# PetscBLASInt
datatype used to represent 'int' parameters to BLAS/LAPACK functions. 
## Notes
Usually this is the same as `PetscIn`t, 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-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
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/include/petscsystypes.h.html#PetscBLASInt">include/petscsystypes.h</A>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/include/petscsystypes.h)


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