Basic Semantics

Basic SemanticsBasic Semantics

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ContentContent

• Names, attributes, and bindings

• Declarations, scope and the symbol table

• Overloading

• Allocation, lifetimes, and the environment

• Variables and constants

• Aliases, dangling references, and garbage

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NamesNames

• A fundamental abstraction mechanism in a programming language is the use of names or identifiers to denote language entities or constructs

• In most languages, constants, variables, and procedures can have names assigned by the programmer

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AttributesAttributes

• The meaning of a name is determined by the attributes associated with the name

const int n = 5; /* n is a constant */

int x; /* x is a variable */

double f(int n) { … } /* f is a function */

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BindingBinding

• The process of associating an attribute to a name is called binding

const int n = 2; /* static binding */

int x; /* static binding */x = 2; /* dynamic binding */

int *y; /* static binding */y = new int; /* dynamic binding */

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Binding TimeBinding Time

• Language definition time: int, char, float

• Language implementation time: sizeof(int)

• Translation time: types of variables

• Link time: code body of external functions

• Load time: locations of global variables

• Execution time: values of variables, locations of local variables

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Symbol TableSymbol Table

• Bindings can be maintained by a data structure called the symbol table

• For an interpreter, the symbol table is a function that maps names to attributes

Symbol TableNames Attributes

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Symbol TableSymbol Table

• For a compiler, the symbol table is a function that maps names to static attributes

Symbol TableNames Static

attributes

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Environment & MemoryEnvironment & Memory

• The dynamic attributes locations and values can be maintained by two functions environment (memory allocation) and memory(assignment)

environmentNames Locations

memoryLocations Values

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DeclarationsDeclarations

• Declarations are a principal method for establishing bindings

• Declarations are commonly associated with a block. These declarations are called local declarations of that block.

• Declarations associated with an enclosing block are called nonlocal declarations

• Declarations can also be external to any block. These declarations are called global declarations

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An ExampleAn Exampleint x; /* global */

main(){ int i, n; /* nonlocal */ i = 1; n = 10; while (i < n) { int m; /* local */ m = i++; f(m); }}

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ScopeScope

• The scope of a binding is the region of the program over which the binding is maintained

• We can also refer to the scope of a declaration if all the bindings established by the declaration have identical scopes

• In a block-structured language, the scope of a binding is limited to the block in which its associated declaration appears. Such a scope rule is called static (or lexical) scope

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An ExampleAn Exampleint x;void p(void){ int i; …}void q(void){ int j; …}main() { int k; …}

i

j

kmain

q

p

x

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VisibilityVisibility

• The local binding of a name will take precedence over the global and nonlocal binding of the name. This causes a scope hole for the global and nonlocal binding of the name

• The visibility of a binding includes only those regions of a program where the binding applies

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An ExampleAn Exampleint i;void p(void){ int i; …}void q(void){ int j; …}main() { int i; …}

i

j

i

i

i

i

scope visibility

scope hole

scope hole

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Scope Resolution OperatorsScope Resolution Operators

• Scope resolution operators can be used to access the bindings hidden by scope holes

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An ExampleAn Example/* An example in C++ */int x;class ex { int x; void f() { int x; x = 1; /* local variable x */ ex::x = 2; /* x field of class ex */ ::x = 3; /* global variable x */ } void g() { x = 4; /* x field of class ex */ }};

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Scope Across FilesScope Across Files

• In C, global variable declarations can actually be accessed across files

File 1: extern int x;/* use x in other files */

File 2: int x; /* x can be used by other files */

File 2: static int x; /* x can only be used in this file */

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The Symbol TableThe Symbol Table

• A symbol table is like a dictionary for names: It must support insertion, lookup, and deletion of names with associated attributes

• The maintenance of scope information in a statically scoped language with block structure requires that declarations be processed in a fashion like stack

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An ExampleAn Exampleint x;char y;

void p(void){ double x; … { int y[10]; …}}

void q(void){ int y; … }

main() { char x; … }

xint

global

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xint

global

ychar

global

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xint

global

ychar

global

pvoid

function

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xdouble

local to pint

global

ychar

global

pvoid

function

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xdouble

local to p1

intglobal

yint array

local to p2

pvoid

function

charglobal

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xint

global

ychar

global

pvoid

function

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xint

global

y

pvoid

function

qvoid

function

charglobal

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xint

global

yint

local to q

pvoid

function

qvoid

function

charglobal

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xint

global

y

pvoid

function

qvoid

function

charglobal

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An ExampleAn Example

int x;char y;




x

y

pvoid

function

qvoid

function

charglobal

mainvoid

function

intglobal

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xchar

local to main

y

pvoid

function

qvoid

function

charglobal

mainvoid

function

intglobal

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x

y

pvoid

function

qvoid

function

charglobal

mainvoid

function

intglobal

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Symbol Tables for StructuresSymbol Tables for Structuresstruct { int a; char b; double c;} x = {1,'a',2.5};

void p(void){ struct { double a; int b; char c; } y = {1.2,2,'b'}; printf("%d, %c, %g\n",x.a,x.b,x.c); printf("%f, %d, %c\n",y.a,y.b,y.c);}

main(){ p(); return 0; }

x

y

pvoid

function

structlocal to p

structglobal

a int

b

c double

char

a double

b

c char

int

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Nested ProceduresNested Proceduresprocedure ex is x: integer := 1; y: character := ‘a’; procedure p is x: float := 2.5; begin put(y); new_line; A: declare y: array (1..10) of integer; begin y(1) := 2; put(y(1)); new_line; put(ex.y); new_line; end A; end p;

procedure q is y: integer := 42; begin put(x); new_line; p; end q;begin B: declare x: character := ‘b’; begin q; put(ex.x); new_line; end B;end ex;

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Nested ProceduresNested Procedures

x float

A

y array of integer

blocky character

p procedure

x integer

ex procedure

q procedure y integer

B block x character

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Dynamic ScopeDynamic Scope

• Using dynamic scope, the binding of a nonlocal name is determined according to the calling ancestors during the execution instead of the static program text

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An Example: Static ScopeAn Example: Static Scope

int x = 1;char y = ‘a’;void p(void){ double x = 2.5; printf(“%c\n”, y); { int y[10]; …}}void q(void){ int y = 42; printf(“%d\n”, x); p(); }main() { char x = ‘b’; q(); return 0; }

a

1

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An Example: Dynamic ScopeAn Example: Dynamic Scope


x

y

pvoid

function

qvoid

function

char = ‘a’global

mainvoid

function

int = 1global

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xchar = ‘b’in main

y

pvoid

function

qvoid

function


mainvoid

function

int = 1global

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y

pvoid

function

qvoid

function

int = 42in q

mainvoid

function

int = 1global


98

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y

pvoid

function

qvoid

function

int = 42in q

mainvoid

function

int = 1global


double = 2.5in p

*

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Issue of Dynamic ScopeIssue of Dynamic Scope

• When a nonlocal name is used in an expression or statement, the declaration that applies to that name cannot be determined by simply reading the program

• Static typing and dynamic scoping are inherently incompatible

• Languages that use dynamic scope are Lisp, APL, Snobol, Perl

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OverloadingOverloading

• An operator or function name is overloaded if it is used to refer to two or more different things in the same scope

2 + 3 integer addition2.1 + 3.2 floating-point addition2 + 3.2 error or floating-point addition

by automatically converting2 to 2.0

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Overload ResolutionOverload Resolution

• Overloading of operators or function names can be resolved by checking the number of parameters (or operands) and the data types of the parameters (or operands)

int max(int x, int y); max(2, 3); double max(double x, double y); max(2.1, 3.2);int max(int x, int y, int z); max(1, 3, 2);

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Overloading & Automatic Overloading & Automatic ConversionConversion

• Automatic conversions complicate the process of overload resolution

max(2.1, 3);

C++: error, both conversions are allowed

Ada: error, no conversion is allowed

Java: only int to double is allowed

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Overloading & Automatic Overloading & Automatic ConversionConversion

• Since there are no automatic conversions in Ada, the return type of a function can also be used to resolve overloading

function max(x:integer; y:integer) return intfunction max(x:integer; y:integer) return floata: integer;b: float;a := max(2, 3); -- call to max #1b := max(2, 3); -- call to max #2

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Operator OverloadingOperator Overloading

• Ada and C++ also allow built-in operators to be extended by overloading

• We must accept the syntactic properties of the operator, i.e., we cannot change their associativity or precedence

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typedef struct { int i; double d; } IntDouble;

bool operator < (IntDouble x, IntDouble y){ return x.i < y.i && x.d < y.d; }

IntDouble operator + (IntDouble x, IntDouble y){ IntDouble z; z.i = x.i + y.i; z.d = x.d + y.d; return z;}

int main(){ IntDouble x = {1, 2.1}, y = {5, 3.4}; if (x < y) x = x + y; else y = x + y; cout << x.i << " " << x.d << endl; return 0;}

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Name OverloadingName Overloading

• A name can be used to refer to completely different things

class A {

A A(A A) {

A: for(;;)

{ if (A.A(A) == A) break A; }

return A;

}

}

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EnvironmentEnvironment

• Depending on the language, the environment may be constructed statically (at load time), dynamically (at execution time), or a mixture of the two

• FORTRAN uses a complete static environment

• LISP uses a complete dynamic environment

• C, C++, Ada, Java use both

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Location AllocationLocation Allocation

• Typically, global variables are allocated statically

• Local variables are usually allocated automatically and dynamically in stack-based fashion

• The lifetime or extent of an allocated location is the duration of its allocation in the environment

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An ExampleAn Examplemain() {

A: { int x; char y; /* ... */

B: { double x; int a; /* ... */ } /* end B */

C: { char y; int b; /* ... */

D: { int x; double y; /* ... */ } /* end D */

/* ... */

} /* end C */

/* ... */

} /* end A */

return 0;

}

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xy

xyxa

xyyb

xy

xyybxy

xyyb

xy

enterA

enterB

exitB

enterC

enterD

exitD

exitC

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Manually Location AllocationManually Location Allocation

• Many languages also provide mechanisms to manually allocate (or deallocate) memory for variables

C: malloc, free library functionsC++: new, delete built-in operatorsJava: new built-in operators

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Structure of an EnvironmentStructure of an Environment

static

stack

heap

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Static Local VariablesStatic Local Variables

• In C, the allocation of a local variable can be changed from stack-based to static

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An ExampleAn Exampleint p(void) { static int p_count = 0; /* initialized only once */ p_count += 1; return p_count;}

main() { int i; for (i = 0; i < 10; i++) if (p() % 3) printf("%d\n",p()); return 0;}

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VariablesVariables

• A variable is an object whose stored value can change during execution

• A variable is specified by its attributes, which include its name, its location, its value, and others

Name Value

Location

OtherAttributes

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AssignmentsAssignments

• The semantics of the assignment x = e are that e is evaluated to a value, which is then copied into the location of x

x = yy

x

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R-Value & L-ValueR-Value & L-Value

• The variable y on the right-hand side of x = y stands for the value of y, while the variable x on the left-hand side stands for the location of x

• We sometimes call the value stored in the location of a variable its r-value, while the location of a variable its l-value

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R-Value & L-ValueR-Value & L-Value

• ML, Algol68, and BLISS are languages that make r-values and l-values explicit. In ML,

val x = ref 0; val y = ref 1;

x := !y;x := !x + 1;

z : int -- not assignable (like a constant) x : int ref -- assignable reference cell

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Address-of & DereferencingAddress-of & Dereferencing

• In C, the address-of operator & explicitly returns the location of a variable, while the dereferencing operator * explicitly takes the value of a variable and returns it as a location

int x, y, *xptr;xptr = &x;*xptr = y;

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Semantics of AssignmentsSemantics of Assignments

• The usual semantics of an assignment, which copies the value of a location to another location, is called storage semantics

• In some languages, like Java and LISP, an assignment copies the location to another location. This semantics is called pointer semantics

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Pointer SemanticsPointer Semantics

• Pointer semantics are usually implemented using implicit pointers and implicit dereferencing

y

x

x = y

y

x

Java

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class ArrTest { public static void main(String[] args) { int[] x = {1, 2, 3}; int[] y = x; x[0] = 42; System.out.println(y[0]); }}

/* y[0] = 42 */

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ConstantsConstants

• A constant is a language entity that has a fixed value for the duration of its existence

• A constant can be a literal like 123 or ‘a’. Or it can be a name for a value, called symbolic constants. Once its value is computed, it cannot change

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Symbolic ConstantsSymbolic Constants

• A symbolic constant is like a variable, except that it has no location attribute, but a value only. The location of a symbolic constant cannot be explicitly referred to

Name ValueOther

Attributes

const int a = 2;int *x;x = &a; /* Illegal C code */

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Initialization of ConstantsInitialization of Constants

/* compile-time constant */const int a = 2; /* manifest constant */const int b = 27 + 2 * 2;const int b = 27 + a * a; /* C illegal, C++ ok *//* static (or load-time) constant */const int c = (int) time(0); /* C illegal, C++ ok *//* dynamic constant */int f(int x) { const int d = x + 1; return b + c;}

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Function ConstantsFunction Constants

• Function definitions are definitions of constants whose values are functions

int gcd(int u, int v) { if (v == 0) return u; else return gcd(v, u % v);}

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Function VariablesFunction Variables

• C has function variables, which must be defined as pointers. However, dereferencing is not required to access the value of function variables

int (*fun_var) (int, int);fun_var = gcd;main() { printf(“%d\n”, fun_var(15, 10)); return 0;}

A little inconsistant

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Functions in Functional LanguagesFunctions in Functional Languages

• Functional languages do a much better job of making clear the distinction between function constants, function variables, and function literals. In ML,

fn(x:int) => x * x (* literal *) (fn(x:int) => x * x) 2; val square = fn(x:int) => x * x; (* constant *) fun square (x:int) = x * x; (* constant *)

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AliasesAliases• An alias occurs when the same location is

bound to two different names at the same time

• An alias can occurs at call-by-reference parameter passing

• An alias can occurs at pointer assignment

• An alias can occurs at assignment with pointer semantics

• An alias can occurs in the EQUIVALENCE declaration of FORTRAN

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An Example: Call by ReferenceAn Example: Call by Reference

void incr(int &i){ i++;}

void f( ){ int x = 1; incr(x); printf(“%d\n”, x);}

/* x = 2 */

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An Example: Call by ReferenceAn Example: Call by Reference

x x

x

i

1

1

x

i

2

x 2

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An Example: Pointer AssignmentAn Example: Pointer Assignment

int *x, *y;x = (int *) malloc(sizeof(int));*x = 1;y = x;*y = 2;printf(“%d\n”, *x); /* x = 2 */

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An Example: Pointer AssignmentAn Example: Pointer Assignment

x

y

x

y

x

y

x

y

1

1 x

y

2

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An Example: Pointer SemanticsAn Example: Pointer Semantics

class ArrTest { public static void main(String[] args) { int[] x = {1, 2, 3}; int[] y = x; x[0] = 42; System.out.println(y[0]); }}

/* y[0] = 42 */

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Dangling ReferencesDangling References

• A dangling reference is a pointer that points to a location that has been deallocated.

int *x, *y;x = (int *) malloc(sizeof(int));*x = 2;y = x;free(x);printf(“%d\n”, *y);

int *dangle(void) { int x; return &x;}

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GarbageGarbage• Garbage is locations that have been allocated

but that have become inaccessible to the program

int *x;x = (int *) malloc(sizeof(int));x = 0;

void p(void) { int *x; x = (int *) malloc(sizeof(int)); *x = 2;}

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Garbage CollectionGarbage Collection

• It is useful to remove the need to deallocate memory explicitly from the programmer, while at the same time automatically reclaiming garbage for further use. This mechanism is called garbage collection

• Most functional. logic, and object-oriented languages provide garbage collection

Basic Semantics

Documents

int x x

static binding

nonlocal binding

dynamic binding

int x void f

new int

global variable x

local variable x