CS201 – C Functions
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Transcript of CS201 – C Functions
CS201 – C Functions
Procedural AbstractionCode Reuse
C Library
What is a C function?
Similar to a mathematical function such as g=f(x)
Takes input Turn the crank Produces output
x
fg
Kinds of Functions No input arguments, no value returned
Output to some place else (screen) Input arguments, no value returned
Void functions (sub-programs) No input arguments, value returned
Values come from some place else (library)
Input arguments, value returned Normal typed functions
No Inputs, No Value Returned void FunctionName (void);
1st void means no value returned 2nd void means no input arguments
A utility function usually used to output messages to the screen.
Called “void function”. Not many of this kind of function in
normal programs.
Inputs, No value returned void FunctionName (arguments);
void indicates no value returned. arguments are input arguments.
A void function. Gets its data from the arguments. Outputs values some place else. Often used to output data to the
screen.
No Inputs, Value Returned
FunctionType FunctionName (void); FunctionType is type of returned value.
A normal typed function. Gets its data from some place else
(library). Not many of this kind of function in
normal programs.
Inputs, Value Returned
FunctionType FunctionName (arguments); FunctionType is type of returned value. arguments are inputs.
A normal typed function. Gets its data from the argument list. This is the most normal kind of
function.
When do you use a function?
If you find yourself retyping the same code lines more than once.
If you want to share your code with another programmer.
Anytime you need to implement a procedural abstraction. (defer implementation details)
Actual Arguments
The name given to the arguments placed in parentheses after a function call.
Must have values before function is called.
Must agree in number, order, and type of the function definition.
Formal Parameters
Name given to the parameters inside the parentheses after a function definition.
Used to receive values from the calling program.
Must agree in number, order and type of the actual arguments.
Example
nice = MyFunction ( 1, 2.5, 3 );
Actual arguments
Calling Program
Called Function
int MyFunction ( int a, float b, int c )
{ int sum;
sum = a + c;
return (sum);}
Formal parameters
Example
nice = MyFunction ( alpha, beta, chuck );
Actual arguments
Calling Program
Called Function
int MyFunction ( int a, float b, int c )
{ int sum;
sum = a + c;
return (sum);}
Formal parameters
int alpha, chuck;float beta;……
Local Data
All formal parameters and any other variables defined within a function have a value ONLY when the function has been activated.
After the call, these variables become undefined. (There is a way to avoid this, but for now, just think of them as only used during the function use.)
Function Prototypes
Listed in front of the main function to tell the compiler what functions you are planning to use. Type of Function Name of Function Types of Formal Parameters
#include “proto.h”
I like to put all my prototypes in a separate file and then include this file in the main program.
When you do it this way, you need some additional pre-processor directives to avoid duplication.
Sample proto.h
/* proto.h – Function Prototypes */#ifndef _PROTO_H#define _PROTO_Hint fun1(int,int,int);float buildit(float,int);float finalone(int);#endif
Avoids Duplication First it checks to see if the variable
_PROTO_H is defined. If it IS defined, the entire file is skipped. If it ISN’T defined, the first thing done is
to define it. Then the rest of the file is processed. If this file is included twice, the compiler
will only see one copy of it!
Funny Variable Name
_PROTO_H is used so that it never conflicts with any normal variables in any programs.
I usually use the file name since that is pretty unique.
You’ll see this done in all sorts of ways.
Drivers A short piece of code used to test a
function. Passes parameters Receives results Maybe print out a message Print out results of function
As long as you don’t change the interface, the function can be reused.
Using Multiple Source Filesproto.h
proga.c fcn2.cfcn1.c fcnn.c
proga.o fcn1.o fcn2.o fcnn.c
gcc -c gcc -c gcc -c gcc -c
library
proga
gcc -o
Example Makefile# Makefile for multipl file example
# CS201 S'05 Ray S. Babcock
#
fire: fire.o fun1.o fun2.o fun3.o
gcc -o fire fire.o fun1.o fun2.o fun3.o
fire.o: fire.c proto.h
gcc -c fire.c
fun1.o: fun1.c
gcc -c fun1.c
fun2.o: fun2.c proto.h
gcc -c fun2.c
fun3.o: fun3.c
gcc -c fun3.c
clean:
rm *.o fire