NAG Library Manual, Mark 28.6
```/* nag_lapacklin_dgttrs (f07cec) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.6, 2022.
*/
#include <nag.h>
#include <stdio.h>

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

/* Arrays */
double *b = 0, *d = 0, *dl = 0, *du = 0, *du2 = 0;
Integer *ipiv = 0;

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

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

INIT_FAIL(fail);

printf("nag_lapacklin_dgttrs (f07cec) 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;
goto END;
}
/* Allocate memory */
if (!(b = NAG_ALLOC(n * nrhs, double)) || !(d = NAG_ALLOC(n, double)) ||
!(dl = NAG_ALLOC(n - 1, double)) || !(du = NAG_ALLOC(n - 1, double)) ||
!(du2 = NAG_ALLOC(n - 2, double)) || !(ipiv = NAG_ALLOC(n, Integer))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

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

/* Read the tridiagonal matrix A from data file */
for (i = 0; i < n - 1; ++i)
scanf("%lf", &du[i]);
scanf("%*[^\n]");
for (i = 0; i < n; ++i)
scanf("%lf", &d[i]);
scanf("%*[^\n]");
for (i = 0; i < n - 1; ++i)
scanf("%lf", &dl[i]);
scanf("%*[^\n]");

/* Read the right hand matrix B */

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

/* Factorize the tridiagonal matrix A using nag_lapacklin_dgttrf (f07cdc). */
nag_lapacklin_dgttrf(n, dl, d, du, du2, ipiv, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dgttrf (f07cdc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Solve the equations AX = B using nag_lapacklin_dgttrs (f07cec). */
nag_lapacklin_dgttrs(order, Nag_NoTrans, n, nrhs, dl, d, du, du2, ipiv, b,
pdb, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dgttrs (f07cec).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print the solution usbing nag_file_print_matrix_real_gen (x04cac). */
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
nrhs, b, pdb, "Solution(s)", 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;
}
END:
NAG_FREE(b);
NAG_FREE(d);
NAG_FREE(dl);
NAG_FREE(du);
NAG_FREE(du2);
NAG_FREE(ipiv);
return exit_status;
}

#undef B
```