How to write mmap input memory to O_DIRECT output file?

Posted by Friedrich on Stack Overflow See other posts from Stack Overflow or by Friedrich
Published on 2011-06-22T18:31:23Z Indexed on 2012/03/19 18:03 UTC
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linux-kernel

|

mmap

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

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