在并行计算中经常需要调用scalapack(并行化的lapack)函数库里面的函数进行编程，这里简单介绍在C语言如何调用scalapck中的矩阵向量乘的函数。
注意：scalapack中的函数是用fortran写的，矩阵是按列进行存储的。
scalapack的链接需要用到blas，因此确保本机上安装好了blas、gfortran
下面是一个矩阵向量乘法的例子(为了简单计算，该程序中设定进程数为4):
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "mpi.h"
#define AA(i,j) AA[(i)*M+(j)]
int main(int argc, char **argv)
{
int i, j, k;
/************ MPI ***************************/
int myrank_mpi, nprocs_mpi;
MPI_Init( &argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &myrank_mpi);
MPI_Comm_size(MPI_COMM_WORLD, &nprocs_mpi);
/************ BLACS ***************************/
int ictxt, nprow, npcol, myrow, mycol,nb;
int info,itemp;
int ZERO=0,ONE=1;
nprow = 2;
npcol = 2;
nb = 1;
Cblacs_pinfo( &myrank_mpi, &nprocs_mpi ) ;
Cblacs_get( -1, 0, &ictxt );
Cblacs_gridinit( &ictxt, "Row", nprow, npcol );
Cblacs_gridinfo( ictxt, &nprow, &npcol, &myrow, &mycol );
int M = 4;
int descA[9],descx[9],descy[9];
int mA = numroc_( &M, &nb, &myrow, &ZERO, &nprow );
int nA = numroc_( &M, &nb, &mycol, &ZERO, &npcol );
int nx = numroc_( &M, &nb, &myrow, &ZERO, &nprow );
int my = numroc_( &M, &nb, &myrow, &ZERO, &nprow );
descinit_(descA, &M, &M, &nb, &nb, &ZERO, &ZERO, &ictxt, &mA, &info);
descinit_(descx, &M, &ONE, &nb, &ONE, &ZERO, &ZERO, &ictxt, &nx, &info);
descinit_(descy, &M, &ONE,&nb, &ONE, &ZERO, &ZERO, &ictxt, &my, &info);
double *x = (double*) malloc(nx*sizeof(double));
double *y = (double*) calloc(my,sizeof(double));
double *A = (double*) malloc(mA*nA*sizeof(double));
int sat,sut;
for(i=0;i<mA;i++)
for(j=0;j<nA;j++)
A[j*mA+i]=1.0;
for(i=0;i<nx;i++)
x[i] = 1.0;
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, &ONE, descy, &ONE);
for(i=0;i<my;i++)
printf("rank=%d ,%f \n", myrank_mpi,y[i]);
MPI_Finalize();
return 0;
}
编译生成可执行文件:
mpicc -o pdgemv pdgemv.c -L/opt/scalapack/scalapck-2.0.2/ -lscalapack -lblas -lgfortran
运行：
mpirun -np 4 ./pdgemv