OpenMP support

[muschellij2]

Not all Mac OSX users have OpenMP support (unless they install the clang toolchain from CRAN).

Please see
https://github.com/USCBiostats/software-dev/wiki/Setting-up-optional-OpenMP-support
Rdatatable/data.table#2161

for some resources on how to add if statements using autoconf for openMP flags.

[muschellij2]

For those on OSX, if you have this in ~/.R/Makevars, you should be fine:

SHLIB_OPENMP_CFLAGS=
SHLIB_OPENMP_CXXFLAGS=
SHLIB_OPENMP_CPPFLAGS=
SHLIB_OPENMP_FCFLAGS= 
SHLIB_OPENMP_FFLAGS=