The Three Attributes of an Identifier
C Part 2 1
Identifiers have three essential attributes:
storage duration
scope
linkage
CS@VT August 2009
Computer Organization I
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Storage Duration
C Part 2 2
storage duration
determines when memory is set aside for the variable and when that memory is
released
automatic
allocated when the surrounding block is executed
deallocated when the block terminates
static
stays in the same storage location as long as the program is running
can retain its value indefinitely (until program terminates)
CS@VT August 2009
Computer Organization I
©2006-09 McQuain, Feng & Ribbens
Scope
C Part 2 3
scope
(of an identifier) the range of program statements within which the identifier is
recognized as a valid name
block scope
visible from its point of declaration to the end of the enclosing block
place declaration within a block
file scope
visible from its point of declaration to the end of the enclosing file
place declaration outside of all blocks (typically at beginning of file)
CS@VT August 2009
Computer Organization I
©2006-09 McQuain, Feng & Ribbens
Linkage
C Part 2 4
linkage
determines the extent to which the variable can be shared by different parts of the
program
external linkage
may be shared by several (or all) files in the program
internal linkage
restricted to a single file, but shared by all functions within that file
no linkage
restricted to a single function
CS@VT August 2009
Computer Organization I
©2006-09 McQuain, Feng & Ribbens
Determining the Attributes
C Part 2 5
The default storage duration, scope and linkage of a variable depend on the location of its
declaration:
inside a block
automatic storage duration, block scope, no linkage
outside any block
static storage duration, file scope, external linkage
When the defaults are not satisfactory, see:
auto
static
extern
register
CS@VT August 2009
Computer Organization I
©2006-09 McQuain, Feng & Ribbens
Function Declaration vs Definition
C Part 2 6
A function name is an identifier, so it must be formally declared before it is used.
Unlike a simple variable or constant, a function has a return type and (possibly) a formal
parameter list. The function declaration must also specify those.
The function declaration is
essentially just a copy of the
function header from the function
definition.
A function declaration must
specify the types of the formal
parameters, but formal parameter
names are optional.
double circleArea(double Radius);
double circleArea(double Radius) {
const double PI = 3.141596224;
}
return (PI * Radius * Radius);
Many authors refer to a
function declaration as a
prototype.
However, it is good practice to
include the formal parameter
names in the function declaration.
Function declarations are typically declared at file scope or within a header file, although
they may be placed anywhere. The placement determines the scope of the function name,
and hence where it may be called.
CS@VT August 2009
Computer Organization I
©2006-09 McQuain, Feng & Ribbens
Formal Parameters are Pass-by-Value
C Part 2 7
Formal parameters have automatic storage duration and block scope, just like local
variables.
Formal parameters are automatically initialized with a copy of the value of the
corresponding actual parameter.
There is no connection between the actual and formal parameters other than that they store
the same value at the time of the function call.
In Java, you can pass a function a reference to an object (in fact, there's no other way to
pass an object) and the function can use the reference to modify the object that's held by the
calling code.
In C, you can't do that until you understand how pointers work…
CS@VT August 2009
Computer Organization I
©2006-09 McQuain, Feng & Ribbens
Typical C Code Organization
C Part 2 8
For now, we'll restrict our attention to single-file programs. The typical organization is:
// include directives
. . .
// file-scoped declarations of functions
//
and constants
. . .
int main() {
. . .
}
// implementations of other functions
CS@VT August 2009
Computer Organization I
©2006-09 McQuain, Feng & Ribbens
Example Program
C Part 2 9
#include <stdio.h>
#include <math.h>
#include <stdbool.h>
int nextPrimeAfter(int Start);
bool isPrime(int Value);
int main() {
int Value;
printf("Enter a positive integer: ");
fflush(stdout);
scanf("%d", &Value);
printf("\n");
while ( Value <= 0 ) {
printf("No, I said a POSITIVE integer: ");
scanf("%d", &Value);
printf("\n");
}
// continues. . .
CS@VT August 2009
Computer Organization I
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Example Program
C Part 2 10
// . . . continued
int Prime = 2;
while ( Value > 1 ) {
bool factorFound = false;
while ( Value % Prime == 0 ) {
if ( !factorFound )
printf("Prime factor: ");
printf("%5d", Prime);
factorFound = true;
Value = Value / Prime;
}
if ( factorFound )
printf("\n");
Prime = nextPrimeAfter( Prime );
}
return 0;
}
CS@VT August 2009
Computer Organization I
©2006-09 McQuain, Feng & Ribbens
Example Program
C Part 2 11
int nextPrimeAfter(int Start) {
int Value = Start + 1;
while ( !isPrime( Value ) ) {
++Value;
}
return Value;
}
CS@VT August 2009
Computer Organization I
©2006-09 McQuain, Feng & Ribbens
Example Program
C Part 2 12
bool isPrime(int Value) {
int Ceiling = (int) sqrt( Value );
for (int Divisor = 2; Divisor <= Ceiling; Divisor++) {
if ( Value % Divisor == 0 )
return false;
}
return true;
}
CS@VT August 2009
Computer Organization I
©2006-09 McQuain, Feng & Ribbens