#include <unistd.h>

是POSIX标准定义的unix类系统定义符号常量的头文件，包含了许多UNIX系统服务的函数原型，例如read函数、write函数和getpid函数

#ifndef _UNISTD_H

#define _UNISTD_H

#include <features.h>

unistd.h含有的常量与函数：

ssize_t read(int, void *, size_t);

int unlink(const char *);

ssize_t write(int, const void *, size_t);

int usleep(useconds_t);

unsigned sleep(unsigned);

int access(const char *, int);

unsigned alarm(unsigned);

int chdir(const char *);

int chown(const char *, uid_t, gid_t);

int close(int);

size_t confstr(int, char *, size_t);

void _exit(int);

pid_t fork(void);

NULL // Null pointer

SEEK_CUR // Set file offset to current plus offset.

SEEK_END // Set file offset to EOF plus offset.

SEEK_SET // Set file offset to offset.