Basic Semantics Content Names attributes and bindings Declarations

Basic Semantics

Content • • • Names, attributes, and bindings Declarations, scope and the symbol table Overloading Allocation, lifetimes, and the environment Variables and constants Aliases, dangling references, and garbage 2

Names • 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 3

Attributes • 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 */ 4

Binding • The process of associating an attribute to a name is called binding const int n = 2; /* static binding */ int x; x = 2; /* static binding */ /* dynamic binding */ int *y; y = new int; /* static binding */ /* dynamic binding */5

Binding 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 6

Symbol 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 Table Names Attributes 7

Symbol Table • For a compiler, the symbol table is a function that maps names to static attributes Symbol Table Names Static attributes 8

Environment & Memory • The dynamic attributes locations and values can be maintained by two functions environment (memory allocation) and memory (assignment) environment Names Locations memory Locations Values 9

Declarations • 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 10

An Example int x; main() { int i, n; i = 1; n = 10; while (i < n) { int m; m = i++; f(m); } } /* global */ /* nonlocal */ /* local */ 11

Scope • 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 12

An Example int x; void p(void) { int i; … } void q(void) { int j; … } main() { int k; … } i x p j q k main 13

Visibility • 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 14

An Example int i; void p(void) { int i; … } void q(void) { int j; … } main() { int i; … } i i scope hole i i j scope hole i scope visibility 15

Scope Resolution Operators • Scope resolution operators can be used to access the bindings hidden by scope holes 16

An 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 */ } }; 17

Scope Across Files • In C, global variable declarations can actually be accessed across files File 1: extern int x; File 2: int x; /* use x in other files */ /* x can be used by other files */ File 2: static int x; /* x can only be used in this file */ 18

The 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 19

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } x int global void q(void) { int y; … } main() { char x; … } 20

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } x int global y char global void q(void) { int y; … } main() { char x; … } 21

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } x int global y char global p void function void q(void) { int y; … } main() { char x; … } 22

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } x double local to p y char global p void function void q(void) { int y; … } main() { char x; … } int global 23

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } x double local to p 1 y int array local to p 2 p void function void q(void) { int y; … } main() { char x; … } int global char global 24

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } x int global y char global p void function void q(void) { int y; … } main() { char x; … } 25

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x; … } x int global y char global p void function q void function 26

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x; … } x int global y int local to q p void function q void function char global 27

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x; … } x int global y char global p void function q void function 28

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x; … } x int global y char global p void function q void function main void function 29

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x; … } x char local to main y char global p void function q void function main void function int global 30

An Example int x; char y; void p(void) { double x; … { int y[10]; …} } void q(void) { int y; … } main() { char x; … } x int global y char global p void function q void function main void function 31

Symbol Tables for Structures struct { int a; char b; double c; } x = {1, 'a', 2. 5}; x void p(void) { struct { double a; int b; char c; p } y = {1. 2, 2, 'b'}; printf("%d, %c, %gn", x. a, x. b, x. c); printf("%f, %d, %cn", y. a, y. b, y. c); } main() { p(); return 0; } y struct global a int b char c double a double b int c char void function struct local to p 32

Nested Procedures procedure 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; 33

Nested Procedures ex procedure x integer y character p procedure x float A block y array of integer q procedure y integer B block x character 34

Dynamic 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 35

An Example: Static Scope int x = 1; char y = ‘a’; void p(void) { double x = 2. 5; printf(“%cn”, y); { int y[10]; …} } void q(void) { int y = 42; printf(“%dn”, x); p(); } main() { char x = ‘b’; q(); return 0; } a 1 36

An Example: Dynamic Scope int x = 1; char y = ‘a’; void p(void) { double x = 2. 5; printf(“%cn”, y); { int y[10]; …} } void q(void) { int y = 42; printf(“%dn”, x); p(); } main() { char x = ‘b’; q(); return 0; } x int = 1 global y char = ‘a’ global p void function q void function main void function 37

An Example int x = 1; char y = ‘a’; void p(void) { double x = 2. 5; printf(“%cn”, y); { int y[10]; …} } void q(void) { int y = 42; printf(“%dn”, x); p(); } main() { char x = ‘b’; q(); return 0; } x char = ‘b’ in main y char = ‘a’ global p void function q void function main void function int = 1 global 38

An Example int x = 1; char y = ‘a’; void p(void) { double x = 2. 5; printf(“%cn”, y); { int y[10]; …} } void q(void) { int y = 42; printf(“%dn”, x); p(); } 98 main() { char x = ‘b’; q(); return 0; } x char = ‘b’ in main int = 1 global y int = 42 in q char = ‘a’ global p void function q void function main void function 39

An Example int x = 1; x char y = ‘a’; void p(void) y { double x = 2. 5; printf(“%cn”, y); * { int y[10]; …} p } void q(void) { int y = 42; q printf(“%dn”, x); p(); } main() { char x = ‘b’; q(); main return 0; } double = 2. 5 in p int = 42 in q char = ‘b’ in main int = 1 global char = ‘a’ global void function 40

Issue 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 41

Overloading • 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 addition 2. 1 + 3. 2 floating-point addition 2 + 3. 2 error or floating-point addition by automatically converting 2 to 2. 0 42

Overload 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); 43

Overloading & Automatic Conversion • 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 44

Overloading & Automatic Conversion • 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 int function max(x: integer; y: integer) return float a: integer; b: float; a : = max(2, 3); -- call to max #1 b : = max(2, 3); -- call to max #2 45

Operator 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 46

An Example typedef struct { int i; double d; } Int. Double; bool operator < (Int. Double x, Int. Double y) { return x. i < y. i && x. d < y. d; } Int. Double operator + (Int. Double x, Int. Double y) { Int. Double z; z. i = x. i + y. i; z. d = x. d + y. d; return z; } int main() { Int. Double 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; } 47

Name 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; } } 48

Environment • 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 49

Location 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 50

An Example main() { 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; } 51

An Example x y x a x y y b x y enter A enter B exit B enter C enter D exit C 52

Manually Location Allocation • Many languages also provide mechanisms to manually allocate (or deallocate) memory for variables C: malloc, free C++: new, delete Java: new library functions built-in operators 53

Structure of an Environment static stack heap 54

Static Local Variables • In C, the allocation of a local variable can be changed from stack-based to static 55

An Example int p(void) { static int p_count = 0; p_count += 1; return p_count; } /* initialized only once */ main() { int i; for (i = 0; i < 10; i++) if (p() % 3) printf("%dn", p()); return 0; } 56

Variables • 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 Other Attributes Value Location 57

Assignments • 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=y y x 58

R-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 59

R-Value & L-Value • ML, Algol 68, 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 60

Address-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; 61

Semantics 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 62

Pointer Semantics • Pointer semantics are usually implemented using implicit pointers and implicit dereferencing Java y y x=y x x 63

An Example class Arr. Test { 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 */ } } 64

Constants • 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 65

Symbolic 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 Other Attributes const int a = 2; int *x; x = &a; /* Illegal C code */ Value 66

Initialization 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; } 67

Function 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); } 68

Function 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; A little inconsistant main() { printf(“%dn”, fun_var(15, 10)); return 0; } 69

Functions 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 *) 70

Aliases • 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 71

An Example: Call by Reference void incr(int &i) { i++; } void f( ) { int x = 1; incr(x); printf(“%dn”, x); } /* x = 2 */ 72

An Example: Call by Reference x x x 1 1 i x 2 i 73

An Example: Pointer Assignment int *x, *y; x = (int *) malloc(sizeof(int)); *x = 1; y = x; *y = 2; printf(“%dn”, *x); /* x = 2 */ 74

An Example: Pointer Assignment x x x y y y x y 1 x 1 2 y 75

An Example: Pointer Semantics class Arr. Test { 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 */ } } 76

Dangling 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(“%dn”, *y); int *dangle(void) { int x; return &x; } 77

Garbage • 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; } 78

Garbage 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 79