In C language, what is the data type of FILE?

In C language, we use files (Files) to handle file operations, and we use pointers of type FILE to operate files. Therefore, FILE is a data type. This is called an opaque data type because its implementation is hidden. The definition of FILE is system specific. The following is the definition of FILE in the 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 look at an example of using FILE. The Chinese translation of

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

Example

is:

Example

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

Output

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

3: This is testing for fputs...

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