#include<unistd.h>头文件的理解

1.百度百科定义

　　unistd.h 是 C 和 C++ 程序设计语言中提供对 POSIX 操作系统 API 的访问功能的头文件的名称。该头文件由 POSIX.1 标准（单一UNIX规范的基础）提出，故所有遵循该标准的操作系统和编译器均应提供该头文件（如 Unix 的所有官方版本，包括 Mac OS X、Linux 等）。

对于类 Unix 系统，unistd.h 中所定义的接口通常都是大量针对系统调用的封装（英语：wrapper functions），如 fork、pipe 以及各种 I/O 原语（read、write、close 等等）。

 

我的定义：

这是一个头文件，封装了类UNIX系统下的很多固定名称的system_call系统调用。所以，这个函数是依赖于编译器，依赖于操作系统的。

　　

2.源码

　　可以从源码中，查看它封装了哪些固定名称的操作系统api。

#ifndef _UNISTD_H
#define _UNISTD_H
#include <features.h>
#ifdef __cplusplus
extern "C" {
#endif
#include <_ansi.h>
#include <sys/types.h>
#include <sys/_types.h>
#define __need_size_t
#define __need_ptrdiff_t
#include <stddef.h>
extern char **environ;
void _EXFUN(_exit, (int __status ) _ATTRIBUTE ((noreturn)));
int _EXFUN(access,(const char *__path, int __amode ));
unsigned _EXFUN(alarm, (unsigned __secs ));
int _EXFUN(chdir, (const char *__path ));
int _EXFUN(chmod, (const char *__path, mode_t __mode ));
int _EXFUN(chown, (const char *__path, uid_t __owner, gid_t __group ));
int _EXFUN(chroot, (const char *__path ));
int _EXFUN(close, (int __fildes ));
char _EXFUN(*ctermid, (char *__s ));
char _EXFUN(*cuserid, (char *__s ));
int _EXFUN(dup, (int __fildes ));
int _EXFUN(dup2, (int __fildes, int __fildes2 ));
int _EXFUN(execl, (const char *__path, const char *, ... ));
int _EXFUN(execle, (const char *__path, const char *, ... ));
int _EXFUN(execlp, (const char *__file, const char *, ... ));
int _EXFUN(execv, (const char *__path, char * const __argv[] ));
int _EXFUN(execve, (const char *__path, char * const __argv[], char * const __envp[] ));
int _EXFUN(execvp, (const char *__file, char * const __argv[] ));
int _EXFUN(fchdir, (int __fildes));
int _EXFUN(fchmod, (int __fildes, mode_t __mode ));
int _EXFUN(fchown, (int __fildes, uid_t __owner, gid_t __group ));
pid_t _EXFUN(fork, (void ));
long _EXFUN(fpathconf, (int __fd, int __name ));
int _EXFUN(fsync, (int __fd));
int _EXFUN(ftruncate, (int __fd, off_t __length));
char _EXFUN(*getcwd, (char *__buf, size_t __size ));
int _EXFUN(getdomainname ,(char *__name, size_t __len));
gid_t _EXFUN(getegid, (void ));
uid_t _EXFUN(geteuid, (void ));
gid_t _EXFUN(getgid, (void ));
int _EXFUN(getgroups, (int __gidsetsize, gid_t __grouplist[] ));
int _EXFUN(__gethostname, (char *__name, size_t __len));
char _EXFUN(*getlogin, (void ));
#if defined(_POSIX_THREAD_SAFE_FUNCTIONS)
int _EXFUN(getlogin_r, (char *name, size_t namesize) );
#endif
char _EXFUN(*getpass, (__const char *__prompt));
int _EXFUN(getpagesize, (void));
pid_t _EXFUN(getpgid, (pid_t));
pid_t _EXFUN(getpgrp, (void ));
pid_t _EXFUN(getpid, (void ));
pid_t _EXFUN(getppid, (void ));
uid_t _EXFUN(getuid, (void ));
char * _EXFUN(getusershell, (void));
char _EXFUN(*getwd, (char *__buf ));
int _EXFUN(isatty, (int __fildes ));
int _EXFUN(lchown, (const char *__path, uid_t __owner, gid_t __group ));
int _EXFUN(link, (const char *__path1, const char *__path2 ));
int _EXFUN(nice, (int __nice_value ));
off_t _EXFUN(lseek, (int __fildes, off_t __offset, int __whence ));
long _EXFUN(pathconf, (const char *__path, int __name ));
int _EXFUN(pause, (void ));
int _EXFUN(pipe, (int __fildes[2] ));
ssize_t _EXFUN(pread, (int __fd, void *__buf, size_t __nbytes, off_t __offset));
ssize_t _EXFUN(pwrite, (int __fd, const void *__buf, size_t __nbytes, off_t __offset));
_READ_WRITE_RETURN_TYPE _EXFUN(read, (int __fd, void *__buf, size_t __nbyte ));
int _EXFUN(readlink, (const char *path, char *buf, size_t bufsiz));
int _EXFUN(rmdir, (const char *__path ));
void * _EXFUN(sbrk, (ptrdiff_t __incr));
int _EXFUN(setegid, (gid_t __gid ));
int _EXFUN(seteuid, (uid_t __uid ));
int _EXFUN(setgid, (gid_t __gid ));
int _EXFUN(setpgid, (pid_t __pid, pid_t __pgid ));
int _EXFUN(setpgrp, (void ));
pid_t _EXFUN(setsid, (void ));
int _EXFUN(setuid, (uid_t __uid ));
unsigned _EXFUN(sleep, (unsigned int __seconds ));
void _EXFUN(swab, (const void *, void *, ssize_t));
int _EXFUN(symlink, (const char *oldpath, const char *newpath));
long _EXFUN(sysconf, (int __name ));
pid_t _EXFUN(tcgetpgrp, (int __fildes ));
int _EXFUN(tcsetpgrp, (int __fildes, pid_t __pgrp_id ));
int _EXFUN(truncate, (const char *, off_t __length));
char * _EXFUN(ttyname, (int __fildes ));
int _EXFUN(ttyname_r, (int __fildes, char *__buf, size_t __len));
int _EXFUN(unlink, (const char *__path ));
int _EXFUN(usleep, (__useconds_t __useconds));
int _EXFUN(vhangup, (void ));
_READ_WRITE_RETURN_TYPE _EXFUN(write, (int __fd, const void *__buf, size_t __nbyte ));
extern char *optarg; /** getopt(3) external variables */
extern int optind, opterr, optopt;
int getopt(int, char * const [], const char *);
extern int optreset; /** getopt(3) external variable */
#ifndef _POSIX_SOURCE
pid_t _EXFUN(vfork, (void ));
extern char *suboptarg; /** getsubopt(3) external variable */
int getsubopt(char **, char * const *, char **);
#endif /** _POSIX_SOURCE */
/** Provide prototypes for most of the _<systemcall> names that are
provided in newlib for some compilers. */
int _EXFUN(_close, (int __fildes ));
pid_t _EXFUN(_fork, (void ));
pid_t _EXFUN(_getpid, (void ));
int _EXFUN(_link, (const char *__path1, const char *__path2 ));
off_t _EXFUN(_lseek, (int __fildes, off_t __offset, int __whence ));
_READ_WRITE_RETURN_TYPE _EXFUN(_read, (int __fd, void *__buf, size_t __nbyte ));
void * _EXFUN(_sbrk, (size_t __incr));
int _EXFUN(_unlink, (const char *__path ));
_READ_WRITE_RETURN_TYPE _EXFUN(_write, (int __fd, const void *__buf, size_t __nbyte ));
int _EXFUN(_execve, (const char *__path, char * const __argv[], char * const __envp[] ));
#define F_OK 0
#define R_OK 4
#define W_OK 2
#define X_OK 1
# define SEEK_SET 0
# define SEEK_CUR 1
# define SEEK_END 2
#include <sys/features.h>
#define STDIN_FILENO 0 /** standard input file descriptor */
#define STDOUT_FILENO 1 /** standard output file descriptor */
#define STDERR_FILENO 2 /** standard error file descriptor */
#include <bits/environments.h>
#include <bits/confname.h>
# define MAXPATHLEN 1024
#ifdef __cplusplus
}
#endif
#endif /** _SYS_UNISTD_H */