> I know this isn't exactly an admin question [...]

   Isn't there a "Languages and Scripting" section?

> Here's a short chunk of code:

   Missing a few pieces.  I'll assume "int dataend".

> The problem is when I increase the value of messagebuf past 8MB, I get
> a segmentation fault (core dumped) [...]

   Without considering the wisdom of using fscanf( , "%c", ) to get each
character from a file, did you ask a debugger to tell you _where_ the
segmentation fault occurred?  Around here (some 64-bit Debian version),
it seems to be pretty early, like before any of the actual executable
statements ("{"), suggesting that the problem really is with the size of
the array.

deb64# cc -o at -g -O0 at.c

deb64# ./at
Segmentation fault (core dumped)

deb64# ulimit -s
8192

deb64# ulimit -s 16384

deb64# ./at
ERROR: could not open file: at.dat

   Apparently, an automatic array like this goes on the stack, and if
your stack is too small, then it can't.  A different storage class can
also make a difference:

deb64# diff at.c at2.c
8c8
<     char messagebuf[10199999];  /* [NOTE THIS CAUSES segmentation faults] */
---
>     static char messagebuf[10199999];

deb64# cc -o at2 -g -O0 at2.c

deb64# ./at
Segmentation fault (core dumped)

deb64# ./at2
ERROR: could not open file: at.dat

   I'm too lazy to try, but I suspect that using malloc() would be
similarly beneficial.  (Or "man mmap".)