Mmap() an entire large file

Question

I am trying to \"mmap\" a binary file (~ 8Gb) using the following code (test.c).

#include 
#include 
#include          


        

Answer 1

You don't have enough virtual memory to handle that mapping.

As an example, I have a machine here with 8G RAM, and ~8G swap (so 16G total virtual memory available).

If I run your code on a VirtualBox snapshot that is ~8G, it works fine:

$ ls -lh /media/vms/.../snap.vdi
-rw------- 1 me users 9.2G Aug  6 16:02 /media/vms/.../snap.vdi
$ ./a.out /media/vms/.../snap.vdi
Size: 9820000256 
[0]=3C [1]=3C [2]=3C [3]=20 [4]=4F [5]=72 [6]=61 [7]=63 [8]=6C [9]=65 

Now, if I drop the swap, I'm left with 8G total memory. (Don't run this on an active server.) And the result is:

$ sudo swapoff -a
$ ./a.out /media/vms/.../snap.vdi
Size: 9820000256 
mmap: Cannot allocate memory

So make sure you have enough virtual memory to hold that mapping (even if you only touch a few pages in that file).