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# 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 may not be the same as `PetscInt`,
except on some 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-indices`

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 uses 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
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/include/petscsystypes.h.html#PetscBLASInt">include/petscsystypes.h</A>


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