[cfe-dev] clang -E -o support

[Shantonu Sen]

To be more compatible with gcc, clang should support outputting  
preprocessed source to a file, instead of relying on shell  
redirection. The attached patch allows "clang -E a.c -o a.i" to work.  
In fact, it supports the following:

> $ echo "" > a.c
> $ clang -E a.c
> # 1 "a.c" 1
> # 0 "a.c"
> # 1 "<predefines>" 1
> # 1 "a.c" 2
>
> $ clang -E a.c -o -
> # 1 "a.c" 1
> # 0 "a.c"
> # 1 "<predefines>" 1
> # 1 "a.c" 2
>
> $ clang -E a.c -o a.i
> $ cat a.i
> # 1 "a.c" 1
> # 0 "a.c"
> # 1 "<predefines>" 1
> # 1 "a.c" 2
> $ clang -E a.c -o /dev/null
> $ clang -E a.c -o /System/unwritable
> Assertion failed: (OutputFD >= 0 && "failed to open output file"),  
> function InitOutputBuffer, file PrintPreprocessedOutput.cpp, line 73.
> 0   clang                               0x0022d655  
> _ZN40_GLOBAL__N_Signals.cpp_00000000_62F12DC115PrintStackTraceEv + 45
> 1   clang                               0x0022d9fb  
> _ZN40_GLOBAL__N_Signals.cpp_00000000_62F12DC113SignalHandlerEi + 323
> ...

I also compiled PrintPreprocessedOutput.cpp with USE_STDIO set to 1 on  
Mac OS X, so I believe that codepath should work as well.

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Shantonu Sen
Apple Inc.