What is the difference between MAP_SHARED and MAP_PRIVATE in the mmap function?

Question

Playing around with mmap for the fun of it, I have the following code:

(.. snip ..)
fd = open("/home/me/straight_a.txt", O_RDONLY);
if (fd == -1) {
    perror("open");
    exit(1);
}

m = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_FILE|MAP_PRIVATE, fd, 0);

if (m == MAP_FAILED) {
    perror("mmap");
    exit(1);
}

printf("m is %p\n", m);

printf("*m = %c\n", *m);
printf("*(m+1) = %c\n", *(m+1));
(.. snip ..)

This works as expected. But before I got to this, I tried...

m = mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_FILE|MAP_SHARED, fd, 0);

... and mmap errored out with:

mmap: Permission denied

In general, what's the difference between the two flags (man page isn't to generous on this subject)? What sort of permission (and where) am I missing?

EDIT

Like it usually happens.. partially figured it out.

Turns out open needed an O_RDWR flag.

So, am I correct to assume that:

• MAP_PRIVATE - changes are made in memory only, not saved to disk?
• MAP_SHARED - changes would be saved to disk...

... but I'm not saving anything to disk anywhere, I thought? Just operating on memory.

Answer 1

You opened the file in read-only mode. Then you attempted to mmap part of it in read/write mode with MAP_SHARED set. In this context MAP_SHARED implies that if you write to the mmap'd region your changes will be committed back to the mapped file itself. You can't do this because you opened the file in read-only mode.

MAP_PRIVATE works because writes to the mmap'd region are not committed back to the original file. When you write to the region the pages that were written to are copied to a different region of memory, possibly backed by swap space.