Changed to memory-mapped region not committing

I have a memory-mapped file and I'm writing changes to the memory region returned, but they don't appear to take.

Code:

	int des = open("processed.dat", O_RDWR | O_CREAT | O_NOATIME, S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH);
	assert(des != -1);
	void *buf = mmap(NULL, SIZEOFTHESUCKA, PROT_READ | PROT_WRITE, MAP_PRIVATE, des, 0);
	assert(buf != MAP_FAILED);

...     //some code that writes to buf

const unsigned char *p = (const unsigned char *)buf;
for (int i = 0; i < SIZEOFTHESUCKA; i++)
	fprintf(stderr, "%02X ", (int)p[i]);
fprintf(stderr, "\n");
	assert(munmap(buf, SIZEOFTHESUCKA) == 0);
	close(des);

The stderr shows the values:

Code:

6D 00 00 04 00 16 33 12

But when I call:

Code:

hexdump processed.dat

I only get:

Code:

0000000 0000 0000 0000 0000
0000008

I should also point out that before the file is opened it is already SIZEOFTHESUCKA bytes long, and is zeroed.
NDEBUG is not defined, so munmap() does get called.

If MAP_PRIVATE is specified, modifications to the mapped data by the calling process will be visible only to the calling process and will not change the underlying object. (man mmap)

I guess changing mmap flags or forcing a synchronization might help.

And here I was thinking it just meant that this region wouldn't be shared with other processes. Yes, changing the flags fixed it. Thanks.

This isn't the only mysteriously useless feature of mmap(). Another is PROT_NONE.

Originally Posted by Angus

And here I was thinking it just meant that this region wouldn't be shared with other processes. Yes, changing the flags fixed it. Thanks.

This isn't the only mysteriously useless feature of mmap(). Another is PROT_NONE.

The features are not useless, just seldom needed.

A private map can be applied to a subset of another mapping to allow hot-patching of shared libraries without affecting the entire system.

PROT_NONE can be used to reserve a region of address space that will be used later but not at the moment. Giving access to the pages would be incorrect, since you could not detect bugs where you improperly access this region.