Question

I would like to how does C++ actually turn the command line arguments into a char array? What “secret” code does this? Where can I view the code that does this (even if it is in assembly, I know some assembly)? I am using Linux, if that helps.

Thank You

Answer 1

It’s an OS job to manage command line arguments and to put it to the stack during process creation.

For POSIX systems the execution path is:

1. in your program you call the execle/execve/… system call, passing the path to the new process executable and command line arguments.
2. this data go to the kernel
3. the kernel updates its internal structures to take the new process identity into account and allocates the address space for the new process (also kernel purges the old address space if it’s not needed anymore). the kernel initializes the process memory with zeros, copying the info from old memory to the new address space at the top of the stack.
4. the kernel puts the new process to the scheduling queue and returns from the exec() system call, transferring execution path to the userspace and eventually to the entry point of the process (this is usually routine from crt0.o object file, which is linked by default to every executable – this routine calls main()).

For Linux, you can see this code here:
http://www.cs.fsu.edu/~baker/devices/lxr/http/source/linux/fs/exec.c#L383 :

383 /*
384  * 'copy_strings()' copies argument/environment strings from the old
385  * processes's memory to the new process's stack.  The call to get_user_pages()
386  * ensures the destination page is created and not swapped out.
387  */

In the do_execve() kernel counterpart of userspace execve() syscall, at line 1345, copy_strings()‘s are called, and copy_strings() routine actually does the job you’re asking about.