Why does the named pipe keep getting modified

fifotimestamps

I have a named fifo created from a C program like this:

res = mkfifo("/home/myfolder/myfifo", 0666);

after that there are only reads and writes.

Now, from this answer : Does a named pipe change the filesystem I concluded that the system should practically never actually write in the actual file on the filesystem, and that everything is handled in RAM.

Whenever I restart my program, the "mkfifo" line returns a "File already exists" error, which is OK.
What bothers me is that when I "ls" the file, the "last modified date time" sometimes does change. If the system never actually writes into the file, shouldn't it remain constant?

Best Answer

The data passing through the pipe is not written or read to and from the filesystem. When creating the named pipe an inode is allocated so that it can be linked to from the directory you created it in; so that's a form of "modifying the filesystem".

Accessing the named pipe will update the inode's access time, writing to the named pipe will update the inode's modification time, so in that indirect way the filesystem is also being modified. That doesn't mean that the data is written to the filesystem; it's just how Linux handles writes to a named pipe.

Related Solutions

Does named pipe modify the filesystem

The file object itself is created in the filesystem, but no data is stored in a file system. From the mkpipe(3) manpage:

   A  FIFO special file is similar to a pipe, except that it is created in
   a different way.  Instead of being an anonymous communications channel,
   a  FIFO  special  file  is  entered  into  the  file  system by calling
   mkfifo().

About the only time the data might be stored on disk is during hibernation when memory is written to the swap space, including buffers - however this an corner case.

Why does a named pipe not get deleted after system restart

No they're written to disk. The command mkfifo pipe21 creates the corresponding device on your filesystem. Often times these devices are kept under /dev but named pipes (aka. FIFOS) don't necessarily have to be kept in this directory.

excerpt from wikipedia article

The named pipe can be deleted just like any file:
$ rm my_pipe

Example

Make a FIFO:

$ pwd
/home/saml

$ mkfifo pipe21

Check out the FIFO device:

$ ls -l | grep pipe
prw-rw-r--   1 saml saml        0 Jul 24 12:22 pipe21

$ file pipe21 
pipe21: fifo (named pipe)

Delete the device:

$ rm pipe21 

$ ls -l | grep pipe

References

Named Pipes - Wikipedia