它通常被称为 opaque data type，这意味着它通常被声明为一个简单的结构，然后在操作系统库的内部，FILE指针被转换为操作系统将使用的数据结构的实际日期类型，访问来自文件的数据。

• What is an opaque value in C++?
• FILE type in C language (Linux)
• What exactly is the FILE keyword in C?

它是一个包含文件句柄状态数据的结构的typedef，该结构的确切内容是系统特定的，但在我的系统（Mac OS X）上，它的定义如下：

/*
 * stdio state variables.
 *
 * The following always hold:
 *
 *      if (_flags&(__SLBF|__SWR)) == (__SLBF|__SWR),
 *              _lbfsize is -_bf._size, else _lbfsize is 0
 *      if _flags&__SRD, _w is 0
 *      if _flags&__SWR, _r is 0
 *
 * This ensures that the getc and putc macros (or inline functions) never
 * try to write or read from a file that is in `read' or `write' mode.
 * (Moreover, they can, and do, automatically switch from read mode to
 * write mode, and back, on "r+" and "w+" files.)
 *
 * _lbfsize is used only to make the inline line-buffered output stream
 * code as compact as possible.
 *
 * _ub, _up, and _ur are used when ungetc() pushes back more characters
 * than fit in the current _bf, or when ungetc() pushes back a character
 * that does not match the previous one in _bf.  When this happens,
 * _ub._base becomes non-nil (i.e., a stream has ungetc() data iff
 * _ub._base!=NULL) and _up and _ur save the current values of _p and _r.
 *
 * NB: see WARNING above before changing the layout of this structure!
 */
typedef struct __sFILE {
        unsigned char *_p;      /* current position in (some) buffer */
        int     _r;             /* read space left for getc() */
        int     _w;             /* write space left for putc() */
        short   _flags;         /* flags, below; this FILE is free if 0 */
        short   _file;          /* fileno, if Unix descriptor, else -1 */
        struct  __sbuf _bf;     /* the buffer (at least 1 byte, if !NULL) */
        int     _lbfsize;       /* 0 or -_bf._size, for inline putc */

        /* operations */
        void    *_cookie;       /* cookie passed to io functions */
        int     (*_close)(void *);
        int     (*_read) (void *, char *, int);
        fpos_t  (*_seek) (void *, fpos_t, int);
        int     (*_write)(void *, const char *, int);

        /* separate buffer for long sequences of ungetc() */
        struct  __sbuf _ub;     /* ungetc buffer */
        struct __sFILEX *_extra; /* additions to FILE to not break ABI */
        int     _ur;            /* saved _r when _r is counting ungetc data */

        /* tricks to meet minimum requirements even when malloc() fails */
        unsigned char _ubuf[3]; /* guarantee an ungetc() buffer */
        unsigned char _nbuf[1]; /* guarantee a getc() buffer */

        /* separate buffer for fgetln() when line crosses buffer boundary */
        struct  __sbuf _lb;     /* buffer for fgetln() */

        /* Unix stdio files get aligned to block boundaries on fseek() */
        int     _blksize;       /* stat.st_blksize (may be != _bf._size) */
        fpos_t  _offset;        /* current lseek offset (see WARNING) */
} FILE;

struct __sbuf是一种结构，由stdio的该实现在内部使用，以存储数据缓冲区，定义为：

struct __sbuf {
        unsigned char   *_base;
        int             _size;
};