NAG Library Manual, Mark 28.5
/* nag_lapacklin_dgesv (f07aac) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.5, 2022.
*/

#include <nag.h>
#include <stdio.h>

int main(void) {
/* Scalars */
Integer exit_status = 0, i, j, n, nrhs, pda, pdb;

/* Arrays */
double *a = 0, *b = 0;
Integer *ipiv = 0;

/* Nag Types */
NagError fail;
Nag_OrderType order;

#ifdef NAG_COLUMN_MAJOR
#define A(I, J) a[(J - 1) * pda + I - 1]
#define B(I, J) b[(J - 1) * pdb + I - 1]
order = Nag_ColMajor;
#else
#define A(I, J) a[(I - 1) * pda + J - 1]
#define B(I, J) b[(I - 1) * pdb + J - 1]
order = Nag_RowMajor;
#endif

INIT_FAIL(fail);

printf("nag_lapacklin_dgesv (f07aac) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n]");
scanf("%" NAG_IFMT " %" NAG_IFMT "%*[^\n]", &n, &nrhs);
if (n < 0 || nrhs < 0) {
printf("Invalid n or nrhs\n");
exit_status = 1;
return exit_status;
}

pda = n;
#ifdef NAG_COLUMN_MAJOR
pdb = n;
#else
pdb = nrhs;
#endif

/* Allocate memory */
if (!(a = NAG_ALLOC(n * n, double)) || !(b = NAG_ALLOC(n * nrhs, double)) ||
!(ipiv = NAG_ALLOC(n, Integer))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

/* Read A and B from data file */
for (i = 1; i <= n; ++i)
for (j = 1; j <= n; ++j)
scanf("%lf", &A(i, j));
scanf("%*[^\n]");

for (i = 1; i <= n; ++i)
for (j = 1; j <= nrhs; ++j)
scanf("%lf", &B(i, j));
scanf("%*[^\n]");

/* Solve the equations Ax = b for x using
* nag_lapacklin_dgesv (f07aac)
*/
nag_lapacklin_dgesv(order, n, nrhs, a, pda, ipiv, b, pdb, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dgesv (f07aac).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print solution */
printf("Solution\n");
for (i = 1; i <= n; ++i)
for (j = 1; j <= nrhs; ++j)
printf("%11.4f%s", B(i, j), j % 7 == 0 || j == nrhs ? "\n" : " ");

/* Print details of factorization using
* nag_file_print_matrix_real_gen (x04cac)
*/
printf("\n");
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
n, a, pda, "Details of factorization", 0,
&fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_file_print_matrix_real_gen (x04cac).\n%s\n",
fail.message);
exit_status = 1;
goto END;
}

/* Print pivot indices */
printf("\nPivot indices\n");
for (i = 1; i <= n; ++i)
printf("%11" NAG_IFMT "%s", ipiv[i - 1], i % 7 == 0 || i == n ? "\n" : " ");
END:
NAG_FREE(a);
NAG_FREE(b);
NAG_FREE(ipiv);
return exit_status;
}

#undef A
#undef B