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32.4.3 Examples of sysconf

We recommend that you first test for a macro definition for the parameter you are interested in, and call sysconf only if the macro is not defined. For example, here is how to test whether job control is supported: 

int
have_job_control (void)
{
#ifdef _POSIX_JOB_CONTROL
  return 1;
#else
  int value = sysconf (_SC_JOB_CONTROL);
  if (value < 0)
    /* If the system is that badly wedged,
       there’s no use trying to go on.  */
    fatal (strerror (errno));
  return value;
#endif
}

Here is how to get the value of a numeric limit: 

int
get_child_max ()
{
#ifdef CHILD_MAX
  return CHILD_MAX;
#else
  int value = sysconf (_SC_CHILD_MAX);
  if (value < 0)
    fatal (strerror (errno));
  return value;
#endif
}