memory - Write large files to disk in Objective C -

I have a large file that I get through a web service that I am trying to write to disk . I can write most files on disk, but this and other files of this size (or more) are causing problems. When I try to change the string of bits to NSDT after receiving memory warnings, the app will crash. Here's the code I'm using:

  NSString * bits = / * string file stream * /; NSDTA * FileData = [[NSDTTA Alloc] InitVithbabs64 encoded string: BITS option: 0]; The app will crash when attempting to create  

file data variable. Is there a standard for doing this process that I can not follow the following or have some tricks to work with big files?

Thank you in advance! This issue is really bothering me.

A bit more info

It seems that due to large quantity of bits, the string will cause the accident due to the data in the file string Not even sure how to get rid of this.