How to write mmap input memory to O_DIRECT output file?

Question

why doesn't following pseudo-code work (O_DIRECT results in EFAULT)

in_fd = open("/dev/mem");
in_mmap = mmap(in_fd);
out_fd = open("/tmp/file", O_DIRECT);
write(out_fd, in_mmap, PAGE_SIZE);

while following does (no O_DIRECT)

in_fd = open("/dev/mem");
in_mmap = mmap(in_fd);
out_fd = open("/tmp/file");
write(out_fd, in_mmap, PAGE_SIZE);

I guess it's something with virtual kernel pages to virtual user pages, which cannot be translated in the write call?

Best regards,

Friedrich

Answer 1

Using mmap() with O_DIRECT is tricky. There are some restrictions. The output to the file should be block aligned. For example, if you set offset in mmap() to 0 your code will work. You have to check the block size of your filesystem to set that value properly.