typedef struct _IO_FILE  FILE;

其中结构体 _IO_FILE的定义在usr/inlucde/libio.h中

struct _IO_FILE {
272   int _flags;           /* High-order word is _IO_MAGIC; rest is flags. */
273 #define _IO_file_flags _flags
274
275   /* The following pointers correspond to the C++ streambuf protocol. */
276   /* Note:  Tk uses the _IO_read_ptr and _IO_read_end fields directly. */
277   char* _IO_read_ptr;   /* Current read pointer */
278   char* _IO_read_end;   /* End of get area. */
279   char* _IO_read_base;  /* Start of putback+get area. */
280   char* _IO_write_base; /* Start of put area. */
281   char* _IO_write_ptr;  /* Current put pointer. */
282   char* _IO_write_end;  /* End of put area. */
283   char* _IO_buf_base;   /* Start of reserve area. */
284   char* _IO_buf_end;    /* End of reserve area. */
285   /* The following fields are used to support backing up and undo. */
286   char *_IO_save_base; /* Pointer to start of non-current get area. */
287   char *_IO_backup_base;  /* Pointer to first valid character of backup area */
288   char *_IO_save_end; /* Pointer to end of non-current get area. */
289
290   struct _IO_marker *_markers;
291
292   struct _IO_FILE *_chain;
293
294   int _fileno;
295 #if 0
296   int _blksize;
297 #else
298   int _flags2;
299 #endif
300   _IO_off_t _old_offset; /* This used to be _offset but it‘s too small.  */
301
302 #define __HAVE_COLUMN /* temporary */
303   /* 1+column number of pbase(); 0 is unknown. */
304   unsigned short _cur_column;
305   signed char _vtable_offset;
306   char _shortbuf[1];
307
308   /*  char* _save_gptr;  char* _save_egptr; */
309
310   _IO_lock_t *_lock;
311 #ifdef _IO_USE_OLD_IO_FILE
312 };