是Unix/Linux系統的基本系統數據類型的頭文件，含有size_t，time_t，pid_t等類型。
在應用程序源文件中包含 <sys/types.h> 以訪問 _LP64 和 _ILP32 的定義。此頭文件還包含適當時應使用的多個基本派生類型。尤其是以下類型更爲重要：
clock_t 表示系統時間（以時鐘週期爲單位）。
dev_t 用於設備號。
off_t 用於文件大小和偏移量。
ptrdiff_t 是一種帶符號整型，用於對兩個指針執行減法運算後所得的結果。
size_t 反映內存中對象的大小（以字節爲單位）。
ssize_t 供返回字節計數或錯誤提示的函數使用。
time_t 以秒爲單位計時。
所有這些類型在 ILP32 編譯環境中保持爲 32 位值，並會在 LP64 編譯環境中增長爲 64 位值。
blkcnt_t Used for file block counts.
blksize_t Used for block sizes.
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.