understanding RCpp compile - r

Question

I was looking at the verbose=TRUE when I tried to sourceCpp a Rcpp file. The last output is:
DIR: C:/Users/xyz/AppData/Local/Temp/RtmpmielLn/sourcecpp_226416891d0e
C:/PROGRA~1/R/R-31~1.0/bin/x64/R CMD SHLIB -o "sourceCpp_22129.dll" --preclean "myfile.cpp"
g++ -m64 -I"C:/PROGRA~1/R/R-31~1.0/include" -DNDEBUG -I"C:/Users/xyz/Documents/R/win-library/3.1/Rcpp/include" -I"d:/RCompile/CRANpkg/extralibs64/local/include" -O2 -Wall -mtune=core2 -c myfile.cpp -o myfile.o
g++ -m64 -shared -s -static-libgcc -o sourceCpp_22129.dll tmp.def myfile.o -Ld:/RCompile/CRANpkg/extralibs64/local/lib/x64 -Ld:/RCompile/CRANpkg/extralibs64/local/lib -LC:/PROGRA~1/R/R-31~1.0/bin/x64 -lR
I have a few questions regarding this:
the 1nd g++ command refers to -I"d:/RCompile/CRANpkg/extralibs64/local/include" and the 2nd command refers to -Ld:/RCompile/CRANpkg/extralibs64/local/lib/x64 and -Ld:/RCompile/CRANpkg/extralibs64/local/lib . But I don't have a D: drive, or a RCompile folder anywhere. What do these things refer to?
I tried to manually run the 1st g++ which ran file and created myfun.o file, but when I tried to manually run the 2nd g++ it gave me an error saying that it couldn't find the tmp.def file. I couldn't find the tmp.def file anywhere on my drives. Where would this tmp.def file located?
I looked under the hood of sourceCpp function. if I directly run the definition of cmd in the sourceCpp function: C:/PROGRA~1/R/R-31~1.0/bin/x64/R CMD SHLIB -o "sourceCpp_22129.dll" --preclean "myfile.cpp" on Windows' command window , I noticed that it does not include -I"C:/Users/xyz/Documents/R/win-library/3.1/Rcpp/include" and the R CMD SHLIB gives me an error.
How does the system(cmd, ..) within the sourceCpp function include this? The value of the cmd variable in the sourceCpp didn't include -I"C:/Users/xyz/Documents/R/win-library/3.1/Rcpp/include"

Answer 1

the 1nd g++ command refers to -I"d:/RCompile/CRANpkg/extralibs64/local/include" and the 2nd command refers to -Ld:/RCompile/CRANpkg/extralibs64/local/lib/x64 and -Ld:/RCompile/CRANpkg/extralibs64/local/lib . But I don't have a D: drive, or a RCompile folder anywhere. What do these things refer to?
AFAIK these are left in as part of the CRAN R Windows distribution; when R binaries are built on Windows they use something in these library paths on the build servers (but stay baked into R anyhow). You can safely ignore it, but it is a bit odd. Unused / non-existent directories passed through gcc / g++ are just ignored.
I tried to manually run the 1st g++ which ran file and created myfun.o file, but when I tried to manually run the 2nd g++ it gave me an error saying that it couldn't find the tmp.def file. I couldn't find the tmp.def file anywhere on my drives. Where would this tmp.def file located?
tmp.def, as it sounds, is a temporary definition file created by R CMD SHLIB on Windows. If you just re-run what you see it does not get generated, so I suppose R does something behind the curtains to generate it. If you are curious about where it's generated, see share/make/winshlib.mk in the R sources.
I looked under the hood of sourceCpp function. if I directly run the definition of cmd in the sourceCpp function: C:/PROGRA~1/R/R-31~1.0/bin/x64/R CMD SHLIB -o "sourceCpp_22129.dll" --preclean "myfile.cpp" on Windows' command window , I noticed that it does not include -I"C:/Users/xyz/Documents/R/win-library/3.1/Rcpp/include" and the R CMD SHLIB gives me an error.
This is because sourceCpp is setting the appropriate environment flags behind the scenes for you as well -- in this case, the CXXFLAGS environment variable. This gets automatically done on package installs as well when the LinkingTo: entry is specified in the DESCRIPTION file.

Answer 2

I had a very similar g++ compilation command as posted in the question, and the compilation of a very simple C function didn't work for me.
The reason it didn't work is the following option shown in the g++ command:
-I"d:/RCompile/CRANpkg/extralibs64/local/include"
which adds an include directory on a drive that does not exist on my computer. Apparently non-existent directories listed in the -I option are ignored by g++ (as stated by Kevin Ushey) but this seems not to be the case for non-existent drives.
The error message I received was that the sdtlib.h header file was not found:
C:/PROGRA~1/R/R-32~1.0/include/R.h:28:20: fatal error:
d:/RCompile/r-compiling/local/local320/include/stdlib.h: Input/output
error #include
In order to remove the offending -I option from the g++ command I had to edit the Makeconf file located in $(R_HOME}/etc$(R_ARCH) (in my case C:\Program Files\R\R-3.2.0\etc\x64) and comment out the line:
LOCAL_SOFT = d:/RCompile/r-compiling/local/local320