What If Two Processes Write to the Same File Simultaneously

What if your open the same file in several processes and you write to it simultaneously? What is the result? Do these process share the same file offset when they try to write? What about writing to the same file in the same process using different threads?(Take Linux for example.)

To understand this, you have to know how the kernel handles file reading/writing internally.

Three Data Structures inside The Kernel

The per-process file description table

For each process, the kernel maintains a table of open file descriptors. Each entry in this table records information about a single file descriptor(the one returned by the open() system call), including:

  • a set of flags controlling the operation of the file descriptor (actually there is just one such flag, the close-on-exec flag)
  • a reference to the open file description

The system-wide table of open file descriptions

An open file description stores all information relating an open file. It's also called open file table or open file handles. Information includes:

  • the current file offset (as updated by read() and write(), or explicitly modified using lseek())
  • status flags specified when opening the file (i.e, the flags argument to open())
  • the file access mode (read-only, write-only, or read-write, as specified in open())
  • setting relating to signal-driven I/O
  • a reference to the i-node object for this file

The file system i-node table

Each file system has a table of i-nodes for all files residing in the file system. Information includes:

  • file type (e.g, regular file, socket or FIFO) and permission
  • a pointer to a list of blocks held on this file
  • various properties of this file, including its size and time stamps, etc. (However, do distinguish between on-disk i-nodes and in-memory i-nodes)

Here is a picture taken from the book The Linux Programming Interface, which clearly depicts the relationship between file descriptors, open file descriptions and i-nodes. In this situation, two processes have a number of open file descriptors.

linux-file-description

What Is Going On

Let us do a little analysis on this diagram and figure out what would happen when more than one processes write to the same file simultaneously.

In this diagram, descriptors 1 and 20 of process A both refer to the same open file description (labeled 23). This situation may arise as a result of a call the dup() , dup2() or fcntl().

Descriptor 2 of process A and descriptor 2 of process B refer to a single open file description(73). This scenario could occur after a call to fork()(i.e, process A is the parent of process B, or vice versa), or if one process passes an open file descriptor to another process using a UNIX domain socket.

Finally, we see that descriptor 0 of process A and descriptor 3 of process B refer to different open file descriptions, but that these descriptions refer to the same i-node table entry (1976) -- in other words, to the same file. A similar situation could occur if a single process open the same file twice.

So, what is the implication? What if two processes/threads write to the same file simultaneously? Well, we can draw some implications from the above analysis:

  • If two different file descriptors refer to the same open file description, then this two file descriptor share the same file offset. Therefore, if the file offset is changed via one file descriptor (as a consequence of call to read(), write(), or lseek()) then this change is visible through the other file descriptors. This applies both when two file descriptors belong to the same process and when they belong to different processes

    Similar scope rules apply when retrieving and changing the open file status flags (e.g, O_APPEND, O_NONBLOCK and O_ASYNC) using the fcntl() F_GETFL and F_SETFL operations.

  • By contrast, the file descriptor flags (i.e, the close-on-exec) are private to the process and file descriptor. Modifying these flags does not affect other descriptors in the same process or a different process.

References


Yubin Ruan, last modified on 2017-02-17

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