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Procedure Activations

Procedure Activations. Chapter 5 CS 331. Procedures: Basic Terminology. Procedures take zero or more arguments, and then do something e.g printf(“Hello”), exit, Functions take zero or more arguments, and return a value of a given type e.g. sqrt(2.), random(), fopen(“foo”)

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Procedure Activations

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  1. Procedure Activations Chapter 5 CS 331

  2. Procedures: Basic Terminology • Procedures take zero or more arguments, and then do something • e.g printf(“Hello”), exit, • Functions take zero or more arguments, and return a value of a given type • e.g. sqrt(2.), random(), fopen(“foo”) • Procedures and functions extend the language (like assembler macros?)

  3. Parameters • When declared, a procedure or function is given zero or more formal parameters • usually a procedure has a certain fixed number of parameters • in C, varargs is used to get around this, e.g. for printf • When the procedure is invoked, the arguments are used

  4. Arguments and Parameters • The formal parameter of foo is x • The argument of foo in line 5 is 2 • In C, everything is a function :-( float foo(x: int) { return sqrt(x); } main(int argc, char **argv) { printf(“%f”, foo(2)); }

  5. Parameter Passing • Call by value • used in C • Call by reference (a.k.a. call by address) • available in Pascal et al • procedure foo(var J: integer) (* J might be changed *) • Call by value-result • like call by reference, except parameter can’t change until the procedure ends • Call by name (historical interest only)

  6. Activation Records • Each invocation of a procedure (unless it’s inline) is associated with an in-memory data structure that indicates • Where the local variables are • The state of the call stack, i.e. to where should control return after this invocation ends? • Prologue and epilogue code is generated to manage local storage, and do copying of parameters

  7. Lexical vs. Dynamic Scope • In lexical scope, identifiers are declared in accordance to nesting in the program text • Can be determined unambiguously at compile-time • In dynamic scope, identifiers are declared in accordance to when the declarations are encountered during execution • Discuss example on pages 161-162

  8. Activation Trees • Each node represents an activation of a procedure • Nodes are listed left to right in order of invocation • Nodes can be “decorated” with values of parameters, local variables, etc. • Exercise 5.2 a,b,c due Tuesday 3/29 • Ignore tail-recursion elimination & quicksort

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