Gnu Debugger: Gdb, Page 1

CS 310 Gnu Debugger: gdb, Page 1

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GNU Debugger: gdb

• A text-based symbolic debugger • Make your program stop on speciﬁed conditions. • Examine what has happened, when your program has stopped. • Change things in your program, so you can experiment with correcting the eﬀects of one bug and go on to learn about another.

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CS 310 Gnu Debugger: gdb, Page 2

' $ An Example Program: main.cc

1 #include 2 class Queue { 3 class ListNode { 4 public: 5 int data; 6 ListNode* next; 7 }; 8 ListNode *front, *rear; 9 public: 10 Queue () {front = rear = NULL;}; ~Queue() {}; 11 void add (int new_item); 12 void remove (int& old_front, bool& empty); 13 };

& % CS 310 Gnu Debugger: gdb, Page 3

' $ 16 void Queue::add (int new_item) 17 { 18 ListNode* p = new ListNode; 19 p->data = new_item; 20 p->next = NULL; 21 22 if (rear==NULL) 23 rear = front = p; 24 else 25 { 26 rear->next = p; 27 rear = p; 28 } 29 }

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CS 310 Gnu Debugger: gdb, Page 4

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32 void Queue::remove (int& old_front, bool& empty) 33 { 34 empty = (front == NULL); 35 if (empty) 36 return; 37 38 ListNode *p = front; 39 front = front->next; 40 old_front = p->data; 41 delete p; 42 }

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46 int main () 47 { 48 Queue q; 49 int i; 50 bool empty; 51 52 q.add (1); 53 q.add (2); 54 q.remove (i,empty); 55 q.remove (i,empty); 56 q.add (3); 57 }

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CS 310 Gnu Debugger: gdb, Page 6

' $ A Debugging Scenario

• g++ -g main.cc • gdb a.out ... We now see the gdb prompt (gdb) ... • list command: see program lines list 46,57 (see lines 46 – 57) • break command: set breakpoints, the points at which we would like the program to pause break Queue::add (stop when entering Queue::add) break Queue::remove (stop when entering Queue::remove) break 48 (stop when reaching line 48)

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'• run (start executing program) $ ... Program stops at line 18, the ﬁrst line of Queue::add ... • print command: examine data print front =⇒ 0x0 (whose front?) print rear =⇒ 0x0 • cont (continue) command: resume program execution ... Program execution resumes and stops again at line 18 ... • print front =⇒ 0x20e88 print rear =⇒ 0x20e88 print front->data =⇒ 1 print rear->data =⇒ ? print front->next =⇒ 0x0 cont ... Program execution resumes and stops again at line 34 ... • print front =⇒ 0x20e88

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CS 310 Gnu Debugger: gdb, Page 8

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print rear =⇒ 0x20e98 print front->data =⇒ 1 print front->next =⇒ 0x20e98 print rear->data =⇒ 2 print rear->next =⇒ 0x0 cont ... Program execution resumes and stops at line 34 ... • print front =⇒ 0x20e98 print rear =⇒ 0x20e98 print front->next =⇒ 0x0 print rear->data =⇒ 2 cont

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... Program execution resumes and stops at line 18 ... • print q.front =⇒ 0x0 print q.rear =⇒ 0x20e98 cont ... Program crashes at line 26 ... • Other useful command: 1. where – show the calling stack 2. display – automatically print some variables when program paused 3. next, step – single step; execute the next statement and pause

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CS 310 Gnu Debugger: gdb, Page 10

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• More complete information available at: http://www.delorie.com/gnu/docs/gdb/gdb toc.html • When encountering a segmentation fault, you probably screwed up pointers. Run your program within gdb, which will show at which line your program crashes. ☞ do not assume that line is the origin of the problem; keep your mind open • In general, when encountering bugs in your programs, use gdb; the instructor and the TA reserve the right not to answer your questions if you have not done so.

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