Question: protecting a page for Read and/or Write access is possible as there are bits in the page table entry that can be turned on and off at kernel level. Is there a way in which certain region of memory be protected from write access, lets say in a C structure there are certain variable(s) which need to be write protected and any write access to them triggers a segfault and a core dump. Its something like the scaled down functionality of mprotect (), as that works at page level, is there a mechanism to similar kind of thing at byte level in user space.

thanks, Kapil Upadhayay.

Answer: 
No, there is no such facility. If you need per-data-object protections, you'll have to allocate at least a page per object (using `mmap`). If you also want to have some protection against access beyond the end of the object (for arrays) you might allocate at least one more page than what you need, align the object so it ends right at a page boundary, and use `mprotect` to protect the one or more additional pages you allocated.

Of course this kind of approach will result in programs that are very slow and waste lots of resources. It's probably not viable except as a debugging technique, and valgrind can meet that need much more effectively without having to modify your program...
One way, although terribly slow, is to protect the whole page in which the object lies. Whenever a write access to that page happens, your *custom handler for invalid page access* gets called and resolves the situation by quickly unprotecting the page, writing the data and then protecting the page again.

This works fine for single-threaded programs, I'm not sure what to do for multi-threaded programs.

This idea is probably not new, so you may be able to find some information or even a ready-made implementation of it.