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First cut at a README for lib/System explaining the #inclusion rules and

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design criteria.


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Design Of lib/System
====================
The software in this directory is designed to completely shield LLVM from any
and all operating system specific functionality. It is not intended to be a
complete operating system wrapper (such as ACE), but only to provide the
functionality necessary to support LLVM.
The software located here, of necessity, has very specific and stringent design
rules. Violation of these rules means that cracks in the shield could form and
the primary goal of the library is defeated. By consistently using this library,
LLVM becomes more easily ported to new platforms since (hopefully) the only thing
requiring porting is this library.
Complete documentation for the library can be found in the file:
llvm/docs/SystemLibrary.html
or at this URL:
http://llvm.org/docs/SystemLibrary.html
However, for the impatient, here's a high level summary of the design rules:
1. No functions are declared with throw specifications. This is on purpose to
make sure that additional exception handling code is not introduced by the
compiler.
2. On error only an instance of std::string that explains the error and possibly
the context of the error may be thrown.
3. Error messages should do whatever is necessary to get a readable message from
the operating system about the error. For example, on UNIX the strerror_r
function ought to be used.
4. Entry points into the library should be fairly high level and aimed at
completing some task needed by LLVM. There should *not* be a 1-to-1
relationship between operating system calls and the library's interface.
Certain implementations of the
5. The implementation of an lib/System interface can vary drastically between
platforms. That's okay as long as the end result of the interface function is
the same. For example, a function to create a directory is pretty straight
forward on all operating system. System V IPC on the other hand isn't even
supported on all platforms. Instead of "supporting" System V IPC, lib/System
should provide an interface to the basic concept of inter-process
communications. The implementations might use System V IPC if that was
available or named pipes, or whatever gets the job done effectively for a
given operating system.
6. Implementations are separated first by the general class of operating system
as provided by the configure script's $build variable. This variable is used
to create a link from $BUILD_OBJ_ROOT/lib/System/platform to a directory in
$BUILD_SRC_ROOT/lib/System directory with the same name as the $build
variable. This provides a retargetable include mechanism. By using the link's
name (platform) we can actually include the operating specific
implementation. For example, support $build is "Darwin" for MacOS X. If we
place:
#include "platform/File.cpp"
into a a file in lib/System, it will actually include
lib/System/Darwin/File.cpp. What this does is quickly differentiate the basic
class of operating system that will provide the implementation.
7. Implementation files in lib/System need may only do two things: (1) define
functions and data that is *TRULY* generic (completely platform agnostic) and
(2) #include the platform specific implementation with:
#include "platform/Impl.cpp"
where Impl is the name of the implementation files.
8. Platform specific implementation files (platform/Impl.cpp) may only #include
other Impl.cpp files found in directories under lib/System. The order of
inclusion is very important (from most generic to most specific) so that we
don't inadvertently place an implementation in the wrong place. For example,
consider a fictitious implementation file named DoIt.cpp. Here's how the
#includes should work for a Linux platform
lib/System/DoIt.cpp
#include "platform/DoIt.cpp" // platform specific impl. of Doit
DoIt
lib/System/Linux/DoIt.cpp // impl that works on all Linux
#include "../Unix/DoIt.cpp" // generic Unix impl. of DoIt
#include "../Unix/SUS/DoIt.cpp // SUS specific impl. of DoIt
#include "../Unix/SUS/v3/DoIt.cpp // SUSv3 specific impl. of DoIt
Note that the #includes in lib/System/Linux/DoIt.cpp are all optional but
should be used where the implementation of some functionality can be shared
across some set of Unix variants. We don't want to duplicate code across
variants if their implementation could be shared.
9. The library does not attempt to shield LLVM from the C++ standard library or
standard template library. These libraries are considered to be platform
agnostic already.
10. LLVM should not include *any* system headers anywhere except in lib/System.
11. lib/System must *not* expose *any* system headers through its interface.