CMSC212: Intro to Low-Level Programming Concepts

Section 0101: 9:00am – 9:50amSection 0102: 10:00am-10:50amMonday & WednesdaysRoom 3118

Announcements/Reminders

•Quiz #2: This Wednesday (12mins)•Reminder: Must submit versions of every

project that work on at least half the public tests to pass the course (Still can submit Project #1 up until 10:00 tonight)

•Today: First, the GNU Project Debugger (GDB) and its innards. Later, a worksheet (based on quiz questions from earlier semesters)

GNU Project Debugger (GDB)

•The main idea▫Lets your step through your code “line-by-

line” during execution▫Lets you look/modify variables and their

values during run-time▫Breakpoints!

Want your program to temporary halt execution on a particular line of code, set a breakpoint

•http://sourceware.org/gdb/current/onlinedocs/gdb_toc.html

GNU Project Debugger (GDB)

•Programs must be compiled with the option –g to be debugged with gdb

•Some common commands with console gdb▫r: run: runs the program from the beginning▫c: cont: continues execution after breakpoint▫n: next: executes next statement, steps over

function calls▫s: step: executes next statement, steeping into

function calls▫finish: completes the execution (returns from

current function)

GNU Project Debugger (GDB)

•Breakpoints▫b: break: sets a breakpoint on current line

or any line▫tb: tbreak: sets a temporary breakpoint on

indicated line▫d: delete: deletes one or all breakpoints▫i b: info breakpoints: lists all currently set

breakpoints

GNU Project Debugger (GDB)

•Looking at data▫p: print: prints a variable’s current value.,

print/x prints in hex▫info locals: prints values of current function’s

local variables▫display: followed by a variable name which is

visible at the current position, prints the value of that variable each time the program execution stops.

▫undisplay: Cancels the display of variables set in prior display commands

GNU Project Debugger (GDB)

•Execution history▫where: shows list of active functions, in

order from most recent-called back to main function

▫up: moves up the function which called the current one (to be able to examine its data)

▫down: move back after calling up•If program crashes during execution (e.g.

segfault, use ‘where’ to find out where it happened

GDB with emacs

•Hit escape-x▫Type ‘gdb’▫The following may appear, just hit enter to

launch: gdb –annotate=3 program.x

•Begin debugging, hit r▫Should see a split window of console and

code▫set breakpoints using cursor in the code

window

▫Other features: Look at menu ‘GuD’, “GDB-UI”, and “Display other Windows” Gdb interaction (console) window display of local variables in the main program Source code Runtime stack and current function call chain List o breakpoints set