Matrix-vector multiplication using BLAS.

gistfile1.c

void doStuff(void) {
    // Les data:
    float A[4][4] = // the matrix to be multiplied
    {
        {1.0f, 1.0f, 1.0f, 1.0f},
        {0.0f, 1.0f, 1.0f, 4.0f},
        {1.0f, 1.0f, 1.0f, 4.0f},
        {6.0f, 2.0f, 1.0f, 1.0f}
    };
    float a = 0.5f;
    float x[4] = {1.0f, 2.0f, 4.0f, 8.0f};
    float y[4] = {0.f, 0.f, 0.f, 0.f}; //result y = a*A*x

    // Le command:
    cblas_sgemv(CblasRowMajor, CblasNoTrans, 4, 4, a, (float*) A, 4, x, 1, 1.0f, y, 1);

    ptrdiff_t i;
    for (i = 0; i < 4; i++) {
        printf("y[%d]=%.4f\n", i, y[i]);
    }
}

See also http://ideone.com/s5Pl4r