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@@ -177,94 +177,3 @@ process the original kernel headers into clean ones:
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prepended to each generated header, contains a message like
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"do not edit directly - file was auto-generated by ...."
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RATIONALE:
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==========
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OVERVIEW OF THE CURRENT KERNEL HEADER MESS:
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-------------------------------------------
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The original kernel headers are not easily usable from userland applications.
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they contain many declarations and construct that will result in a compilation
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failure or even worse, incorrect behaviour. for example:
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- some headers try to define Posix types (e.g. size_t, ssize_t) that can
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conflict with the corresponding definitions provided by your C library.
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- some headers use constructs that cannot be compiled in ANSI C mode.
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- some headers use constructs do not compile with C++ at all.
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- some headers contain invalid "legacy" definitions for the benefit of old
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C libraries (e.g. glibc5) but result in incorrect behaviour if used
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directly.
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e.g. gid_t being defined in <linux/types.h> as a 16-bit type while the
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kernel uses 32-bit ids. this results in problems when getgroups() or
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setgroups() are called, since they operate on gid_t arrays.
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unfortunately, these headers are also the only source of some really extensive
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constant and type definitions that are required by userland applications.
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think any library/program that need to access ALSA, or Video4Linux, or
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anything related to a specific device or Linux-specific system interface
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(e.g. IOCTLS, etc...)
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As a consequence, every Linux distribution provides a set of patched kernel
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headers to be used by userland applications (which installs in
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/usr/include/linux/, /usr/include/asm/, etc...). these are manually maintained
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by distribution packagers, and generated either manually or with various
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scripts. these headers are also tailored to GNU LibC and cannot be reused
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easily by Bionic.
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for a really long period, the kernel authors have stated that they don't want
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to fix the problem, even when someone proposed a patch to start cleaning the
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official headers. from their point of view this is purely a library author
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problem.
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fortunately, enlightnment happened, and the kernel now provides a way to
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install a set of "user-friendly" headers that are generated from the official
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ones by stripping the __KERNEL__ protected declarations.
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unfortunately, this is not enough for Bionic because the result still contains
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a few broken declarations that are difficult to route around. (see below for
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a little bit of details).
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we plan to be able to support these kernel-generated user-land headers in the
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future, but the priority on this issue is very low.
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WHAT WE DO:
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-----------
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so we're doomed to repeat the same effort than anyone else. the big difference
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here is that we want to automate as much as possible the generation of the
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clean headers to easily support additional architectures in the future,
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and keep current with upstream changes in the header definitions with the
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least possible hassle.
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of course, this is only a race to the bottom. the kernel maintainers still
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feel free to randomly break the structure of their headers (e.g. moving the
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location of some files) occasionally, so we'll need to keep up with that by
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updating our build script/original headers as these cases happen.
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what we do is keep a set of "original" kernel headers, and process them
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automatically to generate a set of "clean" headers that can be used from
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userland and the C library.
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note that the "original" headers can be tweaked a little to avoid some subtle
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issues. for example:
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- when the location of various USB-related headers changes in the kernel
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source tree, we want to keep them at the same location in our generated
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headers (there is no reason to break the userland API for something
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like that).
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- sometimes, we prefer to take certain things out of blocks guarded by a
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#ifdef __KERNEL__ .. #endif. for example, on recent kernels <linux/wireless.h>
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only includes <linux/if.h> when in kernel mode. we make it available to
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userland as well since some code out there assumes that this is the case.
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- sometimes, the header is simply incorrect (e.g. it uses a type without
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including the header that defines it before-hand)
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Elliott Hughes