gdb tips - PART1

Some gdb ( GNU debugger) tips

It is very important to understand the debugging tools and their different functionalities to hack the system level programs easily.

How to compile a program to debug in gdb.

1. compile your C program with -g option

   $cc -g helloworld.c

2. Open the executable in gdb
   $gdb a.out

3. Run the program using below commands in gdb prompt using simply ‘r’.
   (gdb)run or (gdb)r

4. Continue run from breakpoint using ‘c’
   (gdb)c

Setting Breakpoint in gdb.

1.Setting up breakpoint in a line number
$b 10
2. breakpoint setting up in an address
$b *0x7c00

(gdb) b *0x7c00
Breakpoint 1 at 0x7c00
(gdb) c
Continuing.
Thread 1 hit Breakpoint 1, 0x00007c00 in ?? ()

3.Setting up breakpoint in a function
$b main

Switch to tui mode using ctrl+x 2

you can switch to tui ( text gui mode) mode using ctrl+x 2
to view the source file in the same screen.

you can ctrl+x 1 to see registers values

ctrl+x n to come back to gdb prompt.

stepping through line and instruction

$si \ steps through instruction
$s \ source level step through next line

Next step over through line

$n
$next

Printing variables

$p foo//print the value of foo
$p car // print the value of car variable

$p/x addr //printing hexa decimal values

information about frames and registers

Current function stack frame details can be displayed using the command ‘f’ or ‘frame’
$f
$frame

The registers details or values can be seen using the command ’ info r’

info r

backtrace of call stacks

How can we backtrace all call stacks in gdb?!. This would be very useful command especially when you are debugging big projects source codes.

$bt
shows the backtrace of all call stacks so far.

You can switch to different frames using below command

$f 1
to switch to the first frame
f 5 switch to 5th frame etc

Some of the helpful commands when you are in a function

$info locals
shows the local variable in a function

$info args
shows arguments of the function

getting help on gdb

(gdb) help

List of classes of commands:

aliases – Aliases of other commands
breakpoints – Making program stop at certain points
data – Examining data
files – Specifying and examining files
internals – Maintenance commands
obscure – Obscure features
running – Running the program

(gdb)h breakpoints //provide help on breakpoints

PART 2 -
https://naveendavisv.blogspot.com/2020/04/gdb-debugging-part-2.html

Have you ever tried Ctrl+x 2 in GDB ?

I got amazed when I tried Ctrl+x  2 in gdb and step through code. I never thought GDB had an option to do this.

gdb debugging with ctrl+x 2

again press ctrl+x 2  .. you will see another magic.

Going to back OS/system related computer courses

I tried to read linux kernel and dropped it long back around the year of 2007 or 2008. Whenever I start touching system level code, programming languages becomes alien language for me. I thought of watching again the OS/computer architecture video courses. Luckily come across xv6 (https://en.wikipedia.org/wiki/Xv6)  operating system. This makes/motivating me to give one more attempt to learn the kernel and system level programming.

This one also helps me a lot.
  https://github.com/Babtsov/jos/tree/master/lab1

Emscripten - C programs to browser

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C programs and libraries to Browser

How nice it would be if I can call my own created C programs especially the computational intensive C programs from my Mozilla or Google Browser ( :-) in fact I don’t have anything that much computational intensive), But still . I would also be amazed if I can port some of the C libraries or C++ Games to browser.

Emscripten and Webassembly(wasm) together make this happen

I was following web-assembly and Rust for a few months now. Since Rust has lot of safety features, I did not think of exploring more on porting C or C++ programs to browser.

Emscripten is an Open Source LLVM to JavaScript compiler. Using Emscripten you can:
Compile C and C++ code into JavaScript
Compile any other code that can be translated into LLVM bitcode into JavaScript.
Compile the C/C++ runtimes of other languages into JavaScript, and then run code in those other languages in an indirect way (this has been done for Python and Lua)!

WebAssembly (often shortened to Wasm) is an open standard that defines a portable binary code format for executable programs, and a corresponding textual assembly language, as well as interfaces for facilitating interactions between such programs and their host environment.

Emscripten Installation

Emcripten is not that complex to install,there is good documentation in the site - Emscripten
You might need to install python2.7 and cmake etc as a prior requisite.We just need to follow the instructions. My Linux machine helped me to install everything with apt-get insall command.
You might come across some installation errors, but just search those errors in google, there will be solution to fix it.

C program to wasm

I wrote a simple C program called emccwasm.c

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<stdint.h>
int main(){
        int a = 1;
        printf("%d",a);
        //string function to put a space in the screen
        puts(" ");
        printf("First C program compiled to browser. a=%d",a);
        return 1;
}

In command line, you might need to use below command to compile if you are using Linux machine.
emcc -s NO_EXIT_RUNTIME=1 -s FORCE_FILESYSTEM=1 -s EXPORTED_FUNCTIONS=[’_main’] -o emccwasm.html emccwasm.c
Emscripten FAQ might help you understand these options and resolve the errors you might encounter quickly.
The above command generated 3 files for me , they are emccwasm.html , emccwasm.js and emccwasm.wasm

Emscripten generated Files.

Emcripten created a compiled webassembly file which is emccwasm.wasm as mentioned earlier.Filename will be same as the C program file name. To know more about webassembly , following documentation link helps - https://webassembly.org/
In a nutshell “WebAssembly (abbreviated Wasm) is a binary instruction format for a stack-based virtual machine. Wasm is designed as a portable target for compilation of high-level languages like C/C++/Rust, enabling deployment on the web for client and server application”
The .js and .html files are glue files created for web. The html file can be rendered to the browser and Javascript file initialize webassembly and rest of the glue work

http server

You might need to install http server to void below error . The following documentation can be used for installation -
https://github.com/thecoshman/http
Note: if you have installed Cargo in your machine, the server installation will be very easy.
"**Failed to execute ‘compile’ on ‘WebAssembly’: Incorrect response MIME type. Expected ‘application/wasm’.
**
We can run the server simply by below command:


http

Emscripten files in localhost

Once the http server is up, the files will be served in the browser in the link: http://localhost:8000/emccwasm.html
Note: the port might vary .
If everything goes well, you should be getting a screen similar to below.

Accessing exported functions.

When we compiled the C program we used EXPORT_FUNCTIONS option with _main. That means the _main function should be available in Module.
ie , we can invoke main function by Module._main()