Basic file system calls

However, it might be easier when you borrow ideas from existing file system designs. I recommend to model your file system after the FAT file allocation table design, although it is also possible though likely more complex to use a Unix inode -based design.

In general, you will likely need a number of data structures on disk, including a super block, a root directory, information about free and empty blocks on disk, file meta-information such as file size , and a mapping from files to data blocks.

The super block is typically the first block of the disk, and it stores information about the location of the other data structures. For example, you can store in the super block the whereabouts of the file allocation table, the directory, and the start of the data blocks. The directory holds the names of the files. When using a FAT-based design, the directory also stores, for each file, its file size and the head of the list of corresponding data blocks.

When you use inodes, the directory only stores the mapping from file names to inodes. The file allocation table FAT is convenient because it can be used to keep track of empty blocks and the mapping between files and their data blocks. When you use an inode-based design, you will need a bitmap to mark disk blocks as used and an inode array to hold file information including the file size and pointers to data blocks.

In addition to the file-system-related data structures on disk, you also need support for file descriptors. A file descriptor is an integer in the range between 0 and 31 inclusive that is returned when a file is opened, and it is used for subsequent file operations such as reading and writing.

A file descriptor is associated with a file, and it also contains a file offset seek pointer. This offset indicates the point in the file where read and write operations start. Note that file descriptors are not stored on disk. They are only meaningful while an application is running and the file system is mounted.

Research Publications. Projects Project 1. Project 2. Project 3. Project 4. Project 5. Previous Classes. Project Goals. Administrative Information. Implementing a simple file system. Implementing a simple file system The goal of this project is to implement a simple file system on top of a virtual disk. Implementation In principle, you can implement the file system in any way that you want as long as 4, blocks of the disk remain available to store file data. Deliverables Please follow the instructions below exactly!

We use gradescope to manage your project submissions and to communicate the results back to you. This type of situation occurs when a parent process creates a child process, and the execution of the parent process remains suspended until its child process executes. The suspension of the parent process automatically occurs with a wait system call.

When the child process ends execution, the control moves back to the parent process. Processes use this system call to create processes that are a copy of themselves. With the help of this system Call parent process creates a child process, and the execution of the parent process will be suspended till the child process executes. This system call runs when an executable file in the context of an already running process that replaces the older executable file.

However, the original process identifier remains as a new process is not built, but stack, data, head, data, etc. The kill system call is used by OS to send a termination signal to a process that urges the process to exit. However, a kill system call does not necessarily mean killing the process and can have various meanings. Save Article. Like Article. Take a step-up from those "Hello World" programs.

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Article Contributed By :. The function returns the number of bytes read, 0 for end of file EOF and -1 in case an error occurred.

It reads noct bytes from the open file referred by the fd descriptor and it puts it into a buffer buf. The pointer current position is incremented automatically after a reading that certain amount of bytes.

The process that executes a read operation waits until the system puts the data from the disk into the buffer. For writing a certain number of bytes into a file starting from the current position we use the write call. Its syntax is:. The function returns the number of bytes written and the value -1 in case of an error. It writes noct bytes from the buffer buf into the file that has as its descriptor fd.

It is interesting to note that the actual writing onto the disk is delayed. This is done at the initiative of the root, without informing the user when it is done. The delayed writing is faster, but it has three disadvantages:. For closing a file and thus eliminating the assigned descriptor we use the system call close.

The function returns 0 in case of success and -1 in case of an error. At the termination of a process an open file is closed anyway. To position a pointer that points to the current position in an absolute or relative way can be done by calling the lseek function. Read and write operations are done relative to the current position in the file.

The syntax for lseek is:. The function returns the displacement of the new current position from the beginning of the file or -1 in case of an error.

The system calls open , creat , write and read execute an lseek by default. To link an existing file to another directory or to the same directory link can be used. To make such a link in fact means to set a new name or a path to an existing file.

The link system call creates a hard link. Creating symbolic links can be done using symlink system call. The syntax of link is:. The argument oldpath has to be a path to an existing file. Only the root has the right to set a link to a directory.

To delete a link a path in a directory we can use the unlink system call. The function returns 0 in case of success and -1 otherwise.

The function decrements the hard link counter in the i-node and deletes the appropriate directory entry for the file whose link was deleted. If the number of links of a file becomes 0 then the space occupied by the file and its i-node will be freed.

Only the root can delete a directory. In order to obtain more details about a file the following system calls can be used: stat , lstat or fstat. These three functions return 0 in case of success and -1 in case of an error. The first two gets as input parameter a name of a file and completes the structure of the buffer with additional information read from its i-node.

The fstat function is similar, but it works for files that were already opened and for which the file descriptor is known. The difference between stat and lstat is that in case of a symbolic link, function stat returns information about the linked refered file, while lstat returns information about the symbolic link file.

The Linux command that the most frequently uses this function is ls. Special device of type character. Special device of type block. When opening a file with system call open the root verifies the access rights in function of the UID and the effective GID. A situation when this can be useful is when a process is executed with other access right using the suid or sgid bit.