#include 的作用

#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.