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GDB commands by function - simple guide

Startup

| Syntax | Description | |—|—| |% gdb -help| print startup help, show switches| |% gdb object|normal debug| |% gdb object core| core debug (must specify core file)| |% gdb object pid| attach to running process |% gdb| use file command to load object

Help

Syntax Description
(gdb) help list command classes
(gdb) help running list commands in one command class
(gdb) help run bottom-level help for a command “run”
(gdb) help info list info commands (running program state)
(gdb) help info line help for a particular info command
(gdb) help show list show commands (gdb state)
(gdb) help show commands specific help for a show command

Breakpoints

| Syntax | Description | |—|—| |(gdb) break main| set a breakpoint on a function |(gdb) break 101| set a breakpoint on a line number |(gdb) break basic.c:101| set breakpoint at file and line (or function) |(gdb) info breakpoints| show breakpoints |(gdb) delete 1| delete a breakpoint by number |(gdb) delete| delete all breakpoints (prompted) |(gdb) clear| delete breakpoints at current line |(gdb) clear function| delete breakpoints at function |(gdb) clear line| delete breakpoints at line |(gdb) disable 2| turn a breakpoint off, but don’t remove it |(gdb) enable 2| turn disabled breakpoint back on |(gdb) tbreak function|line| set a temporary breakpoint |(gdb) commands break-no … end| set gdb commands with breakpoint |(gdb) ignore break-no count| ignore bpt N-1 times before activation |(gdb) condition break-no expression break only if condition is true |(gdb) condition 2 i == 20| example: break on breakpoint 2 if i equals 20 |(gdb) watch expression| set software watchpoint on variable |(gdb) info watchpoints| show current watchpoints

Running the program

Syntax Description
(gdb) run run the program with current arguments
(gdb) run args redirection run with args and redirection
(gdb) set args args… set arguments for run
(gdb) show args show current arguments to run
(gdb) cont continue the program
(gdb) step single step the program; step into functions
(gdb) step count singlestep \fIcount\fR times
(gdb) next step but step over functions
(gdb) next count next \fIcount\fR times
(gdb) CTRL-C actually SIGINT, stop execution of current program
(gdb) attach process-id attach to running program
(gdb) detach detach from running program
(gdb) finish finish current function’s execution
(gdb) kill kill current executing program

Stack backtrace

Syntax Description
(gdb) bt print stack backtrace
(gdb) frame show current execution position
(gdb) up move up stack trace (towards main)
(gdb) down move down stack trace (away from main)
(gdb) info locals print automatic variables in frame
(gdb) info args print function parameters

Browsing source

Syntax Description
(gdb) list 101 list 10 lines around line 101
(gdb) list 1,10 list lines 1 to 10
(gdb) list main list lines around function
(gdb) list basic.c:main list from another file basic.c
(gdb) list - list previous 10 lines
(gdb) list *0x22e4 list source at address
(gdb) cd dir change current directory to \fIdir\fR
(gdb) pwd print working directory
(gdb) search regexpr forward current for regular expression
(gdb) reverse-search regexpr backward search for regular expression
(gdb) dir dirname add directory to source path
(gdb) dir reset source path to nothing
(gdb) show directories show source path

Browsing Data

Syntax Description
(gdb) print expression print expression, added to value history
(gdb) print/x expressionR print in hex
(gdb) print array[i]@count artificial array - print array range
(gdb) print $ print last value
(gdb) print *$->next print thru list
(gdb) print $1 print value 1 from value history
(gdb) print ::gx force scope to be global
(gdb) print ‘basic.c’::gx global scope in named file (>=4.6)
(gdb) print/x &main print address of function
(gdb) x/countFormatSize address low-level examine command
(gdb) x/x &gx print gx in hex
(gdb) x/4wx &main print 4 longs at start of \fImain\fR in hex
(gdb) x/gf &gd1 print double
(gdb) help x show formats for x
(gdb) info locals print local automatics only
(gdb) info functions regexp print function names
(gdb) info variables regexp print global variable names
(gdb) ptype name print type definition
(gdb) whatis expression print type of expression
(gdb) set variable = expression assign value
(gdb) display expression display expression result at stop
(gdb) undisplay delete displays
(gdb) info display show displays
(gdb) show values print value history (>= gdb 4.0)
(gdb) info history print value history (gdb 3.5)

Object File manipulation

Syntax Description
(gdb) file object load new file for debug (sym+exec)
(gdb) file discard sym+exec file info
(gdb) symbol-file object load only symbol table
(gdb) exec-file object specify object to run (not sym-file)
(gdb) core-file core post-mortem debugging

Signal Control

Syntax Description
(gdb) info signals print signal setup
(gdb) handle signo actions set debugger actions for signal
(gdb) handle INT print print message when signal occurs
(gdb) handle INT noprint don’t print message
(gdb) handle INT stop stop program when signal occurs
(gdb) handle INT nostop don’t stop program
(gdb) handle INT pass allow program to receive signal
(gdb) handle INT nopass debugger catches signal; program doesn’t
(gdb) signal signo continue and send signal to program
(gdb) signal 0 continue and send no signal to program

Machine-level Debug

Syntax Description
(gdb) info registers print registers sans floats
(gdb) info all-registers print all registers
(gdb) print/x $pc print one register
(gdb) stepi single step at machine level
(gdb) si single step at machine level
(gdb) nexti single step (over functions) at machine level
(gdb) ni single step (over functions) at machine level
(gdb) display/i $pc print current instruction in display
(gdb) x/x &gx print variable gx in hex
(gdb) info line 22 print addresses for object code for line 22
(gdb) info line *0x2c4e print line number of object code at address
(gdb) x/10i main disassemble first 10 instructions in \fImain\fR
(gdb) disassemble addr dissassemble code for function around addr

History Display

Syntax Description
(gdb) show commands print command history (>= gdb 4.0)
(gdb) info editing print command history (gdb 3.5)
(gdb) ESC-CTRL-J switch to vi edit mode from emacs edit mode
(gdb) set history expansion on turn on c-shell like history
(gdb) break class::member set breakpoint on class member. may get menu
(gdb) list class::member list member in class
(gdb) ptype class print class members
(gdb) print *this print contents of this pointer
(gdb) rbreak regexpr useful for breakpoint on overloaded member name

Miscellaneous

Syntax Description
(gdb) define command … end define user command
(gdb) RETURN repeat last command
(gdb) shell command args execute shell command
(gdb) source file load gdb commands from file
(gdb) quit quit gdb

GDB Notes

1). break main – Breaks main :P, basically sets a breakpoint at main entry point.

2). clear main –> removes bp

3). watch -> to tell whenever a variable changes, VERY critical.

4). run -> runs the program

5). print varname -> prints value of a variable

6). step -> steps into a function

7). next -> steps over a function

8). continue -> continues until the next breakpoint or watch point

9). list -> prints the code

10). quit -> ends

11). disassemble -> prints the assembly

12). set disassemly-flavor intel for intel syntax

13). x/xg x/s x/s etc are for examining the registers

14). set values using set $rax = pewpew