#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.
time_t Used for time in seconds.
timer_t
Used for timer ID returned by timer_create().
trace_attr_t
Used to identify a trace stream attributes object.
trace_event_id_t
Used to identify a trace event type.
trace_event_set_t
Used to identify a trace event type set.
trace_id_t
Used to identify a trace stream.
uid_t Used for user IDs.
useconds_t
Used for time in microseconds.
All of the types shall be defined as arithmetic types of an appropriate length, with the following exceptions:
key_t
Additionally:
* mode_t shall be an integer type.
* nlink_t, uid_t, gid_t, and id_t shall be integer types.
* blkcnt_t and off_t shall be signed integer types.
* fsblkcnt_t, fsfilcnt_t, and ino_t shall be defined as unsigned integer types.
* size_t shall be an unsigned integer type.
* blksize_t, pid_t, and ssize_t shall be signed integer types.
* time_t and clock_t shall be integer or real-floating types.