int getgrnam_r(const char *name, struct group *grp,
char *buf, size_t buflen, struct group **result);
的bufsize 应该多大？
struct group {
char *gr_name; /* group name */
char *gr_passwd; /* group password */
gid_t gr_gid; /* group ID */
char **gr_mem; /* group members */
};
关于getgrnam_r 的说明：
The string fields pointed to by the members of the group structure are stored in the buffer buf of size buflen. A pointer to the result (in case of success) or NULL (in case no entry was found or an error occurred) is stored in *result.
returns either -1, without changing errno, or an initial suggested size for buf. (If this size is too small, the call fails with ERANGE, in which case the caller can retry with a larger buffer.)
buf 的大小需要可以容纳 gr_name, gr_passwd, gr_mem 三部分内容；
gr_mem 是需要存储组成员的，
