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Summary: Calling WriteConsoleW is the most reliable way to print Unicode characters to a Windows console. If binary data gets printed to the console, attempting to re-encode it shouldn't be a problem, since garbage in can produce garbage out. This breaks printing strings in the local codepage, which WriteConsoleA knows how to handle. For example, this can happen when user source code is encoded with the local codepage, and an LLVM tool quotes it while emitting a caret diagnostic. This is unfortunate, but well-behaved tools should validate that their input is UTF-8 and escape non-UTF-8 characters before sending them to raw_fd_ostream. Clang already does this, but not all LLVM tools do this. One drawback to the current implementation is printing a string a byte at a time doesn't work. Consider this LLVM code: for (char C : MyStr) outs() << C; Because outs() is now unbuffered, we wil try to convert each byte to UTF-16, which will fail. However, this already didn't work, so I think we may as well update callers that do that as we find them to print complete portions of strings. You can see a real example of this in my patch to SourceMgr.cpp Fixes PR38669 and PR36267. Reviewers: zturner, efriedma Subscribers: llvm-commits, hiraditya Differential Revision: https://reviews.llvm.org/D51558 llvm-svn: 341433
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 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
While we recommend that you read the more detailed documentation, for the
impatient, here's a high level summary of the library's requirements.
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 returning "true" and setting an optional std::string
7. Library must not throw any exceptions, period.
8. Interface functions must not have throw() specifications.
9. No duplicate function impementations are permitted within an operating
system class.
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
5. Multiple Implementations
6. Minimize Memory Allocation
7. No Virtual Methods