C++学习---cstdio的源码学习分析01-类型定义

标签：__ char 01 FILE int --- 源码 IO struct

引言

cstdio文件是C++对stdio.h头文件的封装，Standard Input and Output Library，定义了一系列标准输入输出函数，包括文件操作(fopen/fclose等)，格式化打印(printf/scanf)等。通过该文件的学习，我们能够洞悉一些底层机制的本质，同时，也能够学习了解这些函数的实现方式，接下来，我们就一一学习这些函数的源码。

代码参考：因为这个头文件是相对比较特殊的，我们没有再采取阅读AOSP源码中bionic代码的方式，取而代之，我们参考glibc中的源码，这也是linux环境下，使用最为广泛的C标准库。

glibc源码下载：www.gnu.org/software/li…

git clone https://sourceware.org/git/glibc.git
cd glibc
git checkout master

后文的源码阅读也都基于glibc代码的实现方式。

头文件位置

在glibc代码中搜索stdio.h,可以看到如下文件：

1      1  /home/sangyu/WorkSpace/Code/C++/glibc/include/bits/stdio.h <<<unknown>>>
   2      1  /home/sangyu/WorkSpace/Code/C++/glibc/include/stdio.h <<<unknown>>>
   3      1  /home/sangyu/WorkSpace/Code/C++/glibc/libio/bits/stdio.h <<<unknown>>>
   4      1  /home/sangyu/WorkSpace/Code/C++/glibc/libio/stdio.h <<<unknown>>>

通过查看头文件的包含关系：

glibc/include/bits/stdio.h include glibc/libio/bits/stdio.h实现

glibc/include/stdio.h include glibc/libio/stdio.h实现

实际上，实现函数定义以及相关接口的是glibc/libio/stdio.h

类型定义与宏定义

stdio.h头文件中定义了后续即将用到的类型与宏定义。

类型定义

FILE类型---文件流对象

/glibc/libio/bits/types/FILE.h，发现了FILE的实际定义_IO_FILE，继续进行追踪

4 struct _IO_FILE;
  5                                                                         
  6 /* The opaque type of streams.  This is the definition used elsewhere.  */   
  7 typedef struct _IO_FILE FILE;

/glibc/libio/bits/types/struct_FILE.h中最终定义了_IO_FILE，其中的数据类型分为以下几类：

int _flags:用于控制文件流的访问权限（如读写执行，注意高位被_IO_MAGIC占用，剩余位用来表示flags）
streambuf控制的：用于标识当前读写位置，buffer大小等。

char *_IO_read_ptr：当前读取的指针
char *_IO_read_end：get区域的末尾
char *_IO_read_base：putback+get区域的开始
char *_IO_write_base：put区域的开始
char *_IO_write_ptr：当前写入的指针
char *_IO_write_end：put区域的末尾
char *_IO_buf_base：剩余数据的开始
char *_IO_buf_end：剩余数据的末尾

备份和回退控制的：用于文件备份和回退控制的指针

char *_IO_save_base：非当前get区域的开始位置
char *_IO_backup_base：备份区域第一个合法字符
char *_IO_save_end：非当前get区域的末尾位置

struct _IO_marker *_markers:记录buffer中的位置(后面查看源码分析)
struct _IO_FILE *_chain:指向链接的下一个_IO_FILE对象
int _fileno：file的number
int _flags2：扩展flags
__off_t _old_offset:旧版的offset，最终定义为(long int,32位)
unsigned short _cur_column：当前列
signed char _vtable_offset：vtable表的偏移
char _shortbuf[1]：只存储一个字符的buffer
_IO_lock_t *_lock:访问IO锁注意：这里针对旧版的IO_FILE做了兼容，下面是新版增加的内容
__off64_t _offset：64位offset(定义为__int64_t,64位)
针对宽字符的处理buff

struct _IO_codecvt *_codecvt:字符转换
struct _IO_wide_data *_wide_data：宽数据

struct _IO_FILE *_freeres_list：剩余的_IO_FILE list
size_t __pad5：对齐信息
int _mode：mode信息
char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)]：预留位置保证追加部分是16个(加上int _mode)sizeof(int)对齐的。

46 /* The tag name of this struct is _IO_FILE to preserve historic
 47    C++ mangled names for functions taking FILE* arguments.
 48    That name should not be used in new code.  */49 struct _IO_FILE50 {
 51   int _flags;       /* High-order word is _IO_MAGIC; rest is flags. */52 
 53   /* The following pointers correspond to the C++ streambuf protocol. */54   char *_IO_read_ptr;   /* Current read pointer */55   char *_IO_read_end;   /* End of get area. */56   char *_IO_read_base;  /* Start of putback+get area. */57   char *_IO_write_base; /* Start of put area. */58   char *_IO_write_ptr;  /* Current put pointer. */59   char *_IO_write_end;  /* End of put area. */60   char *_IO_buf_base;   /* Start of reserve area. */61   char *_IO_buf_end;    /* End of reserve area. */62 
 63   /* The following fields are used to support backing up and undo. */64   char *_IO_save_base; /* Pointer to start of non-current get area. */65   char *_IO_backup_base;  /* Pointer to first valid character of backup area */66   char *_IO_save_end; /* Pointer to end of non-current get area. */67 
 68   struct _IO_marker *_markers;
 69 
 70   struct _IO_FILE *_chain;
 71 
 72   int _fileno;
 73   int _flags2;
 74   __off_t _old_offset; /* This used to be _offset but it's too small.  */75 
 76   /* 1+column number of pbase(); 0 is unknown. */77   unsigned short _cur_column;
 78   signed char _vtable_offset;
 79   char _shortbuf[1];
 80 
 81   _IO_lock_t *_lock;
 82 #ifdef _IO_USE_OLD_IO_FILE83 };
 84 
 85 struct _IO_FILE_complete86 {
 87   struct _IO_FILE _file;
 88 #endif89   __off64_t _offset;
 90   /* Wide character stream stuff.  */91   struct _IO_codecvt *_codecvt;
 92   struct _IO_wide_data *_wide_data;
 93   struct _IO_FILE *_freeres_list;
 94   void *_freeres_buf;
 95   size_t __pad5;
 96   int _mode;
 97   /* Make sure we don't get into trouble again.  */98   char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
 99 };

_IO_marker定义为包含下一个_IO_marker指针，指向当前的FILE* _sbuf，以及标识的位置_pos。

104 /* A streammarker remembers a position in a buffer. */
105 struct _IO_marker {
106   struct _IO_marker *_next;
107   FILE *_sbuf;
108   /* If _pos >= 0
109  it points to _buf->Gbase()+_pos. FIXME comment */
110   /* if _pos < 0, it points to _buf->eBptr()+_pos. FIXME comment */
111   int _pos;
112 };

_IO_lock_t定义包含int lock; int cnt; void *owner;lock用来指定锁，cnt记录访问数量，owner执向对应的IO。

typedef struct { int lock; int cnt; void *owner; } _IO_lock_t;

fpos_t类型---file特定位置的类型

根据具体的offset位数进行定义

82 /* The type of the second argument to `fgetpos' and `fsetpos'.  */83 #ifndef __USE_FILE_OFFSET6484 typedef __fpos_t fpos_t;                                                                                                                             
 85 #else86 typedef __fpos64_t fpos_t;
 87 #endif88 #ifdef __USE_LARGEFILE6489 typedef __fpos64_t fpos64_t;
 90 #endif

对应查看glibc/libio/bits/types/__fpos_t.h，区别__fpos64_t中__pos被定义为__off64_t __mbstate_t是转换状态信息，包括一个__count信息和__value信息(union结构体进行uint32位到4个字符char的转换)

7 /* The tag name of this struct is _G_fpos_t to preserve historic
  8    C++ mangled names for functions taking fpos_t arguments.
  9    That name should not be used in new code.  */10 typedef struct _G_fpos_t11 {
 12   __off_t __pos; //32位偏移，long int13   __mbstate_t __state;
 14 } __fpos_t;
 
 10 typedef struct _G_fpos64_t11 {
 12   __off64_t __pos;//64位偏移，int_64t13   __mbstate_t __state;
 14 } __fpos64_t;
 
 //glibc/wcsmbs/bits/types/__mbstate_t.h8 #ifndef __WINT_TYPE__9 # define __WINT_TYPE__ unsigned int10 #endif11 
 12 /* Conversion state information.  */13 typedef struct14 { 
 15   int __count;
 16   union17   {
 18     __WINT_TYPE__ __wch;
 19     char __wchb[4];
 20   } __value;        /* Value so far.  */21 } __mbstate_t;

size_t类型---unsigned int类型

这个就不需过多解释了，#define size_t unsigned int

标签：__,char,01,FILE,int,---,源码,IO,struct
From： https://blog.51cto.com/u_15830688/5920169