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1Design Of lib/System
2====================
3
4The software in this directory is designed to completely shield LLVM from any
5and all operating system specific functionality. It is not intended to be a
6complete operating system wrapper (such as ACE), but only to provide the
7functionality necessary to support LLVM.
8
9The software located here, of necessity, has very specific and stringent design
10rules. Violation of these rules means that cracks in the shield could form and
11the primary goal of the library is defeated. By consistently using this library,
12LLVM becomes more easily ported to new platforms since the only thing requiring
13porting is this library.
14
15Complete documentation for the library can be found in the file:
16  llvm/docs/SystemLibrary.html
17or at this URL:
18  http://llvm.org/docs/SystemLibrary.html
19
20While we recommend that you read the more detailed documentation, for the
21impatient, here's a high level summary of the library's requirements.
22
23 1. No system header files are to be exposed through the interface.
24 2. Std C++ and Std C header files are okay to be exposed through the interface.
25 3. No exposed system-specific functions.
26 4. No exposed system-specific data.
27 5. Data in lib/System classes must use only simple C++ intrinsic types.
28 6. Errors are handled by returning "true" and setting an optional std::string
29 7. Library must not throw any exceptions, period.
30 8. Interface functions must not have throw() specifications.
31 9. No duplicate function impementations are permitted within an operating
32    system class.
33
34To accomplish these requirements, the library has numerous design criteria that
35must be satisfied. Here's a high level summary of the library's design criteria:
36
37 1. No unused functionality (only what LLVM needs)
38 2. High-Level Interfaces
39 3. Use Opaque Classes
40 4. Common Implementations
41 5. Multiple Implementations
42 6. Minimize Memory Allocation
43 7. No Virtual Methods
44