On Mon, Feb 11, 2013 at 4:51 PM, पारस <beparas@xxxxxxxxx> wrote: > Hi All, > > To read/write data to user-space from kernel-space we use copy_from_user() > and copy_to_user() functions. > > What is the use of these function? > Why kernel can't directly access user address and read/write on to it? > Can any one explain why kernel can't directly access the user-space address. There are a lot resources out there which answer this question adequately in detail (just google for them). I'll give a short answer here: 'User-space' is essentially a particular process' address space, i.e the pages (of physical memory) that have been mapped to this process' virtual memory. Since pages of virtual memory _might_ be swapped out by the kernel, one cannot directly transfer data from the kernel memory to the user mapped memory. For this reason, copy_to/from_user() family of functions is there, which verify first that the user space address (pointed to by the user-space pointer) is valid and accessible and _then_ transfer data to/from it. If the page corresponding to the address pointed to by the user-space pointer has been swapped out, the kernel will first swap in that page and then do the transfer. CMIIW. HTH, -mandeep > > Thanks > > > _______________________________________________ > Kernelnewbies mailing list > Kernelnewbies@xxxxxxxxxxxxxxxxx > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
