NAG Library Manual, Mark 27.2
```/* nag_quad_dim1_quad_inf_1 (d01smc) 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_User *comm);
#ifdef __cplusplus
}
#endif

int main(void) {
static Integer use_comm[1] = {1};
Integer exit_status = 0;
double a;
double epsabs, abserr, epsrel, result;
Integer max_num_subint;
NagError fail;
Nag_User comm;

INIT_FAIL(fail);

/* For communication with user-supplied functions: */
comm.p = (Pointer)&use_comm;

epsabs = 0.0;
epsrel = 0.0001;
a = 0.0;
max_num_subint = 200;

*/
max_num_subint, &result, &abserr, &qp, &comm, &fail);
printf("a      - lower limit of integration = %10.4f\n", a);
printf("b      - upper limit of integration = infinity\n");
printf("epsabs - absolute accuracy requested = %11.2e\n", epsabs);
printf("epsrel - relative accuracy requested = %11.2e\n\n", epsrel);
if (fail.code != NE_NOERROR)
if (fail.code != NE_INT_ARG_LT && fail.code != NE_BAD_PARAM &&
fail.code != NE_ALLOC_FAIL && fail.code != NE_NO_LICENCE) {
printf("result - approximation to the integral = %9.5f\n", result);
printf("abserr - estimate of the absolute error = %11.2e\n", abserr);
printf("qp.fun_count  - number of function evaluations = %4" NAG_IFMT "\n",
qp.fun_count);
printf("qp.num_subint  - number of subintervals used = %4" NAG_IFMT "\n",
qp.num_subint);
/* Free memory used by qp */
NAG_FREE(qp.sub_int_beg_pts);
NAG_FREE(qp.sub_int_end_pts);
NAG_FREE(qp.sub_int_result);
NAG_FREE(qp.sub_int_error);
} else {
exit_status = 1;
goto END;
}

END:
return exit_status;
}

static double NAG_CALL f(double x, Nag_User *comm) {
Integer *use_comm = (Integer *)comm->p;

if (use_comm[0]) {
printf("(User-supplied callback f, first invocation.)\n");
use_comm[0] = 0;
}

return 1.0 / ((x + 1.0) * sqrt(x));
}
```