UNIX Filesystems: Evolution, Design, and Implementation

The FILE Structure

Where system calls such as open() and dup() return a file descriptor through which the file can be accessed, the stdio library operates on a FILE structure, or file stream as it is often called. This is basically a character buffer that holds enough information to record the current read and write file pointers and some other ancillary information. On Linux, the IO_FILE structure from which the FILE structure is defined is shown below. Note that not all of the structure is shown here.

struct _IO_FILE {
   char  *_IO_read_ptr;   /* Current read pointer */
   char  *_IO_read_end;   /* End of get area. */
   char  *_IO_read_base;  /* Start of putback and 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. */
   int   _fileno;
   int   _blksize;
};

typedef struct _IO_FILE FILE;

Each of the structure fields will be analyzed in more detail throughout the chapter. However, first consider a call to the open() and read() system calls:

fd = open(“/etc/passwd”, O_RDONLY);
read(fd, buf, 1024);

When accessing a file through the stdio library routines, a FILE structure will be allocated and associated with the file descriptor fd, and all I/O will operate through a single buffer. For the _IO_FILE structure shown above, _fileno is used to store the file descriptor that is used on subsequent calls to read() or ...