NAG Library Manual, Mark 27.2
Interfaces:  FL   CL   CPP   AD
```/* nag_roots_contfn_brent_interval (c05auc) Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
*
* Mark 27.2, 2021.
*/

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

#ifdef __cplusplus
extern "C" {
#endif
static double NAG_CALL f(double x, Nag_Comm *comm);
#ifdef __cplusplus
}
#endif

int main(void) {
/* Scalars */
Integer exit_status = 0;
double a, b, eps, eta, h, x;
NagError fail;
Nag_Comm comm;
/* Arrays */
static double ruser[1] = {-1.0};

INIT_FAIL(fail);

printf("nag_roots_contfn_brent_interval (c05auc) Example Program Results\n");

x = 1.0;
h = 0.1;
eps = 1e-05;
eta = 0.0;

/* For communication with user-supplied functions: */
comm.user = ruser;

/* nag_roots_contfn_brent_interval (c05auc).
* Locates a simple zero of a continuous function of one variable,
* binary search for an interval containing a zero.
*/
nag_roots_contfn_brent_interval(&x, h, eps, eta, f, &a, &b, &comm, &fail);
if (fail.code == NE_NOERROR) {
printf("Root is %13.5f\n", x);
printf("Interval searched is [%8.5f,%8.5f]\n", a, b);
} else {
printf("%s\n", fail.message);
if (fail.code == NE_PROBABLE_POLE || fail.code == NW_TOO_MUCH_ACC_REQUESTED)
printf("Final value = %13.5f\n", x);
exit_status = 1;
goto END;
}

END:
return exit_status;
}

static double NAG_CALL f(double x, Nag_Comm *comm) {
if (comm->user[0] == -1.0) {
printf("(User-supplied callback f, first invocation.)\n");
comm->user[0] = 0.0;
}
return x - exp(-x);
}
```