~GCC Basics~

Hello Everyone,

In this article we would discuss on yet another wonderful software GCC(GNU
compiler collection). GCC is a portable compiler—it runs on most platforms
available today, and can produce output for many types of processors. In
addition to the processors used in personal computers, it also supports
microcontrollers, DSPs and 64-bit CPUs. "GCC is not only a native compiler—it can also cross-compile any program, "producing executable files for a different system from the one used by GCC "itself. This allows software to be compiled for embedded systems which are not "capable of running a compiler." GCC is written in C with a strong focus on portability, and can compile itself, so it can be adapted to new systems easily.


Compilation refers to the process of converting a program from the textual
source code, in a programming language such as C or C++, into machine code,
the sequence of 1’s and 0’s used to control the central processing unit (CPU) of the computer. This machine code is then stored in a file known as an executable file, sometimes referred to as a binary file.

Lets understand the steps and meaning of different kind of flags which can be passed to. Here is the basic C program (hello.c)


#include"stdio.h"
#include"stdlib.h"

int main(int argc, char *argv[], char *env[]) {

printf ("Hello, world!\n");
return 0;
}


==>
The sequence of commands executed by a single invocation of GCC consists of
the following stages:
• preprocessing (to expand macros)
• compilation (from source code to assembly language)
• assembly (from assembly language to machine code)
• linking (to create the final executable

// The preprocessor step
mantosh@ubuntu:~$cpp hello.c > hello.i

// The compiler step
mantosh@ubuntu:~$gcc -Wall -S hello.i

// The assembler step
mantosh@ubuntu:~$as hello.s -o hello.o

// The linker step
mantosh@ubuntu:~$gcc hello.o


Their is mainly 4 steps which gcc does internally during compilation.After the linker step gcc would generate the final executable file name a.out in the current directory.If we run this a.out we should get output like this:

mantosh@ubuntu:~$./a.out
Hello, world!

However gcc also provided the option "-save-temps" for the user.So if user does not want to run those 4 steps individually, he can give this option and gcc would save all the temporary file generated during whole compilation process. There is one more important flag "-g" which is necessary if we want to use gdb or other debugger for debugging the program. So most of time if we give command like:

mantosh@ubuntu:~$ gcc -save-temps -g hello.c -o hello

gcc would save all temporary files like(hello.i, hello.s,hello.o) in the current working directory. -g flag would add useful symbols in the executable file hello so that later on it can be debugged. There are thousands of different flags/option which are available with gcc. Interested reader can check the official GNU manual for gcc. I tried to put just basic yet very important concepts of gcc.



Hope that this article might be of some use for the reader. Feel free to put yours comment.

~GDB Basic Hacking~

Hello Everyone,

I am going to show basic hack of wonderful wonderful software gdb in this
article. As we know gdb(GNU Debugger) can be used for debugging C/C++ and
many more other languages program.

I am the very frequent user of gdb. There are quite a few commands of gdb which i always set once i am inside gdb session. Once i was going through the official manual of gdb, after reading i got the idea about the file ".gdbinit". Whenever we start gdb session for debugging, it tries to read the .gdbinit file from the user $HOME directory. I searched into my $HOME directory but there was no such file.


For better understanding how gdb gets the information from .gdbinit file, i used the other very very useful tool provided on GNU/Linux machine (strace,ltrace). I strongly recommend to do at least "man strace" and "man ltrace" on your command prompt to know the basics of these stuff.These tools are useful for understanding the flow of program.They are also very special in the sense that even you don't have the source code of a program, you can get good amount of information from these tools.It might take some time before you start to interpret/understand the output of utilities like strace, ltrace.


I did the "strace gdb " from the command line and output was something like :


mantosh@ubuntu:~$ strace gdb
execve("/usr/bin/gdb", ["gdb"], [/* 39 vars */]) = 0
brk(0) = 0x87d3000
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f63000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=96330, ...}) = 0
mmap2(NULL, 96330, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7f4b000
close(3) = 0
access("/etc/ld.so.nohwcap",F_OK) = -1 ENOENT (No such file or directory)
open("/lib/libreadline.so.5", O_RDONLY) = 3
......
......
......
......
......
......
......
......
rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0
stat64("/home/mantosh/.gdbinit",0xbf9fee54)= -1 ENOENT (No such file or directory)

stat64("/home/mantosh/.gdbinit",0xbf9fefe4) = -1 ENOENT (No such file or directory)

stat64(".gdbinit",0xbf9fefe4) = -1 ENOENT (No such file or directory)
open("/home/mantosh/Desktop/LinuxPI/.gdb_history", O_RDONLY) = -1 ENOENT (No such file or directory)
rt_sigprocmask(SIG_BLOCK, NULL, [], 8) = 0
rt_sigprocmask(SIG_BLOCK, [INT], [], 8) = 0


===>
The output of "strace gdb " would be like above. Best way the get information
about the .gdbinit file is to just search name from the the output of strace.
The following line from the strace output is the important for our current discussion.
stat64("/home/mantosh/.gdbinit",0xbf9fee54)=-1 ENOENT (No such file or directory)

The when system calls stat64() system call for the file $HOME/.gdbinit, it gets -1
as return value which says no such file or directory exists.



So i created .gdbinit file into my $HOME directory. I have put some of my favorite
commands into this file so that i don't need to type those every time i am inside
gdb session. My .gdbinit looks something like

mantosh@ubuntu:~$ cat .gdbinit
set height -1
set print pretty on
set print array on
set print array-indexes on
set print repeats 0
set print symbol-filename on
set print elements 0
set listsize 15
set print vtbl on
set print object on



If any of you want to know what these command is all about, please check the
official GNU manual. You can add more commands into the file .gdbinit. Its depend on the user what he/she want to put.

Once i done with the .gdbinit file , i once again did "strace gdb" to confirm that whether gdb is able to read the file .gdbinit of not.This time the strace
output was something like this:


mantosh@ubuntu:~$ strace gdb
execve("/usr/bin/gdb", ["gdb"], [/* 39 vars */]) = 0
brk(0) = 0x9596000
access("/etc/ld.so.nohwcap", F_OK)= -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7fbd000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=96330, ...}) = 0
mmap2(NULL, 96330, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7fa5000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
open("/lib/libreadline.so.5", O_RDONLY) = 3
......
......
......
......
......
......
......
......
......
......

sigprocmask(SIG_BLOCK, NULL, [], 8) = 0
stat64("home/mantosh/.gdbinit",{st_mode=S_IFREG|0644,st_size=204, ...})= 0
open("/home/mantosh/.gdbinit", O_RDONLY) = 5
lstat64("/home", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat64("/home/mantosh", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
fcntl64(5, F_GETFL) = 0 (flags O_RDONLY)


===>
As we can see that now strace64() call return 0 which confirm that there is .gdbinit file in the $HOME directory. In the next line we can see now gdb is able to open the file in the O_RDONLY mode. The return value of open() call is 5 better know as the file descriptor.



Hope that this article might be of some use for the reader. Feel free to put yours comment, so that from next time i could try to incorporate those suggestions.Thanks for reading this article......Happy Hacking!!!!!!!!!!!!!!