c - I Wanna know the Internal Members of struct FILE, the latest ones

Question

When I read pg.176 of The C programming Language by K&R, I was very excited. I found all the members of struct FILE ( which I was searching for ) and its just awesome to know how things work. But guess what, gcc complains, error: ‘FILE’ has no member named ‘fd’. It means things have changed now, I googled but could not find. Please Help, Thank you in advance.

I can use fileno() to get the file descriptor, but I hate working on abstraction level.

int
main ( int argc, char **argv ){

    FILE *fp = fopen ("ct.c", "r");
    printf ("%i", fp->fd);

    return 0;
}

Answer 1

You need to look in your C library's source code.

Since you mention gcc and Linux, you're probably using the GNU libc, which is of course free software.

This file says:

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

And this file declares the _IO_FILE structure:

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
};

Chances are that the above, coming from a "real" production-quality library, is slightly more complicated than the example used in K&R. And, of course, you can't use this since it's library-internal and FILE is an opaque type, just as it says.