```/* nag_roots_contfn_brent (c05ayc) Example Program.
*
* Copyright 2020 Numerical Algorithms Group.
*
* Mark 27.1, 2020.
*/

#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) {
static double ruser[1] = {-1.0};
Integer exit_status = 0;
double a, b;
double x, eta, eps;
NagError fail;
Nag_Comm comm;

INIT_FAIL(fail);

printf("nag_roots_contfn_brent (c05ayc) Example Program Results\n");

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

a = 0.0;
b = 1.0;
eps = 1e-05;
eta = 0.0;

/* nag_roots_contfn_brent (c05ayc).
* Zero of a continuous function using Brent's algorithm
*/
nag_roots_contfn_brent(a, b, eps, eta, f, &x, &comm, &fail);
if (fail.code == NE_NOERROR) {
printf("Zero = %12.5f\n", x);
} else {
printf("%s\n", fail.message);
if (fail.code == NE_TOO_SMALL || fail.code == NE_PROBABLE_POLE)
printf("Final point = %12.5f\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 exp(-x) - x;
}
```