Quickly close mmap discarding unflushed changes

Question

I am using an mmap'ed file as a virtual memory arena - the file is manually allocated because I want to control its location. On munmap, all the current contents of the buffers are flushed to the file, but I don't really need the file contents. Is it possible to simply discard the mmap area without write back?

Linux-specific solutions are OK.

I mean something like

char* myswaparea = "/tmp/myswaparea";
int64_t len = 1LL << 30;
fd = open(myswaparea, O_CREAT|O_RDWR, 0600);
ftruncate(fd, len);
void* arena = mmap(NULL, len, .... fd ...);
/* use arena */
munmap(arena, len);  /* here comes an unnecessary flush */
close(fd);
unlink(myswaparea);

Answer 1

If you don't need / want to write back the changes to the file, just use the MAP_PRIVATE flag when you create the map (4th argument to mmap(2)).

From the manpage:

MAP_PRIVATE

Create a private copy-on-write mapping. Updates to the mapping are not visible to other processes mapping the same file, and are not carried through to the underlying file. It is unspecified whether changes made to the file after the mmap() call are visible in the mapped region.

EXAMPLE

fd = open("myfile", O_RDWR);
if (fd < 0) {
    /* Handle error... */
}
void *ptr;
size_t len = 1024;
ptr = mmap(NULL, len, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
if (ptr == MAP_FAILED) {
    /* Handle error... */
}
/* ... */
if (munmap(ptr, len) < 0) {
    /* Handle error... */
}