• gdb is the GNU debugger on our CS machines. • gdb is most effective when it is debugging a program that

has debugging symbols linked in to it. With gcc and g++, this is accomplished using the -g option, for example, to compile colortest.c and have the executable stored in testcolor, use the following:

gcc -Wall -g -o testcolor colortest.c

• Once you have an executable file (in this case prog.o) you can use gdb to debug it. First you must launch the debugger. To launch the debugger you type

gdb <executable>

• For the above program, this will be

gdb testcolor

gdb

• Valgrind is another suite of tools for debugging and profiling your programs.

• First compile your program using the -g option as above, then launch valgrind:

valgrind [valgrind-options] [your-prog] [your-prog-options]

• For example, if you execute testcolor using the command

./testcolor colordata1.txt

you would could use the following command

valgrind --leak-check=yes ./testcolor colordata1.txt

• For more information about valgrind, refer to the man page:

man valgrind

Valgrind

The gdb debugger is a handy tool for identifying the location at which a program failed.

At the (gdb) prompt you will usually want to tell the debugger to halt the program when it reaches the start of the main() function. The b command is short for breakpoint and tells the debugger where to stop.

After a function is entered, source code line numbers can be used to specify breakpoints.

Using gdb to find the point of failure

(gdb) b main

Breakpoint 1 at 0x804833b: file p18.c, line 11.To start the program use the run command:

(gdb) run

Starting program: /home/rlowe/cs2100/examples/p18

When the program reaches a breakpoint gdb will tell you and display the next line of code to be executed.

Breakpoint 1, main () at p18.c:1111 printf("val of ptr is %p \n", ptr);

Using gdb to find the point of failure

Use the next command to execute a single source statement. The next command will treat a function call as a single statement and not single step into the function being called.

If you want to single step through the function use the step command to step into it. The output of the printf() is intermixed with gdb's output and is shown in blue below.

(gdb) nextval of ptr is (nil)13 *ptr = 99;

Using gdb to find the point of failure

Saying next again will cause the program to execute the flawed assignment. gdb will show you the line that caused the error (line # 13).

(gdb) next

Program received signal SIGSEGV, Segmentation fault.0x08048357 in main () at p18.c:1313 *ptr = 99;

Using gdb to find the point of failure

The where command can show you where the failure occurred (along with a complete function activation trace.

(gdb) where#0 0x08048357 in main () at p18.c:13#1 0x40049917 in __libc_start_main () from /lib/libc.so.6(gdb)

To print the value of a variable use the print command.

(gdb) print ptr$1 = (int *) 0x0

Attempting to print what ptr points to reaffirms what the problem is:(gdb) print *ptrCannot access memory at address 0x0

Using gdb to find the point of failure

Use the q (quit) command to terminate gdb

(gdb) quit

The program is running. Exit anyway? (y or n) yhome/rlowe/cs2100/examples ==>

Using gdb to find the point of failure

Common gdb commands

Command Description

help List gdb command topics.

help topic-classes List gdb command within class.

help command Command description.

apropos search-wordSearch for commands and command topics containing search-word.

info argsi args List program command line arguments

info breakpointsPrint info about breakpoints and watchpoints

info break Print breakpoint numbers.

info break breakpoint-number Print info about specific breakpoint.

info watchpoints Print info about watchpoints

Common gdb commandstbreak Temporary break. Break once only.

Break is then removed. See "break" above for options.

watch condition Suspend processing when condition is met. i.e. x > 5

clearclear functionclear line-number

Delete breakpoints as identified by command option.

deleted

Delete all breakpoints, watchpoints, or catchpoints.

delete breakpoint-numberdelete range

Delete the breakpoints, watchpoints, or catchpoints of the breakpoint ranges specified as arguments.

disable breakpoint-number-or-rangeenable breakpoint-number-or-range

Does not delete breakpoints. Just enables/disables them.Example:Show breakpoints: info breakDisable: disable 2-9

Common gdb commandsBreak and Watch

break funtion-namebreak line-numberbreak ClassName::functionName

Suspend program at specified function of line number.

break +offsetbreak -offset

Set a breakpoint specified number of lines forward or back from the position at which execution stopped.

break filename:function Don't specify path, just the file name and function name.

break filename:line-number Don't specify path, just the file name and line number.break Directory/Path/filename.cpp:62

break line-number if condition Where condition is an expression. i.e. x > 5Suspend when boolean expression is true.

Common gdb commandsenable breakpoint-number once

Enables once

continuec

Continue executing until next break point/watchpoint.

continue number Continue but ignore current breakpoint number times. Usefull for breakpoints within a loop.

finish Continue to end of function.

   

Line Execution

stepsstep number-of-steps-to-perform

Step to next line of code. Will step into a function.

nextnnext number

Execute next line of code. Will not enter functions.

Common gdb commands

untiluntil line-number

Continue processing until you reacha aspecified line number. Also: function name, address, filename:function or filename:line-number.

where Shows current line number and which function you are in.

Source Code

listllist line-numberlist functionlist -list start#,end#list filename:function

List source code.

set listsize countshow listsize

Number of lines listed when list command given.

Common gdb commands

Examine Variables print variable-namep variable-namep file-name::variable-namep 'file-name'::variable-name

Print value stored in variable.

Start and Stop runrrun command-line-argumentsrun < infile > outfile

Start program execution from the beginning of the program. The command break main will get you started. Also allows basic I/O redirection.

continuec

Continue execution to next break point.

killk

Stop program execution.

quitq

Exit GDB debugger.

gdb – Quick reference guid

run -- run the program

run args -- run program with command line args.

break function -- set breakpoint at function entry

break linenum -- set breakpoint at line

break … if cond -- set breakpoint; break if condition

clear funct -- remove breakpoint at function entry

delete bnum -- delete breakpoint bnum

disable bnum -- disable breakpoint bnum

enable bnum -- enable breakpoint bnum

condition bnum -- set conditions for breakpoint bnum

commands bnum -- set commands for breakpoint bnum

cont -- continue execution to next break point

gdb – Quick reference guid

next -- step next source level statement or function

nexti -- step next machine instruction or function

step -- step next source level statement

stepi -- step next machine instruction

print expr -- print value of expression

info data -- information about break, display, registers, functions, variables

list -- list ten source lines

where -- show call stack

kill -- stop program execution

quit -- exit gdb

Note: pressing Enter repeats the last command.