**petsc-3.14.1 2020-11-03**

# PetscBLASInt

datatype used to represent 'int' parameters to BLAS/LAPACK functions.
### Notes

Usually this is the same as PetscInt, but if PETSc was built with --with-64-bit-indices but
standard C/Fortran integers are 32 bit then this is NOT the same as PetscInt it remains 32 bit
(except on very rare BLAS/LAPACK implementations that support 64 bit integers see the notes below).
PetscErrorCode PetscBLASIntCast(a,&b) checks if the given PetscInt a will fit in a PetscBLASInt, if not it
generates a PETSC_ERR_ARG_OUTOFRANGE error

### Installation Notes

./configure automatically determines the size of the integers used by BLAS/LAPACK except when --with-batch is used
in that situation one must know (by some other means) if the integers used by BLAS/LAPACK are 64 bit and if so pass the flag --known-64-bit-blas-indice
MATLAB ships with BLAS and LAPACK that use 64 bit integers, for example if you run ./configure with, the option
--with-blaslapack-lib=[/Applications/MATLAB_R2010b.app/bin/maci64/libmwblas.dylib,/Applications/MATLAB_R2010b.app/bin/maci64/libmwlapack.dylib]

MKL ships with both 32 and 64 bit integer versions of the BLAS and LAPACK. If you pass the flag -with-64-bit-blas-indices PETSc will link
against the 64 bit version, otherwise it use the 32 bit version

OpenBLAS can be built to use 64 bit integers. The ./configure options --download-openblas -with-64-bit-blas-indices will build a 64 bit integer version

External packages such as hypre, ML, SuperLU etc do not provide any support for passing 64 bit integers to BLAS/LAPACK so cannot
be used with PETSc when PETSc links against 64 bit integer BLAS/LAPACK. ./configure will generate an error if you attempt to link PETSc against any of
these external libraries while using 64 bit integer BLAS/LAPACK.

### See Also

PetscMPIInt, PetscInt, PetscBLASIntCast()

### Level

intermediate

### Location

include/petscsystypes.h

Index of all Sys routines

Table of Contents for all manual pages

Index of all manual pages