GDB Debugging Learning

GDB is the program debugging tool under the Unix/linux system, compared with the graphical debugging tools of the IDE (such as VS, Eclipse, etc.), GDB has a shortage of breakpoints and trace display, but it is richer in some aspects than the graphical debugging tools.

GDB Debugging Prerequisites

If you want the program to be debugged by GDB, you need to specify the-G option when compiling the program. GDB Debugging and program release optimization, there is also a level, can be set manually. The default GDB level is 2, when the GDB debug level is set to 3, you can during the GDB debugging process macro Expand/exp to the macros in the program to expand the definition.

Graphical Tool for GDB debugging

GDB itself does not have a graphical interface, which makes it difficult to easily know the current context of the program in debugging. To overcome this weakness, GDB has added a number of graphical toolsets, such as Gdb-tui. Use the $ gdb-tui program name to enter GDB's graphical debug state. Or execute the GDB program name into command-line debugging, and then execute CTRL + X + A to turn on TUI mode, CTRL + X + A to turn off TUI mode.

Common directives for GDB debugging

Pre-Commissioning Preparation

1. Set the operating parameters

Set args sets run-time parameters, set Args-ip "192.168.0.1"-port 6555

Show args can display run-time parameters

2. Operating Environment

Path to set the program's Running paths

Show paths shows the running path of the program

Set Environment VarName = value allows setting environment variables

Show environment varname can display environment variables

3. Working directory

CD equivalent to the shell CD

PWD Displays the directory where you are currently located

4. Input and output of the program

Info Terminal display the terminal mode used by the program

Using the output of the redirect control program, such as R (Run) > outfile

Common commands

GDB Common Commands	Format	Meaning	Shorthand
List	List [Start, end]	Listing code listings for a file	L
Prit	Print variable Name	Print variable Contents	P
Break	break [line number or function name]	Set breakpoints	B
Continue	Continue [Start, end]	Continue running	C
Finish	Finish	Continue running until the program ends
Info	Info variable Name	List information	I
Next	Next	Next line	N
Step	Step	Enter function (Step into)	S
Until	Until line number	Run to the specified row	U
Display	Display variable name/expression	Display the value of a parameter or an expression
File	File file name (can be an absolute path and a relative path)	Loading files
Run	Run args	Run the program	R
Enable	Enable B (NUM)/display (num)	Make breakpoint/Display valid
Disable	Disable B (num)/ display (num)	Make breakpoint/Display invalid
Clear	Clearn filename:linenumber (filename null indicates the current file)	Deletes breakpoints that are set on a specific source file, on a specific line.
BackTrace	Bt	View the stack frame of a function call	Bt
Enter		Repeat the previous command

Info break Displays the list of current breakpoints, including the number of times the breakpoint was reached.
Info files Displays detailed information about the files being debugged.
Info func Displays all the function names.
Info Local Displays the local variable information in the function.
Info prog shows the execution state of the program being debugged.
Info var displays all the global and static variable names.
Kill terminates the program that is being debugged.

L (list) Filename:line_number the line used to display the specified file

Breakpoint

B (Break) line number adds a breakpoint on the specified line

B (Break) function name add a breakpoint before you make a function

B (break) FileName: line number adds a breakpoint before the specified line in the specified file

B (break) FileName: The function name adds a breakpoint before the specified function of the specified file

B (Break) *address stops when the program runs to the specified memory address

B (Break) ... if ... Conditional breakpoint, such as break 337 if i = = 0

Info b Lists all the breakpoint information

D Delete all breakpoints, D number

Condition Break-num Conds When the conds condition is met, Break-num is triggered. Conditions that can be used to set or modify breakpoints.

Condition Break-num makes Break-num's breakpoint unconditional.

Observation point

Watch Stop the program as soon as the expression value changes

Rwatch when an expression/variable is read, stop the program

Awatch Stop a program when an expression/variable is read or written

Info watchpoints lists all the points of observation

Observer deletion, using the ordinal of the D observer (Info B can list all Breakpoints + observation points + case)

Set the Run command for the Stop point (useful function)

commands [Break-num]

.... command list ....

End

Specify a list of commands for breakpoint Break-num, and GDB executes the commands in the list in turn when stopped at the stop point

Debugging child processes

1. Set-follow-fork-mode

GDB can set up multi-process debugging for programs that produce child processes using fork/vfork.

Set-follow-fork-mode Parent continues to debug the parental process after fork/vfork

Set-follow-fork-mode Child continues to debug the subprocess after Fork/vfork

(GDB) Set Follow-fork-mode child
(GDB) Break child process line number

2. Attach (gdb debugging a process, using attach another process PID, debugging another process, after debugging, detach release another process, continue to debug this process)

The attach command can be bound to an external program for debugging, the PID of the attach process

Suppose you want to debug a subprocess of a process proc1, use Ps-ef |grep proc1 to find the PID of the child process, and then (GDB) Attach the subprocess pid.

After debugging, use the detach PID to release the child process and continue debugging the parent process.

Custom Debug Tempo

GDB can customize commands to simplify the debugging process, and the commands are defined in the file. gdbinit file in the format:

Define <command>
<code>
End
Document <command>
End

View macro Definitions

First add the parameter '-gdwarf-2 ' '-g3 ' at compile time:
$ gcc-gdwarf-2-g3 Sample.c-o Sample

Used to notify the GCC compiler to add extended information at compile time.
* Info Macro <macro name>: Display macro Information
* Macro Expand Expression: unfolds expressions. However, the expression results are not displayed.
Suppose there is a definition #define ADD (x, y)
(GDB) macro expand ADD (7,8)
Expands to: (7) + (8)

Common options for GDB tui mode

Executes the GDB-TUI program name, automatically enters the TUI mode layout src mode, namely the window above displays the program source code, below is the command window.

Layout src default mode, above for the program source code, below the command window

Layout ASM Displays assembly code for the program, below the command window

Layout split three windows, above SRC, ASM in center, CMD below

Layout reg Displays the Register window at the top (above src or above asm)

TUI Reg General Display Universal Register

TUI Reg FLOAT Displays floating-point registers

TUI Reg system Display Systems Register

TUI Reg Next Displays the next page of the Register

Focus Src/asm/cmd/reg Switches the cursor to the specified window

Refresh refreshes all windows

Update Updates Source code window and current execution point

Winheight name +/-line adjusts the height of the name window

GDB Debug Core Dump

The program may have a random run-time crash, which is generally difficult to reproduce. You can use the core dump file to debug this problem. The core dump file is a log file generated by the program in the event of a crash.

Ulimit-c View system settings core file size limits, ULIMIT-C size set the system settings for the core file sizes, such as Ulimit-c unlimited set size unrestricted//settings are only valid for the current shell

Debug version of the program at run time error, after generating a core file. Debugging a core file with GDB allows you to debug random errors.

$ GDB Program Name core file

After booting, debug as normal gdb.

GDB Debugging Learning