# Using Cluster MKL PBLAS/ScaLAPACK FORTRAN routine in your C program

Introduction

A set of LAPACK examples is now available online. These examples are based on the ones available with the product in the examples directory, but are more easily browsed and contain no dependencies on other source files or auxiliary libraries. Just paste the code into a file and link with the Intel® MKL libraries.

Intel MKL PBLAS and ScaLAPACK routines are written in FORTRAN 77  (with exception of a few utility routines written
in C to exploit the IEEE arithmetic). It is difficult for the programmer who used to C language convention. This article describles a sample to call the Intel® Math Kernel Library (Intel® MKL) PBLAS routines from a C program to show the difference between C calling converstion and Fortran calling conversion .
Please see the detail discussion on MKL forum.

1. Problem
The sample is using PDGEMV(), which computes a distributed matrix-vector product y =alphaAx+beta*y.
A= [1  4  7  10  13 ; 3   6   9   12  15 ;  5  8   11  14  17 ; 7  10  13  16  19;  9  12  15  18  21]
and x=[1 ; 1; 0; 0; 1]T, x is a column vector, Call PDGEMV() routine to compute y=Ax.
The right result is y=[18; 24; 30; 36; 42]T

2. Code sample in C:pdgemv.c
The function call in C is like,

double alpha = 1.0; double beta = 0.0;
pdgemv_("N",&M,&M,&alpha,A,&ONE,&ONE,descA,x,&ONE,&ONE,descx,&ONE,&beta,y,&ONE,& amp;ONE,descy,&ONE);

3. Build and Run it

Build it under linux environment, assume intel 64bit application.
! set environment and build pdgemv.c
\$source /opt/intel/mkl/10.x.x.0xx/tools/environment/mklvarsem64t.sh
\$source /opt/intel/mpi/3.x.x/bin64/mpivars.sh
\$source /opt/intel/Compiler/11.x/0xx/bin/iccvars.sh intel64
\$mpiicc -w -o pdgemv pdgemv.c -I/opt/intel/mkl/10.x.x.0xx/include -L/opt/intel/mkl/10.x.x.0xx/lib/em64t

! run the application
\$ mpirun -n 4 ./pdgemv

4. Notes

To call Fortran function in C, you need to take care of two things:

1. Pass variables by address, not by value.   for example,

double alpha = 1.0;
pdgemv_("N",&M,&M,&alpha

2. Store your data in Fortran style, that is, column-major rather than row-major order

Intel MKL PBLAS/ScaLAPACK routines are written in FORTRAN interface, so Column-major are used and row-major is not acceptable.  If you gives the matrix to these routines in row-major - even your code is completely right from C (Row-major) point of view,  the result will be almost always wrong.

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