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"Program" operating system concept (find and execute programs). git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16084 91177308-0d34-0410-b5e6-96231b3b80d8 |
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README.txt |
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.