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CSCI0330 Intro Computer Systems Doeppner
gdb Cheatsheet
Fall 2018
1 Introduction 1
2 Program Execution 1
3 TUI (Text User Interface) Mode 4
4 Viewing Variables, Registers and Memory 4
5 More Information 5 5.1 Official Documentation 6 5.2 Tutorials 6
6 Tips 6
1 Introduction
This document contains several gdb commands which you will find useful throughout your x86- and C-programming career.
The commands contained within this document are by no means exhaustive; gdb contains many features which are not documented here. Consult the man pages (man gdb) or the internet if you require further information.
Throughout this document, commands shall be listed in the form
[c]ommand (optional arg) This is what the command does. This is an example use of this command.
where the character(s) in [brackets] are the abbreviated version of the command.
2 Program Execution
[b]reak <function name or filename:line# or *memory address> Sets a breakpoint on either a function, a line given by a line number, or the instruction located at a particular address.
If you do not have access to the source code of a function and wish to set a breakpoint on a particular instruction, call disassemble function_name (where function_name is the name of the procedure); this command will allow you to see the memory address of each instruction. See section 4 for further information.
(gdb) break main Breakpoint 1 at 0x80488f6: file main.c, line 67.
[d]elete <breakpoint #> Removes the indicated breakpoint. To see breakpoint numbers, run info break , or i b.
(gdb) delete 4
[condition] <breakpoint #> Updates the breakpoint indicated by the given number so that execution of the program stops at that point only if condition is true. condition is expressed in C syntax, and can only use variables and functions that are available in the scope of the breakpoint location
(gdb) break main Breakpoint 1 at 0x80488f6: file main.c, line 48 (gdb) condition 1 argc <= 2 || !strcmp(argv[1], "jasmine")
[i]nfo (about) Lists information about the argument (about), or lists what possible arguments are if none are provided. about can be one of the following^1 : โ [f]rame โ list information about the current stack frame, including the address of the current frame, the address of the previous frame, locations of saved registers, function arguments, and local variables. โ [s]tack โ list the stack backtrace, showing what function calls have been made, and their arguments. You can also use the commands backtrace or where to do the same. โ [r]egisters โlists the contents of each register. [all-r]egisters lists even more registers. โ [b]reak โ lists the number and address of each breakpoint, and what function the breakpoint is in. โ [fu]nctions - lists all of the function signatures, if the program was compiled with the gcc flag -g. This is useful for setting breakpoints in functions.
[file] Loads the specified file into gdb.
[r]un (arg1 arg2 ... argn) Runs the loaded executable program with program arguments arg1 ... argn.
[c]ontinue Resumes execution of a stopped program, stopping again at the next breakpoint.
Prints a stack trace, listing each function and its arguments. This does the same thing as the commands info stack and where.
(gdb) bt #0 fibonacci (n=1) at main.c: #1 fibonacci (n=2) at main.c: #3 main (argc=2, argv=0xbffff6e4) at main.c:
[where] Prints a stack trace, listing each function and its arguments. This is the same as the commands info stack and backtrace.
[q]uit Quits gdb.
3 TUI (Text User Interface) Mode
layout is a terminal interface which allows the user to view the source file while debugging. The TUI mode is enabled by default when you invoke gdb as gdb tui. You can also switch in and out of TUI mode while gdb runs by using various TUI commands and key bindings, such as tui enable or Ctrl-x Ctrl-a. To disable TUI mode, you can type tui disable. If the layout of the TUI becomes unreadable, pressing Ctrl-l will reload it.
Once you are running TUI mode, there are several commands you can use to change the display. One of them is layout name. The name parameter controls which additional windows are displayed, and can be any of the following: โ next will display the next layout. โ prev will display the previous layout. โ src will display the source and command windows. โ asm will display the assembly and command windows. โ split will display the source, assembly, and command windows. โ regs will display the register, source, and command windows when in src layout. When in asm or split layout, will display the register, assembler, and command windows.
When you have multiple windows open, you can then use the command focus name to switch focus between windows. The name parameter controls which window is focused, and can be any of the following: โ next will make the next window active for scrolling. โ prev will make the previous window active for scrolling. โ src will make the source window active for scrolling. โ asm will make the assembly window active for scrolling. โ regs will make the register window active for scrolling.
โ cmd will make the command window active for scrolling. When the command window is active for scrolling, for example, using the arrow keys allows you to scroll through gdb commands instead of moving the text window.
4 Viewing Variables, Registers and Memory
[p]rint Prints the value which the indicated expression evaluates to. expression can contain variable names (from the current scope), memory addresses, registers, and constants as its operands to various operators. It is written in C syntax, which means that in addition to arithmetic operations, you can also use casting operations and dereferencing operations.
To access the value contained in a register, replace the % character prefix with $ , e.g. $eax instead of %eax. (gdb) print *(char *)($esp + $eax + my_ptr_array[13]) โeโ
[p]rint/x Prints the value which the indicated expression evaluates to as a hexadecimal number. expression is evaluated the same way as it is in print. (gdb) p/x my_var $1 = 0x1b
[x]/(number)(format)(unit_size)
Examines the data located in memory at address. โ number optionally indicates that several contiguous elements, beginning at address , should be examined. This is very useful for examining the contents of an array. By default, this argument is 1. โ format indicates how data should be printed. In most cases, this is the same character that you would use in a call to printf(). One exception is the format i, which prints an instruction rather than a decimal integer. โ unit_size indicates the size of the data to examine. It can be [b]ytes , [h]alfwords (2 bytes), [w]ords , or [g]iant words. By default, this is bytes, which is perfect for examining instructions.
A variation of this command is the display command. This command takes the same arguments, but repeats execution every time gdb waits for input. For example, display/I $pc would display the next instruction after each step. (gdb) x/4x argv
