

• I.e., where types and functions are declared

• Physically:–• A file of C code that is copied into your program at

compile time– By the C preprocessor

• Spells out the contract of the interface between implementer and client

aka Header file


#include <stdio.h>

• Declares everything that your program needs to know about the “standard I/O facilities” of C …

• … and conceals everything that your program does not need to know about those same facilities

• Doesn’t change very often

And when it does change,every program that dependson it must be recompiled!



#include <stdio.h>

int main (void) {


return 0;

}

• Body of the function• Defines what the

function “does”• Sequence of

statements• Each does a step of the

function

• Enclosed in curly brackets { }

• Indistinguishable from a compound statement



#include <stdio.h>

int main (void) {


return 0;

}

• Call to another function • In this case, a function defined by the system

• Prints some data on standard output



#include <stdio.h>

int main (void) {


return 0;

}

• Argument to printf – a constant string• Enclosed in straight double quotes

• Note the new-line character ′\n′ at the end



#include <stdio.h>

int main (void) {


return 0;

}

• A return statement• return is a reserved word in C

• main should return zero if no error; non-zero if error



#include <stdio.h>

int main (void) {


return 0;

}

• Note that statements typically end with semicolons• So compiler can tell where end of statement is


Questions?

Write, compile, and execute this program in Lab session today and

tomorrow


What happens to your program …

…after it is compiled, but before it can be run?


Example

#include <stdio.h>

int main (void) {

printf (″Hello,″ ″ world\n″);

return 0;

}

• Symbol defined in your program and used elsewhere

•main

• Symbol defined elsewhere and used by your program

•printf


Static Linking and LoadingPrintf.c

Printf.o

Librarygcc

ar

Linker

Memory

HelloWorld.c

gcc

HelloWorld.o

Loader

a.out(or file name ofyour command)


Static Linking and Loading

Linker

Printf.c

Printf.o

Librarygcc

ar

Memory

HelloWorld.c

gcc

HelloWorld.o

Loader


Part of gcc


Static Linking and Loading

Linker

Printf.c

Printf.o

Librarygcc

ar

Memory

HelloWorld.c

gcc

HelloWorld.o

Loader


Part of gcc

Part of operating system


Compiling Your Program• gcc HelloWorld.c

• Compiles the program in HelloWorld.c, links with any standard libraries, puts executable in a.out

• You should find HelloWorld.o in your directory

• gcc –o hello_world HelloWorld.c• Same as above, but names the executable file hello_world instead of a.out

• gcc –lrt HelloWorld.c• Searches library named rt.a for functions to link

(in addition to standard libraries)


Compiling Your Program (continued)

• gcc foo.c bar.c help.c• Compiles the programs foo.c, bar.c, and help.c, links

with standard libraries, executable in a.out• You should find foo.o, bar.o, and help.o in your directory

• gcc –o Lab2 foo.c bar.c help.c• Same as above, but names the executable file Lab2

• gcc –c foo.c bar.c help.c• Compiles foo.c, bar.c, and help.c to foo.o, bar.o, and help.o but does not link together

• gcc –o Lab2 foo.o bar.o help.o• Links together previously compiled foo.o, bar.o, help.o,

stores result in Lab2


Questions?


Definition – Function

• A fragment of code that accepts zero or more argument values, produces a result value, and has zero or more side effects.

• A method of encapsulating a subset of a program or a system

• To hide details

• To be invoked from multiple places

• To share with others

A function in C is equivalent to a method in Java, but without the surrounding class


Functions – a big Topic

• Examples

• Function definition

• Function prototypes & Header files

• Pre- and post-conditions

• Scope rules and storage classes

• Implementation of functions

• Recursive functions


Textbook References

• Chapter 4 of K&R

• Chapter 5 of D&D


Common Functions in C

#include <math.h>– sin(x) // radians– cos(x) // radians– tan(x) // radians– atan(x) – atan2(y,x)– exp(x) // ex

– log(x) // loge x

– log10(x) // log10 x

– sqrt(x) // x 0– pow(x, y) // xy

– ...

#include <stdio.h>– printf()– fprintf()– scanf()– sscanf()– ...

#include <string.h>– strcpy()– strcat()– strcmp()– strlen()– ...


Common Functions (continued)

• In K&R, also in D&D– <assert.h> // for diagnostics, loop invariants,

etc.– <stdarg.h> // for parsing arguments– <time.h> // time of day and elapsed time– <limits.h> // implementation dependent

numbers– <float.h> // implementation dependent numbers.

– <setjmp.h> // beyond scope of this course– <signal.h> // beyond scope of this course


Common Functions (continued)

• See also the man pages of your system for things like

•<pthread.h> // concurrent execution•<socket.h> // network communications•... // many, many other facilities

• Fundamental Rule: if there is a chance that someone else had same problem as you, …

• … there is probably a package of functions to solve it in C!


Functions in C

resultType functionName(type1 param1, type2 param2, …) {

…

body

…

}

• If no result, resultType should be void• Warning if not!

• If no parameters, use void between ()


Functions in C

resultType functionName(type1 param1, type2 param2, …) {

…

body

…

} // functionName

• If no result, resultType should be void• Warning if not!

• If no parameters, use void between ()

It is good style to always

end a function with a

comment showing its name


Using Functions

• Let int f(double x, int a) be (the

beginning of) a declaration of a function.

• Then f(expr1, expr2) can be used in any expression where a value of type int can be used – e.g.,

N = f(pi*pow(r,2), b+c) + d;





N = f(pi*pow(r,2), b+c) + d;

Using Functions (continued)This is a parameter

This is an argumentThis is also an argument


Definitions

• Parameter:– a declaration of an identifier within the '()' of a function declaration

• Used within the body of the function as a variable of that function

• Initialized by the caller to the value of the corresponding argument.

• Argument:– an expression passed when a function is called; becomes the initial value of the corresponding parameter


Definitions

• Parameter:– a declaration of an identifier within the '()' of a function declaration

• Used within the body of the function as a variable of that function

• Initialized by the caller to the value of the corresponding argument.

• Argument:– an expression passed when a function is called; becomes the initial value of the corresponding parameter

Note: Changes to parameters within thefunction do not propagate back to caller!

All parameters in C are “call by value.”





N = f(pi*pow(r,2), b+c) + d;

Using Functions (continued)

The first argument expression is

evaluated, converted to double,

and assigned to parameter x

The second argument expression

is evaluated, converted to int,

and assigned to parameter a





N = f(pi*pow(r,2), b+c) + d;

Using Functions (continued)

Function f is executed and returns a value of type int

Result of f is added to dSum is assigned to N


Note

• Functions in C do not allow other functions to be declared within them

• Like C++, Java

• Unlike Algol, Pascal

• All functions defined at “top level” of C programs• (Usually) visible to linker

• Can be linked by any other program that knows the function prototype

• Can “see” anything declared previously in same program (or in an included interface)


Note on printf parameters

• int printf(char *s, ...) {body

} // printf

• In this function header, “…” is not a professor’s shorthand

• (as often used in these slides)

• …but an actual sequence of three dots (no spaces between)

• Meaning:– the number and types of arguments is indeterminate

• Use <stdarg.h> to extract the arguments


Questions?


Function Prototypes

• There are many, many situations in which a function must be used separate from where it is defined –

• before its definition in the same C program

• In one or more completely separate C programs

• This is actually the normal case!

• Therefore, we need some way to declare a function separate from defining its body.

• Called a Function Prototype


Function Prototypes (continued)

• Definition:– a Function Prototype in C is a language construct of the form:–

return-type function-name (parameter declarations) ;

• I.e., exactly like a function definition, except with a ';' instead of a body in curly brackets

• Essentially, the method of a Java interface.

aka function header


Purposes of Function Prototype

• So compiler knows how to compile calls to that function, i.e.,– number and types of arguments– type of result

• As part of a “contract” between developer and programmer who uses the function

• As part of hiding details of how it works and exposing what it does.

• A function serves as a “black box.”


Questions?


“Contract” between Developer and User of a Function

1. Function Prototype

2. The pre- and post-conditions– I.e., assertions about what is true before the

function is called and what is true after it returns.

– A logical way of explaining what the function does


Definitions

• Pre-condition:–a characterization or logical statement about

• the values of the parameters, and • values of relevant variables outside the function

prior to calling the function

• Post-condition:–a logical statement or characterization about

• the result of the function in relation to the values of the parameters and pre-conditions, and

• changes to values of variables outside the function

after the function returns


Example 1

• double sin (double angle);– Pre:– angle is expressed in radians– Post:– result is the familiar sine of angle– Note: this function does not use or change any

other variables


Example 2

• int printf (string, arg1, arg2, …)– Pre:– string terminated with '\0' and containing

conversion specifiers

– Pre:– a buffer maintained by the file system contains zero or more unprinted characters from previous calls.

– Post:– args are substituted for conversion codes in copy of string; resulting string is added to buffer

– Post:– if '\n' is anywhere in buffer, line is “printed” up to '\n'; printed characters are cleared from buffer

– Post:– result is number of characters added to buffer by printf


Example 3

float total = 0;int count = 0;

int GetItem(void) {float input;int rc;printf("Enter next item:- ");if ((rc = scanf("%f", &input)) != EOF

&& (rc > 0)) {total += input;count++;

}; // ifreturn rc;

} // GetNewItem


Example 3

float total = 0;int count = 0;

int GetItem(void) {float input;int rc;...;if ((rc = scanf("%f", &input)) != EOF

&& (rc > 0)) {total += input;count++;

}; // ifreturn rc;

} // GetItem

Pre:– total is sum of all previous inputs, or zero if none

Pre:– count is number of previous inputs, or zero if none

Post:– if valid input is received total = totalprev + input, count = countprev + 1


Important

• Pre- and post-conditions are analogous to loop invariants

• I.e., they describe something about the data before and after a function is called and the relationship that the function preserves

• Often are used together with loop invariants• … to show that loop invariant is preserved from one

iteration to the next


Questions?

functions and interfacescs-2303, c-term 20101 functions and interfaces in c cs-2303, system...

Documents