Review Request 130090: Fix incorrect definition of major(3)/minor(3) macros

[KJ Tsanaktsidis]

> On April 22, 2017, 10:27 a.m., Lamarque Souza wrote:
> > autotests/CMakeLists.txt, line 66
> > <https://git.reviewboard.kde.org/r/130090/diff/5/?file=494817#file494817line66>
> >
> >     CMake's developers recommend using else() instead of else(<expression>). The <expression> part used to be required with cmake 2.6.x, that is not true with cmake 3.x that we use nowadays.
> 
> KJ Tsanaktsidis wrote:
>     I'm not sure I understand here - elseif() needs to have the expression to match to enter the elseif() block? In any case I've gone ahead and deleted this because I worked out a cleaner way to do this using cmake generator expressions.
> 
> Lamarque Souza wrote:
>     Oh sorry, I did a mistake. I thought it wase a else(). The expression is required for elseif(), the old code was correct. You can revert to the old code, which require less lines of code. Sorry again.

Actually it was not quite correct - it was defining `HAVE_XXX_MAJOR_MINOR=1` for when the header was found, but not `HAVE_XXX_MAJOR_MINOR=0` when it wasn't. It worked on Linux because `sys/sysmacros.h` was first, but I don't think it would work on most BSD's which have it in `sys/types.h` (the `#if HAVE_SYSMACROS_MAJOR_MINOR` check wouldn't compile). This version of it sets each `HAVE_XXX` macro to either 1 or 0 on all platforms.


- KJ


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On April 23, 2017, 9:56 a.m., KJ Tsanaktsidis wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://git.reviewboard.kde.org/r/130090/
> -----------------------------------------------------------
> 
> (Updated April 23, 2017, 9:56 a.m.)
> 
> 
> Review request for KDE Frameworks.
> 
> 
> Repository: solid
> 
> 
> Description
> -------
> 
> Previously, udesksblock.cpp was attempting to find a definition for
> major/minor on Linux in <sys/kdev_t.h> by checking Q_OS_LINUX before
> importing the header. Q_OS_LINUX is however only set when
> qsystemdetection.h is included, and the macro was being checked first.
> 
> Even had this check worked, it would still be wrong. On a modern version
> of the userspace linux-headers, <sys/kdev_t.h> includes definitions for
> major and minor that assume each is limited to 8 bits and that dev_t is
> 16 bits. This is no longer true anymore; on Linux, major numbers can be
> up to 12 bits at present and minor numbers up to 20. Calling these
> macros with dev_t values > 2^16 would give incorrect results.
> 
> Because the Q_OS_LINUX check failed, a fallback version of the macros
> were defined for use on all platforms. The code is allegedly copied from
> kdev_t.h, except it is copied from the *kernel* version of the header,
> not the userspace version. Linux internally uses a different
> representation of dev_t than it exposes to userspace - the kernelspace
> version is 20 bits of minor/12 bits of major contiguously, but the
> userspace version packs the bits in a different order to maintain
> compatability with old 16-bit device numbers. Thus, this code also does
> not work for dev_t values > 2^16.
> 
> To fix this, we add CMake rules to search for a system-provided
> definition of the major/minor macros - on various systems, these can be
> in a few different places. As a fallback, we assume old-style 16-bit
> dev_t (although I suspect that is only used for Windows, where
> major/minor numbers are pretty meaningless anyway).
> 
> 
> Diffs
> -----
> 
>   autotests/CMakeLists.txt 54adeea62b954b9169b37f1eab8fa3e215fafafa 
>   autotests/fakeUdisks2.h PRE-CREATION 
>   autotests/fakeUdisks2.cpp PRE-CREATION 
>   autotests/solidudisks2test.cpp PRE-CREATION 
>   src/solid/devices/backends/udisks2/CMakeLists.txt 34390064af29ace07cbb3470945be098cc606d04 
>   src/solid/devices/backends/udisks2/udisksblock.cpp 0622ec77fcf670a2005d34b7a6c31ca8b53a18d8 
> 
> Diff: https://git.reviewboard.kde.org/r/130090/diff/
> 
> 
> Testing
> -------
> 
> I've written a little snippet to iterate through block devices, print their major/minor number, and their device properties. It was previously incorrectly labeling all my disks with major 0 and minor == device_number (since it was using the first 20 bits for the minor). It now correctly identifies their major/minor number.
> 
> 
> Thanks,
> 
> KJ Tsanaktsidis
> 
>

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