NAG Library Manual, Mark 28.5
```/* nag_matop_real_gen_matrix_log (f01ejc) Example Program.
*
* Copyright 2022 Numerical Algorithms Group.
*
* Mark 28.5, 2022.
*/

#include <math.h>
#include <nag.h>

int main(void) {

/* Scalars */
Integer exit_status = 0;
Integer i, j, n, pda;
double imnorm;

/* Arrays */
double *a = 0;

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

INIT_FAIL(fail);

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

/* Output preamble */
printf("nag_matop_real_gen_matrix_log (f01ejc) ");
printf("Example Program Results\n\n");
fflush(stdout);

/* Skip heading in data file */
scanf("%*[^\n]");

/* Read in the problem size */
scanf("%" NAG_IFMT "%*[^\n]", &n);

pda = n;

if (!(a = NAG_ALLOC(pda * n, double))) {
printf("Allocation failure\n");
exit_status = -1;
goto END;
}

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

/* Find log(A) using
* nag_matop_real_gen_matrix_log (f01ejc)
* Real matrix logarithm
*/
nag_matop_real_gen_matrix_log(order, n, a, pda, &imnorm, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_matop_real_gen_matrix_log (f01ejc)\n%s\n",
fail.message);
exit_status = 1;
goto END;
}

/* Print solution using
* nag_file_print_matrix_real_gen (x04cac)
* Print real general matrix (easy-to-use)
*/
nag_file_print_matrix_real_gen(order, Nag_GeneralMatrix, Nag_NonUnitDiag, n,
n, a, pda, "log(A)", NULL, &fail);
if (fail.code != NE_NOERROR) {
printf("Error from nag_file_print_matrix_real_gen (x04cac)\n%s\n",
fail.message);
exit_status = 2;
goto END;
}

END:
NAG_FREE(a);
return exit_status;
}
```