Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Institute of Radio Physics and Electronics and Indian GNU/Linux Users Group Kolkata Chapter

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Here We Start  Crashes, errors and warnings are part of a C programmer’s life  Debugger allows the programmer to take a look at the program in execution to deduce the cause of crashes and erroneous results

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Debugger over naïve printf  printf pollutes the code  Difficult to probe the cause of a failure if printfs are absent at appropriate places  Inserting a new code may change program behavior and therefore the nature and/or existance of error and/or crash  Inserting new debug code implies recompilation

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Debugger over naïve printf (contd.)  With debugger, program needs to be compiled only once with debug flag  The program run from within a debugger may be stopped at specified points and the state of the program (variables, etc) observed  Allows post-mortem of program crashes

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB GDB  GDB is the GNU Source-Level Debugger for C/C++ programs  GDB may be used to debug programs that either crash or produce incorrect results when run  Allows for debugging of single as well as multi-threaded programs

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Compiling for Debugging  Use “ -g ” or “ -ggdb ” compile option to add debugging information to the object files  Preserves type information from variables  Preserves function prototypes  provides correspondence between instructions and source code line numbers  A numerical argument may follow “ –g ” or “ –ggdb ”, 1 meaning least amount of debugging information and 3 maximum. Default is 2  With “ –g3 ” or “ -ggdb3 ”, preprocessor macros can be accessed

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Compiling for Debugging (contd.)  Best to turn off compiler optimizations  With high optimization levels, variables may get optimized out of existence and functions get inlined  Example: $ gcc –Wall –g3 –O0 –o hello hello.c

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Running gdb  Common ways to run GDB :  gdb Ex: $ gdb hello  gdb Ex: $ gdb hello core.123  gdb Ex: $ gdb hello 123  Once program is loaded, run it:  (gdb) r hello  You can also specify arguments to the program:  (gdb) r hello GNU

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Debugger Features  Stop program execution (breakpoints)  Stop program execution under certain conditions (conditional breakpoints)  Display values of variables, pointers, and contents of structures  Execute program line by line  Hop from stack frame to stack frame  Create watchpoints  Alter data and program execution

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Breakpoints  Specifies point in the program at which the debugger should stop executing (gdb) b main (gdb) b HelloWorld.c:5 The above is useful in multifile projects (gdb) b printHelloWorld  Each breakpoint gets an identifier which can be used later to enable, disable or delete the breakpoint

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Deleting Breakpoints  Breakpoints can be removed in the following ways: To remove a breakpoint by its location, use clear To remove a breakpoint by its identifier, use delete (gdb) clear main Deleted breakpoint 1 (gdb) clear HelloWorld.c:5 Deleted breakpoint 2  delete n deletes the breakpoint with identifier n (gdb) delete 1 Deleted breakpoint 1

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Stepping and Resuming ● Once a program has stopped at a specified point, it can be made to resume execution using one of the following ways: ● Execute next program line (after stopping); step over any function calls in the line (gdb) next ● Execute next program line (after stopping); step into any function calls in the line (gdb) step ● Continue running your program (after stopping, e.g. at a breakpoint) (gdb) c

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Inspecting Variables ● Values and types of variables can be inspected with gdb ● ptype allows looking at the types of variables (can be structures) (gdb) ptype mystring type = unsigned char * (gdb) ptype mystruct type = struct map { int key; char* val; } ●

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Displaying variables Values of variables can be printed: (gdb) print num With display, variables are printed whenever they change (gdb) display num

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Stack Frames  Stack Frame: A structure which holds execution information for a function call  Components of the structure are: - return pointer - space for the function’s return value (to be populated) - arguments to the function - local variables  A stack frame is created for each function call and placed on the top of the stack  When a function call returns, the frame corresponding to the function call is removed from the top of the stack

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Navigating Stack Frames ● The commands to list the program stacks are: ● Display backtrace (that is the program stack upto the current point) (gdb) bt ● Move up the stack frame (gdb) up ● Move down the stack frame (gdb) down ● Display frame 1 (gdb) frame 1

Institute of Radio Physics and Electronics ILug-Cal Introduction to GDB Wrapping up ● To complete the function being currently executed, use “finish”. It also shows the value the function returned. (gdb) finish ● To quit gdb, use: (gdb) quit