当我们取消引用 FILE 指针时会发生什么？

Question

假设我有一个文件指针

FILE* infile = fopen("<somefilepath", "r");

现在当我取消引用 gdb 中的文件指针时，我得到了

打印 *infile

│$2 = {_flags = -72539000, _IO_read_ptr = 0x0, _IO_read_end = 0x0,
│ _IO_read_base = 0x0, _IO_write_base = 0x0, _IO_write_ptr = 0x0,
│ _IO_write_end = 0x0, _IO_buf_base = 0x0, _IO_buf_end = 0x0,
│ _IO_save_base = 0x0, _IO_backup_base = 0x0, _IO_save_end = 0x0, _markers = 0x0,
│ _chain = 0x7ffff7dd41c0 <_io_2_1_stderr_>, _fileno = 3, _flags2 = 0,
│ _old_offset = 0, _cur_column = 0, _vtable_offset = 0 '\000', _shortbuf = "",
│ _lock = 0x6020f0, _offset = -1, __pad1 = 0x0, __pad2 = 0x602100, __pad3 = 0x0,
│ __pad4 = 0x0, __pad5 = 0, _mode = 0, _unused2 = '\000' }

谁能帮我理解这意味着什么？

Answer 1

FILE * 应该被视为一个不透明的值...取消引用它将取决于平台/库（因此不可移植）。

Answer 2

C11 7.21.1/2 将FILE 描述为

…一种能够记录所有需要的信息的对象类型 控制一个流，包括它的文件位置指示器，一个指向 其关联的缓冲区（如果有），记录的错误指示器 是否发生读/写错误，以及文件结束指示符 记录是否已到达文件末尾；

但没有提到具体的成员。这是您观察到的 glibc 的实现（为了清楚起见，我删除了未使用的预处理器分支）：

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;
  int _flags2;
  _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;

  _IO_off64_t _offset;
  void *__pad1;
  void *__pad2;
  void *__pad3;
  void *__pad4;
  size_t __pad5;
  int _mode;
  /* Make sure we don't get into trouble again.  */
  char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
};

Answer 3

FILE 在 C 标准第 7.21.1 节中有所描述：

2 声明的类型是 size_t（在 7.19 中描述）；

    文件

这是一种能够记录所有信息的对象类型 需要控制一个流，包括它的文件位置指示器，一个 指向其关联缓冲区（如果有）的指针，错误指示器 记录是否发生读/写错误，以及文件结束 记录是否已到达文件末尾的指示器； ...

在您的特定情况下，您似乎正在使用 glibc。 FILE 的实现可以在 libio.h 中找到：

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
};

请记住，完整的实现跨越许多文件、函数、数据结构等。