#include <sys/types.h>
基本系统数据类型
是Unix/Linux系统的基本系统数据类型的头文件，含有size_t，time_t，pid_t等类型。
NAME
sys/types.h - data types
SYNOPSIS
#include <sys/types.h>
DESCRIPTION
The <sys/types.h> header shall include definitions for at least the following types:
blkcnt_t
Used for file block counts.
blksize_t
Used for block sizes.
clock_t
Used for system times in clock ticks or CLOCKS_PER_SEC; see <time.h> .
clockid_t
Used for clock ID type in the clock and timer functions.
dev_t Used for device IDs.
fsblkcnt_t
Used for file system block counts.
fsfilcnt_t
Used for file system file counts.
gid_t Used for group IDs.
id_t Used as a general identifier; can be used to contain at least a pid_t, uid_t, or gid_t.
ino_t Used for file serial numbers.
key_t Used for XSI interprocess communication.
mode_t Used for some file attributes.
nlink_t
Used for link counts.
off_t Used for file sizes.
pid_t Used for process IDs and process group IDs.
size_t Used for sizes of objects.
ssize_t
Used for a count of bytes or an error indication.
suseconds_t
Used for time in microseconds.