Lei Tang's List: HowTos

•     4.3  The Main gdb Commands
           4.3.1  Invoking/Quitting gdb
           4.3.2  The r (Run) Command
           4.3.3  The l (List) Command
           4.3.4  The b (Breakpoint) and c (Continue) Commands
           4.3.5  The d (Display) and p (Print) Commands:
           4.3.6  The printf Command
           4.3.7  The n (Next) and s (Step) Commands
           4.3.8  The bt (Backtrace) Command
           4.3.9  The set Command
       4.4  The call Command

• $ gdb  (gdb) attach 17399

• To set a breakpoint 2 lines down from the current line, use break +2. Similarly, you can set a breakpoint 3 lines up from the current line by break -3.
• The break command without any argument gives a 7th form (only one more to go). It sets a break point at the very next instruction
• break +offset or break linenumber is more convenien
• break ... if cond
• Form 1:break linenumberSet a breakpoint at line number linenumberForm 2:break functionSet a breakpoint at function function.Form 3:break filename:linenumberSet a breakpoint at line linenum in source file filename.Form 4:break *addressSet a breakpoint at address <address>. Use this to set breakpoints in parts of a program that doesn't have debugging information or source files.
• That's quite a structure! You can abbreviate ptype by pt.
• you can use the abbreviation p for print:
• print 5 elements of myIntArray, starting at element 3
• (gdb) p myIntArray[3]@5  $49 = {3, 4, 5, 6, 7}
• You can set pretty printing of structures by set print pretty:

• You can print things using a format specifier:

    

  	print /FMT variable 

    

  Where FMT is:

        

  o	octal		x	hex		d	decimal		u	unsigned decimal
  t	binary		f	float		a	address		c	char
• There are two ways you can change the value of a variable in gdb. Let's change the value of double myvariable to 10.0. Firstly, you can use the set command:

• step: 

  Execute a single line in the program. If the current statement calls a function, the function is single stepped. 

  next: 

  Execute a single line in the program but treat function calls as a single line. This command is used to skip over function calls. 

    
• To find out where you are at any time, type the command:

• If you're listing lines and decide you want to see the 10 lines previous to the 10 lines you just displayed:

    

  	list - 

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• • A breakpoint stops your program whenever a particular point in the program is reached. We will discuss breakpoints  momentarily.
   
  • A watchpoint stops your program whenever the value of a variable or expression changes. We'll discuss watchpoints  later on in this chapter.
   
  • A catchpoint stops your program whenever a particular event occurs. We won't discuss catchpoints until I get a chance  to write about them.
• • If you want to remove the breakpoint by its location, use clear.
   
  • If you want to remove the breakpoint by its identifier, use delete.
• You can enable or disable breakpoints using the enable and disable commands which take an argument of the breakpoint identifier for the breakpoint you want to enable or disab
• ignore  command, which disables a breakpoint "for n crossings"

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• We can ask GDB to tell us what the stack looks like with the backtrace command. We can also find out which frame GDB's context is in using the frame command. Lastly, we can change GDB's context to the n'th frame using the frame n command.
• Executables don't contain references to object (function and variable) names or source code line numbers. It would be painful to debug a program without these things, so to debug a program, we generate an augmented symbol table using gcc's -g option.
• the break command and execute one line of source code using the step command
• "list -" works like list, except in reverse
• starting with some line number(gdb) list 5,   ending with some line number (gdb) list ,28   between two numbers: (gdb) list 21,25   by function name: (gdb) list f   functions in the other file: (gdb) list CentralDiff   by filename and line number: (gdb) list derivative.c:12   filename and function name: (gdb) list derivative.c:ForwardDiff
• set listsize 5
• You can put commands to be executed for all your programming projects in $HOME/.gdbinit and project-specific commands in $PWD/.gdbinit.
• If you do give an argument to set args, those arguments will be passed to the program the next time you use run
• 1. Use the kill command to stop the program, and run to restart it.
   
  2. Use the GDB command run. GDB will tell you the program is already running and ask if  you want to re-run the program from the beginning.
   
    

  The last two options will leave everything intact: breakpoints, watchpoints, commands, convenience variables,

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• For many members, “friends” now means a mish-mash of real friends, former friends, friends of friends, and non-friends; younger and older relatives; colleagues and, if cursed, a nosy boss or two. Everyone accepted as a “friend” gets the same access

• I suggest you always use -ggdb3 to produce an enhanced symbol table.
• To be on the safe side, turn off optimization when you're debugging a program
• We can tell GDB to switch from frame 0 to frame 1 using the frame command with the frame number as an argument

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