Ftruncate and Msync

C code

int　ftruncate(int　fd,　off_t　length)　
int　msync(void　*addr,　size_t　len,　int　flags)

First of all, the man manual inside Ftruncate said that when length is larger than the size of the file itself, will enlarge the file and make up the "", but I ARM-LINU-GCC 3.4 is not.

And then there's the msync. The starting address (i.e. addr) must be a multiple of the page size, so that we do this every time:

Assuming that the starting address is mmap and the page size is pagesize, the length of the synchronization required is long. At this point, the starting address we need to sync is:

(mmap/pagesize) * pagesize. And the length we need to expand, when the length is long + mmap%pagesize. Then you can sync properly.