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A general description is here.

~~Header ‘Rconfig.h’ contains the value of ‘FC_LEN_T’ deduced at installation which is used by the prototypes in headers ‘BLAS.h’ and ‘Lapack.h’ but to avoid extensive breakage this is only exposed when ‘USE_FC_LEN_T’ is defined.~~If a package's C/C++ calls to BLAS/LAPACK allow for the ‘hidden’ arguments use by most Fortran compilers to pass the lengths of Fortran character arguments, define ‘USE_FC_LEN_T’ and include ‘Rconfig.h’ (possibly _via_ ‘R.h’) before including ‘BLAS.h’ or ‘Lapack.h’.

Header ‘Rconfig.h’ contains the value of ‘FC_LEN_T’ deduced at installation which is used by the prototypes in headers ‘BLAS.h’ and ‘Lapack.h’ but to avoid extensive breakage this is only exposed when ‘USE_FC_LEN_T’ is defined.

If a package's C/C++ calls to BLAS/LAPACK allow for the ‘hidden’ arguments used by most Fortran compilers to pass the lengths of Fortran character arguments, define ‘USE_FC_LEN_T’ and include ‘Rconfig.h’ (possibly _via_ ‘R.h’) before including ‘BLAS.h’ or ‘Lapack.h’.

‘apply(diag(3), 2:3, mean)’ now gives a helpful error message.