Understanding LAPACK calls in C++ with a simple example

You need to factor the matrix (by calling dgetrf) before you can solve the system using dgetrs. Alternatively, you can use the dgesv routine, which does both steps for you.

By the way, you don’t need to declare the interfaces yourself, they are in the Accelerate headers:

// LAPACK test code

#include <iostream>
#include <vector>
#include <Accelerate/Accelerate.h>

using namespace std;

int main()
{
    char trans="N";
    int dim = 2;    
    int nrhs = 1;
    int LDA = dim;
    int LDB = dim;
    int info;

    vector<double> a, b;

    a.push_back(1);
    a.push_back(1);
    a.push_back(1);
    a.push_back(-1);

    b.push_back(2);
    b.push_back(0);

    int ipiv[3];

    dgetrf_(&dim, &dim, &*a.begin(), &LDA, ipiv, &info);
    dgetrs_(&trans, &dim, &nrhs, & *a.begin(), &LDA, ipiv, & *b.begin(), &LDB, &info);


    std::cout << "solution is:";    
    std::cout << "[" << b[0] << ", " << b[1] << ", " << "]" << std::endl;
    std::cout << "Info = " << info << std::endl; 

    return(0);
}