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049500715e
This is strictly user-space material at this point, so put it with the other user-space API documentation. Signed-off-by: Jonathan Corbet <corbet@lwn.net>
32 lines
830 B
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32 lines
830 B
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==============================
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Decoding an IOCTL Magic Number
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==============================
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To decode a hex IOCTL code:
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Most architectures use this generic format, but check
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include/ARCH/ioctl.h for specifics, e.g. powerpc
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uses 3 bits to encode read/write and 13 bits for size.
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====== ==================================
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bits meaning
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====== ==================================
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31-30 00 - no parameters: uses _IO macro
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10 - read: _IOR
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01 - write: _IOW
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11 - read/write: _IOWR
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29-16 size of arguments
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15-8 ascii character supposedly
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unique to each driver
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7-0 function #
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====== ==================================
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So for example 0x82187201 is a read with arg length of 0x218,
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character 'r' function 1. Grepping the source reveals this is::
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#define VFAT_IOCTL_READDIR_BOTH _IOR('r', 1, struct dirent [2])
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