What is data type of FILE in C?

In C we have used Files. To handle files, we use the pointer of type FILE. So the FILE is a datatype. This is called the Opaque datatype. So its implementation is hidden. The definition of the FILE is system specific. This is the definition of FILE in Ubuntu System −

FILE Definition

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
};

Now let us see one example using FILE.

Text file (/tmp/test.txt):
This
is testing for fprintf...
This is testing for fputs...

Example

#include <stdio.h>
main() {
   FILE *fp;
   char buff[255];
   fp = fopen("/tmp/test.txt", "r");
   fscanf(fp, "%s", buff);
   printf("1 : %s
", buff );
   fgets(buff, 255, (FILE*)fp);
   printf("2: %s
", buff );
   fgets(buff, 255, (FILE*)fp);
   printf("3: %s
", buff );
   fclose(fp);
}

Output

1 : This
2: is testing for fprintf...

3: This is testing for fputs...