This is the path earlier i was getting , at which i was unable to write file
/var/mobile/Applications/8584F54E-75D2-4833-8826-29C125E53DBC/Library/Documentation/291013193758w.png
This morning, I just run my code once again ,. now its showing path
/var/mobile/Applications/DDA14123-6A88-4756-B2E4-C4A3AA39AA5B/Documents/291013081335test.png
on this path am able to write my file
the Difference between two paths is that, first one is of Library/Documentation , where as second one is of Documents
dont know the difference, but it is working now