GDB The GNU Project Debugger

1
GDBThe GNU Project Debugger

• CS202
• Dec 7, 2006
• Semsak Buntha
• Office 205A
• Email buntha_at_cse.unr.edu

2
Get ready before a ride

• Download
• www.cse.unr.edu/buntha/202/gdb/gdb.tgz
• Extract files
• tar xvfz gdb.tgz
• Enter gdb directory
• cd gdb

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Compilation for gdb

• g -g Wall -c file1.cpp
• g -g Wall -c file2.cpp
• g -o myout file1.o file2.o

or

• g -g -o myout file1.cpp file2.cpp

-g causes the compiler to generate an augmented
symbol table Without -g, we cannot use gdb
4
Start Stop

• Start gdb
• gdb program core.xxxx
• e.g. gdb myout
• Stop gdb
• (gdb) quit

5
Basic commands

• list
• break, info, delete
• run, continue where
• finish, next, step
• print display
• whatis ptype

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list

• (gdb) l
• - show 10 lines surrounding the code that has
  just executed
• - if the program has not run yet, it will show
  at main().
• (gdb) l 5
• - show 10 lines start from 1 of current file
• (gdb) l div.cpp1, 13
• - show the lines between 1 and 15 of file
  div.cpp

7
break, info, delete

• break - set a breakpoint
• (gdb) b main.cpp 5
• (gdb) b div.cpp 6
• (gdb) b 8
• Info break - see all breakpoints
• (gdb) I b
• delete - delete a breakpoint
• (gdb) d 3
• note we can disable and enable a breakpoint

8
Source code
div.cpp
main.cpp
1 include "div.h" 2 using namespace
std 3 4 template ltclass Tgt 5
PairltTgtPair(T xx, T yy) 6 x
xx 7 y yy 8 9
10 template ltclass Tgt 11 void
PairltTgtresult() 12 cout ltlt x/y
ltlt endl 13
1 include "div.h" 2 include
"div.cpp" 3 4 int main() 5
Pairltdoublegt a(18, 4) 6
Pairltintgt b(18, 4) 7 a.result()
8 b.result() 9 return
0 10
Existing breakpoints
Deleted breakpoint
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run, continue where

• run start running the program
• (gdb) r
• Note stop program with CTRL-C
• exit gdb with (gdb) q
• continue execute until next break point
• (gdb) c
• where list all functions called in hierarchy
• (gdb) where
• Note most recent first

10
Source code
div.cpp
main.cpp
1 include "div.h" 2 using namespace
std 3 4 template ltclass Tgt 5
PairltTgtPair(T xx, T yy) 6 x
xx 7 y yy 8 9
10 template ltclass Tgt 11 void
PairltTgtresult() 12 cout ltlt x/y
ltlt endl 13
1 include "div.h" 2 include
"div.cpp" 3 4 int main() 5
Pairltdoublegt a(18, 4) 6
Pairltintgt b(18, 4) 7 a.result()
8 b.result() 9 return
0 10
First stop
Second stop after continue
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finish, next, step

• finish continues until the function return
• (gdb) finish
• Next executes the next line. Not stop inside
  the function
• (gdb) n
• step enters the function and executes one line
  at a time
• (gdb) s

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Source code
div.cpp
main.cpp
1 include "div.h" 2 using namespace
std 3 4 template ltclass Tgt 5
PairltTgtPair(T xx, T yy) 6 x
xx 7 y yy 8 9
10 template ltclass Tgt 11 void
PairltTgtresult() 12 cout ltlt x/y
ltlt endl 13
1 include "div.h" 2 include
"div.cpp" 3 4 int main() 5
Pairltdoublegt a(18, 4) 6
Pairltintgt b(18, 4) 7 a.result()
8 b.result() 9 return
0 10
Current stop
Next stop after command finish
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Source code
div.cpp
main.cpp
1 include "div.h" 2 using namespace
std 3 4 template ltclass Tgt 5
PairltTgtPair(T xx, T yy) 6 x
xx 7 y yy 8 9
10 template ltclass Tgt 11 void
PairltTgtresult() 12 cout ltlt x/y
ltlt endl 13
1 include "div.h" 2 include
"div.cpp" 3 4 int main() 5
Pairltdoublegt a(18, 4) 6
Pairltintgt b(18, 4) 7 a.result()
8 b.result() 9 return
0 10
Next command
Step command
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print display

• print show value of variable
• (gdb) print x
• (gdb) n
• (gdb) print a
• display show value whenever stops
• (gdb) display a
• (gdb) display b
• (gdb) n
• (gdb) undisplay 2

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whatis ptype

• whatis show datatype
• (gdb) whatis a.x
• (gdb) whatis b.x
• (gdb) whatis a
• ptype show structure
• (gdb) ptype a
• (gdb) ptype b

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Question?

• Office SEM 205A 9am-4pm
• Email buntha_at_cse.unr.edu