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design criteria. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16054 91177308-0d34-0410-b5e6-96231b3b80d8
97 lines
4.7 KiB
Plaintext
97 lines
4.7 KiB
Plaintext
Design Of lib/System
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====================
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The software in this directory is designed to completely shield LLVM from any
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and all operating system specific functionality. It is not intended to be a
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complete operating system wrapper (such as ACE), but only to provide the
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functionality necessary to support LLVM.
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The software located here, of necessity, has very specific and stringent design
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rules. Violation of these rules means that cracks in the shield could form and
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the primary goal of the library is defeated. By consistently using this library,
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LLVM becomes more easily ported to new platforms since (hopefully) the only thing
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requiring porting is this library.
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Complete documentation for the library can be found in the file:
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llvm/docs/SystemLibrary.html
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or at this URL:
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http://llvm.org/docs/SystemLibrary.html
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However, for the impatient, here's a high level summary of the design rules:
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1. No functions are declared with throw specifications. This is on purpose to
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make sure that additional exception handling code is not introduced by the
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compiler.
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2. On error only an instance of std::string that explains the error and possibly
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the context of the error may be thrown.
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3. Error messages should do whatever is necessary to get a readable message from
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the operating system about the error. For example, on UNIX the strerror_r
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function ought to be used.
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4. Entry points into the library should be fairly high level and aimed at
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completing some task needed by LLVM. There should *not* be a 1-to-1
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relationship between operating system calls and the library's interface.
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Certain implementations of the
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5. The implementation of an lib/System interface can vary drastically between
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platforms. That's okay as long as the end result of the interface function is
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the same. For example, a function to create a directory is pretty straight
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forward on all operating system. System V IPC on the other hand isn't even
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supported on all platforms. Instead of "supporting" System V IPC, lib/System
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should provide an interface to the basic concept of inter-process
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communications. The implementations might use System V IPC if that was
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available or named pipes, or whatever gets the job done effectively for a
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given operating system.
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6. Implementations are separated first by the general class of operating system
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as provided by the configure script's $build variable. This variable is used
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to create a link from $BUILD_OBJ_ROOT/lib/System/platform to a directory in
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$BUILD_SRC_ROOT/lib/System directory with the same name as the $build
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variable. This provides a retargetable include mechanism. By using the link's
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name (platform) we can actually include the operating specific
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implementation. For example, support $build is "Darwin" for MacOS X. If we
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place:
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#include "platform/File.cpp"
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into a a file in lib/System, it will actually include
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lib/System/Darwin/File.cpp. What this does is quickly differentiate the basic
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class of operating system that will provide the implementation.
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7. Implementation files in lib/System need may only do two things: (1) define
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functions and data that is *TRULY* generic (completely platform agnostic) and
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(2) #include the platform specific implementation with:
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#include "platform/Impl.cpp"
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where Impl is the name of the implementation files.
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8. Platform specific implementation files (platform/Impl.cpp) may only #include
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other Impl.cpp files found in directories under lib/System. The order of
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inclusion is very important (from most generic to most specific) so that we
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don't inadvertently place an implementation in the wrong place. For example,
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consider a fictitious implementation file named DoIt.cpp. Here's how the
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#includes should work for a Linux platform
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lib/System/DoIt.cpp
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#include "platform/DoIt.cpp" // platform specific impl. of Doit
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DoIt
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lib/System/Linux/DoIt.cpp // impl that works on all Linux
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#include "../Unix/DoIt.cpp" // generic Unix impl. of DoIt
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#include "../Unix/SUS/DoIt.cpp // SUS specific impl. of DoIt
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#include "../Unix/SUS/v3/DoIt.cpp // SUSv3 specific impl. of DoIt
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Note that the #includes in lib/System/Linux/DoIt.cpp are all optional but
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should be used where the implementation of some functionality can be shared
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across some set of Unix variants. We don't want to duplicate code across
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variants if their implementation could be shared.
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9. The library does not attempt to shield LLVM from the C++ standard library or
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standard template library. These libraries are considered to be platform
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agnostic already.
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10. LLVM should not include *any* system headers anywhere except in lib/System.
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11. lib/System must *not* expose *any* system headers through its interface.
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