linux FILE 类型.

　　stdio.h 头文件中,有以下内容(用内部行号解释):

 /* The opaque type of streams.  This is the definition used elsewhere.  */
   typedef struct _IO_FILE FILE;
 ...
 #include <libio.h>
 ...
 /* Standard streams.  */
  extern struct _IO_FILE *stdin;          /* Standard input stream.  */
  extern struct _IO_FILE *stdout;         /* Standard output stream.  */
  extern struct _IO_FILE *stderr;         /* Standard error output stream.  */ #ifdef __STDC__
  /* C89/C99 say they're macros.  Make them happy.  */
  #define stdin stdin
  #define stdout stdout
  #define stderr stderr
  #endif

　　由上,知道 FILE 是 struct _IO_FILE 类型的别名,或者说 FILE 类型就是 struct _IO_FILE 类型,由 第143行中的 _IO_FILE,往上追溯至 include 预处理指令,我们可以在 libio.h 头文件中看到 _IO_FILE 结构体的定义:

 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[];

    /*  char* _save_gptr;  char* _save_egptr; */

    _IO_lock_t *_lock;
  #ifdef _IO_USE_OLD_IO_FILE
  };

　　稍微注意的是(外侧行号解释), stdio.h 头文件中的第 4行优先于第 2 行执行,虽然第2行在第4行的前面.