IO_FILE浅析

前言

由于glibc的不断更新,hook在glibc2.34最终被舍去了
IO_FILE就越发受关注了

_IO_FILE

struct _IO_FILE
{
  int _flags;		/* 高位字为_IO_MAGIC，剩余的部分是flag */
  char *_IO_read_ptr;	/* 正在使用的input缓冲区的input地址 */
  char *_IO_read_end;	/* input缓冲区的结束地址 */
  char *_IO_read_base;	/* input缓冲区的基址 */
  char *_IO_write_base;	/* output缓冲区的基址 */
  char *_IO_write_ptr;	/*  指向还没输出的那个字节 */
  char *_IO_write_end;	/* output缓冲区的结束地址 */
  char *_IO_buf_base;	/* input和output缓冲区的基址 */
  char *_IO_buf_end;	/* input和output缓冲区的结束地址 */

  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; /* 存放着一个单链表，用于串联所有的file stream */

  int _fileno; /* 与文件相关的文件描述符 */
  int _flags2;
  __off_t _old_offset; /* This used to be _offset but it's too small.  */

  /* 1+column number of pbase(); 0 is unknown. */
  unsigned short _cur_column;
  signed char _vtable_offset; /* 存放虚表(virtual table)的偏移 */
  char _shortbuf[1];

  _IO_lock_t *_lock;
#ifdef _IO_USE_OLD_IO_FILE
};

struct _IO_FILE_complete
{
  struct _IO_FILE _file;
#endif
  __off64_t _offset; /* 存放当前文件的偏移 */
  /* Wide character stream stuff.  */
  struct _IO_codecvt *_codecvt;
  struct _IO_wide_data *_wide_data;
  struct _IO_FILE *_freeres_list;
  void *_freeres_buf;
  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)];
};

_IO_FILE_plus

struct _IO_FILE_plus
{
  FILE file;
  const struct _IO_jump_t *vtable;
};

这里的FILE就是_IO_FILE.
是利用宏定义换名了

#ifndef __FILE_defined
#define __FILE_defined 1

struct _IO_FILE;

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

#endif

_IO_jump_t

_IO_FILE_plus的后半部分就是应该_IO_jump_t的指针.
所谓 vtable 就是 visual table(虚表)

struct _IO_jump_t
{
    JUMP_FIELD(size_t, __dummy);
    JUMP_FIELD(size_t, __dummy2);
    JUMP_FIELD(_IO_finish_t, __finish);
    JUMP_FIELD(_IO_overflow_t, __overflow);
    JUMP_FIELD(_IO_underflow_t, __underflow);
    JUMP_FIELD(_IO_underflow_t, __uflow);
    JUMP_FIELD(_IO_pbackfail_t, __pbackfail);
    /* showmany */
    JUMP_FIELD(_IO_xsputn_t, __xsputn);
    JUMP_FIELD(_IO_xsgetn_t, __xsgetn);
    JUMP_FIELD(_IO_seekoff_t, __seekoff);
    JUMP_FIELD(_IO_seekpos_t, __seekpos);
    JUMP_FIELD(_IO_setbuf_t, __setbuf);
    JUMP_FIELD(_IO_sync_t, __sync);
    JUMP_FIELD(_IO_doallocate_t, __doallocate);
    JUMP_FIELD(_IO_read_t, __read);
    JUMP_FIELD(_IO_write_t, __write);
    JUMP_FIELD(_IO_seek_t, __seek);
    JUMP_FIELD(_IO_close_t, __close);
    JUMP_FIELD(_IO_stat_t, __stat);
    JUMP_FIELD(_IO_showmanyc_t, __showmanyc);
    JUMP_FIELD(_IO_imbue_t, __imbue);
};