在utmp.h文件中似乎也没找到定义的地方，UTMP_FILE到底是什么？
grep -n -R 'UTMP_FILE' /usr/include
grep -n -R '#define.*UTMP_FILE' /usr/include
#include <bits/utmp.h>
/* Compatibility names for the strings of the canonical file names. */
#define UTMP_FILE _PATH_UTMP
#define UTMP_FILENAME _PATH_UTMP
#define WTMP_FILE _PATH_WTMP
#define WTMP_FILENAME _PATH_WTMP
/usr/include/paths.h:#define _PATH_UTMP "/var/run/utmp"