Going forward with NTS

[Eric S. Raymond]

Hal Murray <hmurray at megapathdsl.net>:
> I'm not enough of a Unix file system wizard.  Are there any guarantees about 
> multi-threaded writes to the same file not getting scrambled?  Will it work if 
> we buffer everything into a big buffer and do a single write?

Under Linux, yes, up to 4096 bytes. Look for "I wrote a script to test" on
this page:

https://stackoverflow.com/questions/1154446/is-file-append-atomic-in-unix

There's a great deal of confusion abroad about this. The first answer reflects
a widespread belief that 512-byte writes are guaranteed atomic by POSIX.
That belief is wrong, as is pointed out in the comments. PIPE_BUF is a real
thing in POSIX but has nothing to do with file writes.

I am here to add that I believe the myth about POSIX persists in
practice because 512 byte file write(2) operations *are* in fact atomic on
every Unix. So the mythology about PIPE_BUF is only half wrong; POSIX
does not require it but it's true anyway.

Yes, I have used this assumption in production code under multiple
Unixes and not been bitten.

The way I would recommend handling this is with a PIPE_BUF sized buffer,
write(2), and a comment that explains that PIPE_BUF is accidentally
correct here.  Been there, done that myself.  This is very old lore.
-- 
		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>

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