Using OpenBLAS LAPACKE in Visual Studio

Question

i need some linear algebra in my project and want use OpenBLAS for this. I downloaded the precompiled version (64bit version) and unpacked it to my projectfolder. In Visual Studio, i added include-, bin-, and lib-folder to my Project and ran the this example without problems.

Next, i wanted to look at LAPACK, so i added lapacke.h to the includes, which is in the same directory as cblas.h and is included in the official download. But now i get hundreds of errors, for every function, as if a lib file was missing or something. E.g. for this line

85 lapack_complex_float lapack_make_complex_float( float re, float im );

i get

PATH\include\lapacke.h(85): error C2146: syntax error: missing ';' before identifier 'lapack_make_complex_float'

I can't find any further information on how to set up OpenBLAS/LAPACK, they usually just say 'include the files', which i have. Otherwise the cblas example wouldn't run either. And the (relevant) examples i can find only use cblas.h, not lapacke.h

Can some tell me what i'm doing wrong?

Answer 1

Using std::complex is problematic unless OpenBLAS was built with LAPACK_COMPLEX_CPP otherwise the library uses a different complex type internally, usually C99 _Complex.

Modern versions of the Microsoft compiler (e.g. the one in VSTUDIO 2022) support a similar "_Complex" mechanism. Therefore I include the following header file prior to "lapacke.h"

#ifdef _MSC_VER

#include <complex.h>

#define LAPACK_COMPLEX_CUSTOM

typedef _Dcomplex lapack_complex_float;
typedef _Fcomplex lapack_complex_double;

#define lapack_complex_float_real(z)       (real(z))
#define lapack_complex_float_imag(z)       (imag(z))

#endif // _MSC_VER

See the lapack.h header file that ships with OpenBLAS how #define LAPACK_COMPLEX_CUSTOM overrides complex variable type definitions

Answer 2

The problem is that OpenBlas uses C99 _Complex by default. This is not supported by Visual C++. You can solve this by using standard library definitions before including lapacke.h:

#include <complex>
#define lapack_complex_float std::complex<float>
#define lapack_complex_double std::complex<double>
#include <lapacke.h>