The following code shows how to organize debug printing with minimal impact on the loaded system

#include <syslog.h>
#include <stdarg.h>
#include <stdlib.h>

static void print_at(const char *srcfile, int line, const char *format, ...) {
  va_list ap;
  const char *envvar="JAVA_SHOULD_PRINT_AT";
  static int requested_line = -1;

  if (requested_line == -1) {
    const char *should_print = getenv(envvar);
    requested_line = (should_print != NULL) ? atoi(should_print) : 0;
    openlog("javadebug", LOG_PID, LOG_UUCP);
  }

  if (requested_line == line) {
     va_start(ap, format);
     vsyslog(LOG_DEBUG, format, ap);
     va_end(ap);
  }
}