NAG Library Manual, Mark 27.3
```/* nag_lapacklin_dtrrfs (f07thc) Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
*
* Mark 27.3, 2021.
*/

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

int main(void) {

/* Scalars */
Integer i, j, n, nrhs, berr_len, ferr_len;
Integer pda, pdb, pdx;
Integer exit_status = 0;
Nag_UploType uplo;

NagError fail;
Nag_OrderType order;
/* Arrays */
char nag_enum_arg;
double *a = 0, *b = 0, *berr = 0, *ferr = 0, *x = 0;

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

INIT_FAIL(fail);

printf("nag_lapacklin_dtrrfs (f07thc) Example Program Results\n\n");

/* Skip heading in data file */
scanf("%*[^\n] ");
scanf("%" NAG_IFMT "%" NAG_IFMT "%*[^\n] ", &n, &nrhs);
berr_len = nrhs;
ferr_len = nrhs;
#ifdef NAG_COLUMN_MAJOR
pda = n;
pdb = n;
pdx = n;
#else
pda = n;
pdb = nrhs;
pdx = nrhs;
#endif

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

/* Read A and B from data file, and copy B to X */
scanf(" %39s%*[^\n] ", nag_enum_arg);
/* nag_enum_name_to_value (x04nac).
* Converts NAG enum member name to value
*/
uplo = (Nag_UploType)nag_enum_name_to_value(nag_enum_arg);

if (uplo == Nag_Upper) {
for (i = 1; i <= n; ++i) {
for (j = i; j <= n; ++j)
scanf("%lf", &A(i, j));
}
scanf("%*[^\n] ");
} else {
for (i = 1; i <= n; ++i) {
for (j = 1; j <= i; ++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] ");
for (i = 1; i <= n; ++i) {
for (j = 1; j <= nrhs; ++j)
X(i, j) = B(i, j);
}

/* Compute solution in the array X */
/* nag_lapacklin_dtrtrs (f07tec).
* Solution of real triangular system of linear equations,
* multiple right-hand sides
*/
nag_lapacklin_dtrtrs(order, uplo, Nag_NoTrans, Nag_NonUnitDiag, n, nrhs, a,
pda, x, pdx, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dtrtrs (f07tec).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Compute backward errors and estimated bounds on the */
/* forward errors */

/* nag_lapacklin_dtrrfs (f07thc).
* Error bounds for solution of real triangular system of
* linear equations, multiple right-hand sides
*/
nag_lapacklin_dtrrfs(order, uplo, Nag_NoTrans, Nag_NonUnitDiag, n, nrhs, a,
pda, b, pdb, x, pdx, ferr, berr, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_lapacklin_dtrrfs (f07thc).\n%s\n", fail.message);
exit_status = 1;
goto END;
}

/* Print solution */

printf("\n");
/* nag_file_print_matrix_real_gen (x04cac).
* Print real general matrix (easy-to-use)
*/
fflush(stdout);
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
nrhs, x, pdx, "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;
}
printf("\nBackward errors (machine-dependent)\n");

for (j = 1; j <= nrhs; ++j)
printf("%11.1e%s", berr[j - 1], j % 7 == 0 ? "\n" : " ");
printf("\nEstimated forward error bounds "
"(machine-dependent)\n");
for (j = 1; j <= nrhs; ++j)
printf("%11.1e%s", ferr[j - 1], j % 7 == 0 ? "\n" : " ");
printf("\n");
END:
NAG_FREE(a);
NAG_FREE(b);
NAG_FREE(berr);
NAG_FREE(ferr);
NAG_FREE(x);

return exit_status;
}
```