©2004 Joel Jones1

CS 403: Programming Languages

Lecture 4

Fall 2004

Department of Computer Science

University of Alabama

Joel Jones

Lecture 4 ©2004 Joel Jones2

Outline

• Symbol Tables• Activation Records• GDB• Reading & Questions for Next Class

Lecture 4 ©2004 Joel Jones3

Symbol Tables (Cont.)• A hash table can be added to the previous

data structure to accelerate the search.• Elements with the same name are linked from

top to bottom.• Search start at the entry of the hash table and

proceeds through the linked list until the end of the list is reached (old_id) or until the link list refers to an element below scope_marker(LL - 1) (new_id)

Lecture 4 ©2004 Joel Jones4

Symbol Tables (Cont.)• This approach does not work in some cases.• Consider the with statement of Pascal and Modula 2.Date = RECORD day: [1..31]; mo: month; yr: CARDINAL ENDd1: Date;

WITH d1 DO day:=10; mo:=Sep; yr:=1981ENDis equivalent to

d1.day:=10; d1.mo:=Sep; d1.yr:=1981

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Symbol Tables

Lecture 4 ©2004 Joel Jones6

Symbol Tables (Cont.)

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The binding of referencing environments

• Shallow binding: the referencing environment of a routine is not created until the subroutine is actually called.

• Deep binding: the program binds the environment at the time the subroutine is passed as a parameter.

• Deep binding is implemented by creating an explicit representation of a referencing environment and bundling it together with a reference to the subroutine. Closure

Lecture 4 ©2004 Joel Jones8

Activation Records for Nested RoutinesP1(){ REAL X { /* B1 */ { /* B2 */ { /* B3 */ P2(P3) }

P3() { x } } P2(PX) { PX() }

}

}

ARP1

ARB1

ARB2

ARB3

ARP2

ARP3

PX

Lecture 4 ©2004 Joel Jones9

GDB

• GDB – Gnu DeBugger• Preparation

• Compile with gcc -g• Example: gcc -g -o myProgram myprogram.c

• Run the program under control of gdb• Example: gdb myProgram

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MP1

• Complete write-up available on the class website

• Basic Idea: write 1 or more programs and associated GDB scripts that demonstrate object (generic, not OO) lifetimes and scopes

• GDB scripts should print out addresses, values of variables, etc.

Lecture 4 ©2004 Joel Jones11

Demonstration of GDB

• Create program• Compile it• Run under gdb• Print some variables• Show automatically running script

• gdb -x gdbCmdFile myProgram

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Reading for Next Class

• Skim the following, paying particular attention to how to print the addresses of variables:• Debugging with GDB: The GNU Source

Level Debugger• http://www.gnu.org/manual/gdb-5.1.1/gdb.html

• GDB Quick Reference Card• http://www.refcards.com/download/gdb-refcard-

letter.pdf