Project 6: A Simple File Server

Important Info and Dates

You can have a partner if you'd like.

Questions about the project? Send them to .

Due: Friday 5/7 by 9pm.


  • 04/27: For directory entries that are not yet in use, the inode number should be set to -1. This way, utilities can scan through the entries to check if they are valid.


In this assignment, you will be developing a working file server. We provide you with only the bare minimal UDP communication code - you have to build the rest. Thus, this project has a bigger design component than the previous projects - so think about your design! And feel free to consult with me or the TA if you have questions.

A Basic File Server

Your file server is built as a stand-alone UDP-based server. It should wait for a message and then process the message as need be, replying to the given client.

Your file server will store all of its data in an on-disk file which will be referred to as the file system image . This image contains the on-disk representation of your data structures; you should use these system calls to access it: open(), read(), write(), lseek(), close(), fsync() .

To access the file server, you will be building a client library. The interface that the library supports is defined in mfs.h . The library should be called, and any programs that wish to access your file server will link with it and call its various routines.

On-Disk File System

Your on-disk file system structures should be very simple. You should have a simple inode table which consists of 4096 inodes. You should have a data region which consists of 4096 data blocks. Each inode should contain a type field (directory or regular file), a size field (number of bytes in the file), a blocks field (number of blocks allocated to the file), and 10 (direct) pointers to data blocks. The data block size is fixed at 4096 bytes, and hence the max file size is 10 * 4096 or 40KB. To track what is allocated and what isn't, you should also have a couple of bit maps. One bitmap for the inodes, and one for the data blocks.

When your server is started, it is passed the name of the file system image file. If this file does not exist, the file server should create it, and initialize it properly, and force it to disk. Such initialization includes creating a space big enough for the inodes and the data blocks, and initializing a root directory with proper . and .. entries. The root inode number should be 0.

Client library

The client library should export the following interfaces:

  • int MFS_Init(char *hostname, int port): MFS_Init() takes a host name and port number and uses those to find the server exporting the file system.
  • int MFS_Lookup(int pinum, char *name): MFS_Lookup() takes the parent inode number (which should be the inode number of a directory) and looks up the entry name in it. The inode number of name is returned. Success: return inode number of name; failure: return -1. Failure modes: invalid pinum, name does not exist in pinum.
  • int MFS_Stat(int inum, MFS_Stat_t *m): MFS_Stat() returns some information about the file specified by inum. Upon success, return 0, otherwise -1. The exact info returned is defined by MFS_Stat_t. Failure modes: inum does not exist.
  • int MFS_Write(int inum, char *buffer, int block): MFS_Write() writes a block of size 4096 bytes at the block offset specified by block . Returns 0 on success, -1 on failure. Failure modes: invalid inum, invalid block, not a regular file (you can't write to directories).
  • int MFS_Read(int inum, char *buffer, int block): MFS_Read() reads a block specified by block into the buffer from file specified by inum . The routine should work for either a file or directory; directories should return data in the format specified by MFS_DirEnt_t. Success: 0, failure: -1. Failure modes: invalid inum, invalid block.
  • int MFS_Creat(int pinum, int type, char *name): MFS_Creat() makes a file ( type == MFS_REGULAR_FILE) or directory ( type == MFS_DIRECTORY) in the parent directory specified by pinum of name name . Returns 0 on success, -1 on failure. Failure modes: pinum does not exist. If name already exists, return success (think about why).
  • int MFS_Unlink(int pinum, char *name): MFS_Unlink() removes the file or directory name from the directory specified by pinum . 0 on success, -1 on failure. Failure modes: pinum does not exist, directory is NOT empty. Note that the name not existing is NOT a failure by our definition (think about why this might be).

Server Idempotency

The key behavior implemented by the server is idempotency. Specifically, on any change to the file system state (such as a Write, Creat, or Unlink), all the dirtied buffers in the server are committed to the disk. The server can achieved this end by calling fsync() on the file system image. Thus, before returning a success code, the file system should always fsync() the image.

Now you might be wondering: why do this? Simple: if the server crashes, the client can simply timeout and retry the operation and know that it is OK to do so. We'll be talking about this more when we talk about NFS, the Network File System from the company formerly known as Sun.

Now you might be wondering: how do I implement a timeout? Simple, with the select() interface. The select() calls allows you to wait for a reply on a certain socket descriptor (or more than one, though that is not needed here). You can even specify a timeout so that the client does not block forever waiting for data to be returned from the server. By doing so, you can wait for a reply for a certain amount of time, and if nothing is returned, try the operation again until it is successful.

Program Specifications

Your server program must be invoked exactly as follows:

prompt> server [portnum] [file-system-image]

The command line arguments to your file server are to be interpreted as follows.

  • portnum: the port number that the file server should listen on.
  • file-system-image: a file that contains the file system image.

If the file system image does not exist, you should create it and properly initialize it to include an empty root directory.

Your client library should be called and be built as usual. It should implement the interface as specified by mfs.h , and in particular deal with the case where the server does not reply in a timely fashion; the way it deals with that is simply by retrying the operation, after a timeout of some kind (default: five second timeout).

Some Helper Code

To get you going, we have written some simple UDP code that can send a message and then receive a reply from a client to a server. It can be found in the ~cs537-1/public/p6/ directory.


Hand in your source code in the p6 directory, as usual. Please put a copy of everything (described below) in BOTH partners directories (if you have a partner). Also, in a README, specify WHO your partner is.

You should copy all of your server source files (*.c and *.h) and a Makefile to your p6 handin directory. Do not submit any .o files.