Can I create a memory-mapped FILE * in C/Objective-C on iOS?

Because ObjC is a superset of C, all you have to do is #import/#include <stdio.h> to gain access to the funopen() which in itself contains the functions readfn, writefn, seekfn, and closefn. And fwopen which has an example showing how to write to two streams at this other SO question.

Mac OSX and iOS don't include fmemopen and open_memstreambecause they are apparently unportable linux functions

As of macos 10.13, ios 11.0, tvos 11.0, and watchos 4.0, fmemopen and open_memstream along with a few other helpful POSIX.1-2008 standard functions are available in stdio.

Have a look at https://github.com/shyuep/pyhull/tree/master/src/fmemopen, I've tested it myself on a Mac OSX 10.8.2 and it is working ok.

Author states it should also work on iOS.