using files as IPC on linux

Question

I have one writer which creates and sometimes updates a file with some status information. The readers are implemented in lua (so I got only io.open) and possibly bash (cat, grep, whatever). I am worried about what would happen if the status information is updated (which means a complete file rewrite) while a reader has an open handle to the file: what can happen? I have also read that if the write/read operation is below 4KB, it is atomic: that would be perfectly fine for me, as the status info can fit well in such dimension. Can I make this assumption?

Answer 1

A read or write is atomic under 4Kbytes only for pipes, not for disk files (for which the atomic granularity may be the file system block size, usually 512 bytes).

In practice you could avoid bothering about such issues (assuming your status file is e.g. less than 512 bytes), and I believe that if the writer is opening and writing quickly that file (in particular, if you avoid open(2)-ing a file and keeping the opened file handle for a long time -many seconds-, then write(2)-ing later -once, a small string- inside it), you don't need to bother.

If you are paranoid, but do assume that readers are (like grep) opening a file and reading it quickly, you could write to a temporary file and rename(2)-ing it when written (and close(2)-ed) in totality.

As Duck suggested, locking the file in both readers and writers is also a solution.

Answer 2

I may be mistaken, in which case someone will correct me, but I don't think the external readers are going to pay any attention to whether the file is being simultaneously updated. They are are going to print (or possibly eof or error out) whatever is there.

In any case, why not avoid the whole mess and just use file locks. Have the writer flock (or similar) and the readers check the lock. If they get the lock they know they are ok to read.