-System Utilities Interface
-==========================
+Design Of lib/System
+====================
-The design of this library has several key constraints aimed at shielding LLVM
-from the vagaries of operating system differences. The goal here is to provide
-interfaces to operating system concepts (files, memory maps, sockets, signals,
-locking, etc) efficiently and in such a way that the remainder of LLVM is
-completely operating system agnostic.
+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.
-PLEASE READ AND COMPREHEND FULLY THE DOCUMENTATION in
+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 the only thing requiring
+porting is this library.
-llvm/docs/SystemLibrary.html
+Complete documentation for the library can be found in the file:
+ llvm/docs/SystemLibrary.html
+or at this URL:
+ http://llvm.cs.uiuc.edu/docs/SystemLibrary.html
-before making changes to this library.
+While we recommend that you read the more detailed documentation, for the
+impatient, here's a high level summary of the library's requirements.
-Briefly, the rules are:
+ 1. No system header files are to be exposed through the interface.
+ 2. Std C++ and Std C header files are okay to be exposed through the interface.
+ 3. No exposed system-specific functions.
+ 4. No exposed system-specific data.
+ 5. Data in lib/System classes must use only simple C++ intrinsic types.
+ 6. Errors are handled by throwing std::string *only*.
+ 7. Library must not throw any exceptions except std::string.
+ 8. Interface functions must not have throw() specifications.
+ 9. No duplicate function impementations are permitted within an operating
+ system class.
-1. This library may not expose no system-specific header files (STL excepted).
-2. This library may not allocate memory (directly).
-3. This library may not throw exceptions.
-4. This library may not use virtual methods in its classes.
-5. This library may not expose system specific types, data or functions through
- its interface
+To accomplish these requirements, the library has numerous design criteria that
+must be satisfied. Here's a high level summary of the library's design criteria:
+
+ 1. No unused functionality (only what LLVM needs)
+ 2. High-Level Interfaces
+ 3. Use Opaque Classes
+ 4. Common Implementations</a></li>
+ 5. Multiple Implementations</a></li>
+ 6. Minimize Memory Allocation</a></li>
+ 7. No Virtual Methods
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