```/* nag_roots_sys_func_rcomm (c05qdc) Example Program.
*
* Copyright 2019 Numerical Algorithms Group.
*
* Mark 27.0, 2019.
*/

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

#ifdef __cplusplus
extern "C"
{
#endif
static void NAG_CALL fcn(Integer n, const double x[], double fvec[]);
#ifdef __cplusplus
}
#endif

int main(void)
{
Integer exit_status = 0, i, n = 9, irevcm, ml, mu;
double *diag = 0, *fjac = 0, *fvec = 0, *qtf = 0, *r = 0, *x = 0,
*rwsav = 0;
Integer *iwsav = 0;
double epsfcn, factor, xtol;
/* Nag Types */
NagError fail;
Nag_ScaleType scale_mode;

INIT_FAIL(fail);

printf("nag_roots_sys_func_rcomm (c05qdc) Example Program Results\n");
if (n > 0) {
if (!(diag = NAG_ALLOC(n, double)) ||
!(fjac = NAG_ALLOC(n * n, double)) ||
!(fvec = NAG_ALLOC(n, double)) ||
!(qtf = NAG_ALLOC(n, double)) ||
!(r = NAG_ALLOC(n * (n + 1) / 2, double)) ||
!(x = NAG_ALLOC(n, double)) ||
!(iwsav = NAG_ALLOC(17, Integer)) ||
!(rwsav = NAG_ALLOC(4 * n + 10, double)))
{
printf("Allocation failure\n");
exit_status = -1;
goto END;
}
}
else {
printf("Invalid n.\n");
exit_status = 1;
goto END;
}

/* The following starting values provide a rough solution. */
for (i = 0; i < n; i++)
x[i] = -1.0;

/* nag_machine_precision (x02ajc).
* The machine precision
*/
xtol = sqrt(nag_machine_precision);

for (i = 0; i < n; i++)
diag[i] = 1.0;

ml = 1;
mu = 1;
epsfcn = 0.0;
scale_mode = Nag_ScaleProvided;
factor = 100.0;
irevcm = 0;

/* nag_roots_sys_func_rcomm (c05qdc).
* Solution of a system of nonlinear equations (function values only,
* reverse communication)
*/

do {
nag_roots_sys_func_rcomm(&irevcm, n, x, fvec, xtol, ml, mu,
epsfcn, scale_mode, diag, factor, fjac, r,
qtf, iwsav, rwsav, &fail);

switch (irevcm) {
case 1:
/* x and fvec are available for printing */
break;
case 2:
fcn(n, x, fvec);
break;
}

} while (irevcm != 0);

if (fail.code != NE_NOERROR) {
printf("Error from nag_roots_sys_func_rcomm (c05qdc).\n%s\n",
fail.message);
exit_status = 1;
if (fail.code != NE_TOO_SMALL && fail.code != NE_NO_IMPROVEMENT)
goto END;
}

printf(fail.code == NE_NOERROR ? "Final approximate" : "Approximate");
printf(" solution\n\n");
for (i = 0; i < n; i++)
printf("%12.4f%s", x[i], (i % 3 == 2 || i == n - 1) ? "\n" : " ");

if (fail.code != NE_NOERROR)
exit_status = 2;

END:
NAG_FREE(diag);
NAG_FREE(fjac);
NAG_FREE(fvec);
NAG_FREE(qtf);
NAG_FREE(r);
NAG_FREE(x);
NAG_FREE(iwsav);
NAG_FREE(rwsav);
return exit_status;
}

static void NAG_CALL fcn(Integer n, const double x[], double fvec[])
{
Integer k;

for (k = 0; k < n; ++k) {
fvec[k] = (3.0 - x[k] * 2.0) * x[k] + 1.0;
if (k > 0)
fvec[k] -= x[k - 1];
if (k < n - 1)
fvec[k] -= x[k + 1] * 2.0;
}
}
```