What is data type of FILE in C ?

Prerequisite : Basics of File Handling
In C language, while file handling is done a word FILE is used. What is FILE?
Example

FILE *fp1, *fp2;

While doing file handling we often use FILE for declaring the pointer in order to point to the file we want to read from or to write on. As we are declaring the pointer of type FILE so we can say it is data type, but what kind of data type? A FILE is a type of structure typedef as FILE. It is considered as opaque data type as its implementation is hidden. We don’t know what constitutes the type, we only use pointer to the type and library knows the internal of the type and can use the data.

Definition of FILE is in stdio although it is system specific.



Following is the definition of FILE in ubuntu

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
};

struct _IO_FILE_complete
{
  struct _IO_FILE _file;
#endif
#if defined _G_IO_IO_FILE_VERSION && _G_IO_IO_FILE_VERSION == 0x20001
  _IO_off64_t _offset;
# if defined _LIBC || defined _GLIBCPP_USE_WCHAR_T
  /* Wide character stream stuff.  */
  struct _IO_codecvt *_codecvt;
  struct _IO_wide_data *_wide_data;
  struct _IO_FILE *_freeres_list;
  void *_freeres_buf;
  size_t _freeres_size;
# else
  void *__pad1;
  void *__pad2;
  void *__pad3;
  void *__pad4;
  size_t __pad5;
# endif
  int _mode;
  /* Make sure we don't get into trouble again.  */
  char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
#endif
};

Example of C program to showing the use of FILE

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code

// Program beased on FILE handling.
#include<stdio.h>
int main()
{
      
    // declaring pointer of FILE type
    FILE *fp1, *fp2;
    char c;
      
    // pointing fp1 to a file geeky.txt
    // to read from it.
    fp1 = fopen("geeky.txt", "r");
      
    // pointing fp2 to a file outgeeky.txt
    // to write to it. 
    fp2 = fopen("outgeeky.txt", "w");
      
    // reading a character from file.
    fscanf(fp1, "%c", &c);
      
    // writing a character to file.
    fprintf(fp2, "%c", c);
      
    return 0;
}

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NOTE: The files should be in the same directory where program exist or specify the path of files.



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