用户态与内核态 & 文件流与文件描述符 简介

用户态和内核态

程序代码的依赖和调用关系如下图所示:

  • Lib:标准ASCI C函数,几乎所有的平台都支持该库函数,因此依赖该库的程序可移植性好;
  • System Function:系统调用函数,与系统内核进行交互,不同平台具备不同的函数接口,因此可移植性较差

区分用户态和内核态主要是由于系统资源的有限性,不能无限制的随意分配给用户使用,必须由系统进行统一管理

  • User mode:不能直接对系统资源进行访问,如果要操作系统资源,必须转化为内核态
  • Kernel mode:管理系统资源,可直接对系统资源进行控制和访问

内核为用户提供了统一的API供其使用,不同的系统的API接口不同,为了便于代码的移植,出台了POSIX标准,类Unix系统(Unix、Linux、BSD、SunOS等)均支持该标准。

文件流与文件描述符

  • 问题: 由上图我们可看到,每执行一次系统调用,都要涉及到CPU状态的切换,即从用户态切换到内核态,即从用户空间切换到内核空间,实现上下文切换的过程,会消耗相当一部分的CPU资源,因此频繁的磁盘访问对程序的执行效率将造成很大影响。
  • 解决方案: 为了解决以上的难题,采用了缓冲区的概念,当对磁盘文件进行操作时,可一次性从磁盘文件中读出大量的数据暂放到缓冲区中,以后对这部分数据的访问就不需要再进行系统调用了;当对文件行操作后,可将处理后的数据暂存到输出缓冲区,待文件缓冲区满后,一次性写入到磁盘。

以上,数据的输入输出就像是水在流动一样,因此我们采用了的概念。

  • 文件流 : 简单来说就是建立在面向对象基础上的一种抽象的处理数据的工具。在流中,定义了一些处理数据的基本操作,如读取数据,写入数据等,程序员是对流进行所有操作的,而不用关心流的另一头数据的真正流向; 文件流用结构体表示:struct FILE. FILE的结构体又是怎么样的呢?我们可以进行查找一下: [niesh@niesh ~]$ vim /usr/include/stdio.h

我们看到了 stdio.h的文件中有一行:

__BEGIN_NAMESPACE_STD
 /* The opaque type of streams.  This is the definition used elsewhere.  */
 typedef struct _IO_FILE FILE;
 __END_NAMESPACE_STD

显然,FILE_IO_FILE的类型替换,那么我们找一下 _IO_FILE在哪里呢?

[niesh@niesh ~]$ grep -rn "\<_IO_FILE\>" /usr/include/
/usr/include/c++/4.8.2/streambuf:178:       *  This is based on _IO_FILE, just reordered to be more consistent,
/usr/include/libio.h:145:struct _IO_jump_t;  struct _IO_FILE;
/usr/include/libio.h:163:  struct _IO_FILE *_sbuf;
/usr/include/libio.h:246:struct _IO_FILE {              //此处正解
/usr/include/libio.h:267:  struct _IO_FILE *_chain;
/usr/include/libio.h:291:  struct _IO_FILE _file;
/usr/include/libio.h:299:  struct _IO_FILE *_freeres_list;
/usr/include/libio.h:316:typedef struct _IO_FILE _IO_FILE;
/usr/include/libio.h:325:#define _IO_stdin ((_IO_FILE*)(&_IO_2_1_stdin_))
/usr/include/libio.h:326:#define _IO_stdout ((_IO_FILE*)(&_IO_2_1_stdout_))
/usr/include/libio.h:327:#define _IO_stderr ((_IO_FILE*)(&_IO_2_1_stderr_))
/usr/include/libio.h:329:extern _IO_FILE *_IO_stdin attribute_hidden;
/usr/include/libio.h:330:extern _IO_FILE *_IO_stdout attribute_hidden;
/usr/include/libio.h:331:extern _IO_FILE *_IO_stderr attribute_hidden;
/usr/include/libio.h:391:extern int __underflow (_IO_FILE *);
/usr/include/libio.h:392:extern int __uflow (_IO_FILE *);
/usr/include/libio.h:393:extern int __overflow (_IO_FILE *, int);
/usr/include/libio.h:395:extern _IO_wint_t __wunderflow (_IO_FILE *);
/usr/include/libio.h:396:extern _IO_wint_t __wuflow (_IO_FILE *);
/usr/include/libio.h:397:extern _IO_wint_t __woverflow (_IO_FILE *, _IO_wint_t);
/usr/include/libio.h:435:extern int _IO_getc (_IO_FILE *__fp);
/usr/include/libio.h:436:extern int _IO_putc (int __c, _IO_FILE *__fp);
/usr/include/libio.h:437:extern int _IO_feof (_IO_FILE *__fp) __THROW;
/usr/include/libio.h:438:extern int _IO_ferror (_IO_FILE *__fp) __THROW;
/usr/include/libio.h:440:extern int _IO_peekc_locked (_IO_FILE *__fp);
/usr/include/libio.h:446:extern void _IO_flockfile (_IO_FILE *) __THROW;
/usr/include/libio.h:447:extern void _IO_funlockfile (_IO_FILE *) __THROW;
/usr/include/libio.h:448:extern int _IO_ftrylockfile (_IO_FILE *) __THROW;
/usr/include/libio.h:465:extern int _IO_vfscanf (_IO_FILE * __restrict, const char * __restrict,
/usr/include/libio.h:467:extern int _IO_vfprintf (_IO_FILE *__restrict, const char *__restrict,
/usr/include/libio.h:469:extern _IO_ssize_t _IO_padn (_IO_FILE *, int, _IO_ssize_t);
/usr/include/libio.h:470:extern _IO_size_t _IO_sgetn (_IO_FILE *, void *, _IO_size_t);
/usr/include/libio.h:472:extern _IO_off64_t _IO_seekoff (_IO_FILE *, _IO_off64_t, int, int);
/usr/include/libio.h:473:extern _IO_off64_t _IO_seekpos (_IO_FILE *, _IO_off64_t, int);
/usr/include/libio.h:475:extern void _IO_free_backup_area (_IO_FILE *) __THROW;
/usr/include/libio.h:478:extern _IO_wint_t _IO_getwc (_IO_FILE *__fp);
/usr/include/libio.h:479:extern _IO_wint_t _IO_putwc (wchar_t __wc, _IO_FILE *__fp);
/usr/include/libio.h:480:extern int _IO_fwide (_IO_FILE *__fp, int __mode) __THROW;
/usr/include/libio.h:514:extern int _IO_vfwscanf (_IO_FILE * __restrict, const wchar_t * __restrict,
/usr/include/libio.h:516:extern int _IO_vfwprintf (_IO_FILE *__restrict, const wchar_t *__restrict,
/usr/include/libio.h:518:extern _IO_ssize_t _IO_wpadn (_IO_FILE *, wint_t, _IO_ssize_t);
/usr/include/libio.h:519:extern void _IO_free_wbackup_area (_IO_FILE *) __THROW;
/usr/include/stdio.h:44:struct _IO_FILE;
/usr/include/stdio.h:48:typedef struct _IO_FILE FILE;
/usr/include/stdio.h:64:typedef struct _IO_FILE __FILE;
/usr/include/stdio.h:168:extern struct _IO_FILE *stdin;         /* Standard input stream.  */
/usr/include/stdio.h:169:extern struct _IO_FILE *stdout;                /* Standard output stream.  */
/usr/include/stdio.h:170:extern struct _IO_FILE *stderr;                /* Standard error output stream.  */

(⊙o⊙)…,还挺多啊,不过仔细观察,发现只有 libio.h的246行是对该结构体的定义,我们打开瞅瞅!

[niesh@niesh ~]$ vim /usr/include/libio.h

struct FILE 的结构体成员如下代码所示:

    struct _IO_FILE {
      int _flags;       /* High-order word is _IO_MAGIC; rest is flags. */
    #define _IO_file_flags _flags
    
      /* The following pointers correspond to the C++ streambuf protocol. */
      /* Note:  Tk uses the _IO_read_ptr and _IO_read_end fields directly. */
      char* _IO_read_ptr;   /* Current read pointer */
      char* _IO_read_end;   /* End of get area. */
      char* _IO_read_base;  /* Start of putback+get area. */
      char* _IO_write_base; /* Start of put area. */
      char* _IO_write_ptr;  /* Current put pointer. */
      char* _IO_write_end;  /* End of put area. */
      char* _IO_buf_base;   /* Start of reserve area. */
      char* _IO_buf_end;    /* End of reserve area. */
      /* The following fields are used to support backing up and undo. */
      char *_IO_save_base; /* Pointer to start of non-current get area. */
      char *_IO_backup_base;  /* Pointer to first valid character of backup area */
      char *_IO_save_end; /* Pointer to end of non-current get area. */
    
      struct _IO_marker *_markers;
    
      struct _IO_FILE *_chain;
    
      int _fileno;
    #if 0
      int _blksize;
    #else
      int _flags2;
    #endif
      _IO_off_t _old_offset; /* This used to be _offset but it's too small.  */
    
    #define __HAVE_COLUMN /* temporary */
      /* 1+column number of pbase(); 0 is unknown. */
      unsigned short _cur_column;
      signed char _vtable_offset;
      char _shortbuf[1];
    
      /*  char* _save_gptr;  char* _save_egptr; */
    
      _IO_lock_t *_lock;
    #ifdef _IO_USE_OLD_IO_FILE
    };
  • 文件描述符: 每个进程,当打开一个文件后,内核会为其建立一个打开文件的数组 (数组的前三个为stdin,stdout,stderr),然后返回打开文件位于数组的索引值(下标),该所以只即为文件描述符,只要文件不关闭,用户便可以根据该描述符对文件进行访问和操作。
  • 不同点文件描述符:表示为int类型的对象:如stdin对应文件描述符0,stdout对应文件描述符1; :表示为指向结构FILE的指针FILE* ,因此流也称为文件指针 若需要对特定设备进行控制操作,必须使用文件描述符方式,没有函数能对流进行这类操作 如果需要按照特殊的方式进行I/O操作(例如非阻塞的方式),必须使用文件描述符方式,也没有函数能对流进行这类操作。
  • 联系: 流给用户程序提供了更高一级的(功能更强大,使用更简化)的I/O接口,它处在文件描述符方式的上层,也就是说,流函数是通过文件描述符函数来实现的。

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