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A general description is here.
‘USE_FC_LEN_T’ is the default: this uses the correct prototypes for Fortran BLAS/LAPACK routines called from C/C++, and requires adjustment of most such calls - see ‘Writing R Extensions’ §6.6.1.
[In progress: setting environment variable ‘_R_INSTALL_USE_FC_LEN_T_’ to a true value enables this _pro tem_. This is set by ‘R CMD check --as-cran’ when it installs a package.]
‘USE_FC_LEN_T’ will become the default: this uses the correct prototypes for Fortran BLAS/LAPACK routines called from C/C++, and requires adjustment of most such calls - see ‘Writing R Extensions’ §6.6.2. (This has been supported since R 3.6.2.)