It would be much easier to answer this question if you could describe what you are trying to achieve, rather than a particular mechanism you thought of to achieve it.
Modifying the running executable is typically prevented by modern operating systems, and for good reasons (security, integrity, etc). And modifying the executable on-disk is also inadvisable, for similar reasons. I'm sure there are other ways to achieve what you wish without resorting to self-modifying code.
I have a c code with a character array initialized to "hello world'. I would like to know if there is a possibility to re-initialize a this character array upon each execution of the code, to some other random string.
Yes, it's possible, but you would need to give the user write access to the executable to do so, find the correct offset in the binary, patch the file and save it intact atomically, etc, etc. There are many ways this can go wrong. Don't do it this way.
Instead of a statically allocated string in the executable itself, just have a separate resource file that contains the string(s) and whatever other state is required. You can change this resource file (INI file, data file, whatever) to modify the string as and when required (or even delete it). To provide security, you can digitally sign the file, which allows you to verify the contents are legitimate. If the signature fails, the software can refuse to work. You can also encrypt the contents so that it cannot be read out by an inquisitive user. (Unless they are handy with a debugger!)
If not C, may I know if such an implementation is possible in any other programming language?
It's mainly an OS restriction, including a regular user being able to write to an installed app in a system folder (typically something you do not want to allow). And if you can't do it in C, you probably can't do it in another language!
After executing this code once, I want the char array c to be initialized automatically to some other random string (and not "hello world") and this should be a permanent change. ... May I know if such a thing is even possible?
Yes indeed, but use a separate file as described above. Don't modify the executable itself.
This need is for security reasons.
The reason self-modifying code is disallowed in the first place is for security reasons. If you are attempting to implement some kind of copy protection, you would do well to research existing methods and tools and best practices, and possibly even use an off-the-shelf solution. This stuff is hard, and there are people who crack software for fun who could easily get around all but the most sophisticated protection schemes.
If not, may I know if it is possible to let the code self-destroy after executing it once?
You might be able to get the executable to delete itself, but this would typically require elevated privileges too.
Just get the program to check the signature on the resource file and refuse to run if it isn't valid.
