4 The software in this directory is designed to completely shield LLVM from any
5 and all operating system specific functionality. It is not intended to be a
6 complete operating system wrapper (such as ACE), but only to provide the
7 functionality necessary to support LLVM.
9 The software located here, of necessity, has very specific and stringent design
10 rules. Violation of these rules means that cracks in the shield could form and
11 the primary goal of the library is defeated. By consistently using this library,
12 LLVM becomes more easily ported to new platforms since (hopefully) the only thing
13 requiring porting is this library.
15 Complete documentation for the library can be found in the file:
16 llvm/docs/SystemLibrary.html
18 http://llvm.org/docs/SystemLibrary.html
20 However, for the impatient, here's a high level summary of the design rules:
22 1. No functions are declared with throw specifications. This is on purpose to
23 make sure that additional exception handling code is not introduced by the
26 2. On error only an instance of std::string that explains the error and possibly
27 the context of the error may be thrown.
29 3. Error messages should do whatever is necessary to get a readable message from
30 the operating system about the error. For example, on UNIX the strerror_r
31 function ought to be used.
33 4. Entry points into the library should be fairly high level and aimed at
34 completing some task needed by LLVM. There should *not* be a 1-to-1
35 relationship between operating system calls and the library's interface.
36 Certain implementations of the
38 5. The implementation of an lib/System interface can vary drastically between
39 platforms. That's okay as long as the end result of the interface function is
40 the same. For example, a function to create a directory is pretty straight
41 forward on all operating system. System V IPC on the other hand isn't even
42 supported on all platforms. Instead of "supporting" System V IPC, lib/System
43 should provide an interface to the basic concept of inter-process
44 communications. The implementations might use System V IPC if that was
45 available or named pipes, or whatever gets the job done effectively for a
46 given operating system.
48 6. Implementations are separated first by the general class of operating system
49 as provided by the configure script's $build variable. This variable is used
50 to create a link from $BUILD_OBJ_ROOT/lib/System/platform to a directory in
51 $BUILD_SRC_ROOT/lib/System directory with the same name as the $build
52 variable. This provides a retargetable include mechanism. By using the link's
53 name (platform) we can actually include the operating specific
54 implementation. For example, support $build is "Darwin" for MacOS X. If we
56 #include "platform/File.cpp"
57 into a a file in lib/System, it will actually include
58 lib/System/Darwin/File.cpp. What this does is quickly differentiate the basic
59 class of operating system that will provide the implementation.
61 7. Implementation files in lib/System need may only do two things: (1) define
62 functions and data that is *TRULY* generic (completely platform agnostic) and
63 (2) #include the platform specific implementation with:
65 #include "platform/Impl.cpp"
67 where Impl is the name of the implementation files.
69 8. Platform specific implementation files (platform/Impl.cpp) may only #include
70 other Impl.cpp files found in directories under lib/System. The order of
71 inclusion is very important (from most generic to most specific) so that we
72 don't inadvertently place an implementation in the wrong place. For example,
73 consider a fictitious implementation file named DoIt.cpp. Here's how the
74 #includes should work for a Linux platform
77 #include "platform/DoIt.cpp" // platform specific impl. of Doit
80 lib/System/Linux/DoIt.cpp // impl that works on all Linux
81 #include "../Unix/DoIt.cpp" // generic Unix impl. of DoIt
82 #include "../Unix/SUS/DoIt.cpp // SUS specific impl. of DoIt
83 #include "../Unix/SUS/v3/DoIt.cpp // SUSv3 specific impl. of DoIt
85 Note that the #includes in lib/System/Linux/DoIt.cpp are all optional but
86 should be used where the implementation of some functionality can be shared
87 across some set of Unix variants. We don't want to duplicate code across
88 variants if their implementation could be shared.
90 9. The library does not attempt to shield LLVM from the C++ standard library or
91 standard template library. These libraries are considered to be platform
94 10. LLVM should not include *any* system headers anywhere except in lib/System.
96 11. lib/System must *not* expose *any* system headers through its interface.