Lab 7 Linux Debugging. EECS 448: Software Engineering I Mark Calnon October 17, 2011

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1 Lab 7 Linux Debugging EECS 448: Software Engineering I Mark Calnon October 17, 2011

2 GDB Getting Started To start gdb from the command line, first browse to the directory containing the core dump to debug and the program executable. Next, start gdb specifying both the program executable and the core dump. For example: $ gdb main core.8596 GDB will first attempt to load all of the appropriate symbol files, and then place you at a command prompt where you can enter additional GDB commands.

3 GDB Back Trace To display the call stack of the running program at the time of the crash, use where or bt. (gdb) bt #0 0x c in?? () #1 0x f8b in CoffeeShop::chatUpCustomer (this=0x1a94260, customer=0x7fff704dc7f0) at CoffeeShop.cpp:44 #2 0x b in CoffeeShop::addPushyCustomer (this=0x1a94260) at CoffeeShop.cpp:74 #3 0x a3 in main (argc=2, argv=0x7fff704dcb98) at main.cpp:45

4 GDB Full Back Trace To display more information about the call stack, including function parameters and local variables, use bt full. (gdb) bt full #0 0x c in?? () No symbol table info available. #1 0x f8b in CoffeeShop::chatUpCustomer (this=0x1a94260, customer=0x7fff704dc7f0) at CoffeeShop.cpp:44 No locals. #2 0x b in CoffeeShop::addPushyCustomer (this=0x1a94260) at CoffeeShop.cpp:74 customer = {<Customer> = {_vptr.customer = 0x406151, Name = ""}, <No data fields>} customerdelay = 0 #3 0x a3 in main (argc=2, argv=0x7fff704dcb98) at main.cpp:45 in = <incomplete type> LydiasLattes = 0x1a94260 input = "PushyCustomer"

5 GDB Back Trace (Threaded) To display the call stacks of other running threads, use the following commands: thread apply all bt or thread apply all bt full - displays the call stacks for all running threads. thread apply 5 bt or thread 5 (switches to thread 5) and bt (displays the call stack) - displays the call stack for a specific thread.

6 GDB Displaying Program Data To display the value of a variable at the current point in the program s execution, use print, following by the name of the variable or a c++ expression. (gdb) print *customer $1 = {_vptr.customer = 0x406151, Name = ""} (gdb) print this $2 = (CoffeeShop * const) 0x1a94260 (gdb) print *this $3 = {CustomerQueue = std::queue wrapping: std::deque with 2 elements = {0x1a94410, 0x1a94390}, CustomerStack = std::stack wrapping: std::deque with 1 elements = {0x1a94460}}

7 GDB Back Trace Program Data To display the value of a variable at a prior point in the program s execution, use up and down to traverse the active call stack, or use frame n to specify an exact the point in the call stack. (gdb) frame 2 #2 0x b in CoffeeShop::addPushyCustomer (this=0x1a94260) at CoffeeShop.cpp:74 74 chatupcustomer(&customer); (gdb) print customer $1 = {<Customer> = {_vptr.customer = 0x406151, Name = ""}, <No data fields>}

8 GDB Disassembly To display a disassembly of the executable, use disassemble. (gdb) disassemble Dump of assembler code for function CoffeeShop::chatUpCustomer(Customer*): 0x f64 <+0>: push %rbp 0x f65 <+1>: mov %rsp,%rbp 0x f68 <+4>: sub $0x10,%rsp 0x f6c <+8>: mov %rdi,-0x8(%rbp) 0x f70 <+12>: mov %rsi,-0x10(%rbp) 0x f74 <+16>: mov -0x10(%rbp),%rax 0x f78 <+20>: mov (%rax),%rax 0x f7b <+23>: add $0x10,%rax 0x f7f <+27>: mov (%rax),%rdx 0x f82 <+30>: mov -0x10(%rbp),%rax 0x f86 <+34>: mov %rax,%rdi 0x f89 <+37>: callq *%rdx => 0x f8b <+39>: leaveq 0x f8c <+40>: retq End of assembler dump.

9 GDB Registers To display the state of the registers (such as for determining the exact instruction running at the time of the crash), use info registers. (gdb) info registers rax 0x7fff704dc7f rbx 0x0 0 rcx 0x3220cd30f rdx 0x401c rsi 0x7fff704dc7f rdi 0x7fff704dc7f rbp 0x7fff704dc7c0 0x7fff704dc7c0 rsp 0x7fff704dc7b0 0x7fff704dc7b0 r8 0x7f828aab r9 0x7f828aab r10 0xcccccccccccccccd r11 0x r12 0x7fff704dca r13 0x7fff704dcb r14 0x0 0 r15 0x0 0 rip 0x401f8b 0x401f8b <CoffeeShop::chatUpCustomer(Customer*)+39> eflags 0x10202 [ IF RF ] cs 0x33 51 ss 0x2b 43 ds 0x0 0 es 0x0 0 fs 0x0 0 gs 0x0 0

10 GDB Memory To display the contents in memory at a specific location, use x. Note: To see additional formatting arguments that can be provided, use help x. (gdb) x 0x401f8b 0x401f8b <CoffeeShop::chatUpCustomer(Customer*)+39>: 0x5590c3c9 (gdb) x/8 0x401f8b 0x401f8b <CoffeeShop::chatUpCustomer(Customer*)+39>: 0x5590c3c9 0x48e x4810ec83 0x48f87d89 0x401f9b <CoffeeShop::addCustomer(Customer*)+13>: 0x48f x48f8458b 0x0ae8c789 0x83ffffff

11 GDB Debugging To debug the executable, use run. Note: Command-line arguments can be provided as arguments to run. (gdb) run input.txt Starting program:./main input.txt "Welcome back, John!" Now serving John Done serving John "Welcome back, Mary!" Now serving Mary "Good afternoon, Mr. President!" Now serving VIP5 (Pushy customer delays line 2 minutes) "..." Done serving VIP5 "Welcome back, Mary!" Now serving Mary Done serving Mary "How's the weather?" Done serving Customer1 No one in line [Inferior 1 (process 9607) exited normally]

12 GDB Breakpoints To set a breakpoint in the program, use b <line number> or b <function name>. (gdb) b CoffeeShop::addPushyCustomer Breakpoint 1 at 0x40209d: file CoffeeShop.cpp, line 68. (gdb) run input.txt Starting program:./main input.txt "Welcome back, John!" Now serving John Done serving John "Welcome back, Mary!" Now serving Mary "Good afternoon, Mr. President!" Now serving VIP5 Breakpoint 1, CoffeeShop::addPushyCustomer (this=0x60c260) at CoffeeShop.cpp:68 68 PushyCustomer customer;

13 GDB Debugging To step over the next line (i.e., execute function calls as a single instruction), use next. To step into the next line (i.e., execute the next instruction in a function call) use step. (gdb) run input.txt... Breakpoint 1, CoffeeShop::addPushyCustomer (this=0x60c260) at CoffeeShop.cpp:68 68 PushyCustomer customer; (gdb) next 69 unsigned int customerdelay = 0; (gdb) next 70 customer.getcustomerdelay(&customerdelay); (gdb) step PushyCustomer::GetCustomerDelay (this=0x7fffffffda90, customerdelay=0x7fffffffda8c) at PushyCustomer.cpp:25 25 srand(time(null));

Binghamton University. CS-220 Spring X86 Debug. Computer Systems Section 3.11

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