Q:

how standard specify atomic write to regular file(not pipe or fifo)?

The posix standard specified that when write less than PIPE_BUF bytes to pipe or FIFO are granted atomic, that is, our write doesn't mix with other processes'. But I failed to find out how standard specify about regular file. I mean it's true that when we write less than PIPE_BUF, it will also granted be atomic. But I want to know does regular file have such limitation? I mean, the pipe has the capacity, so that when write to the pipe and beyond its capacity, kernel will put the writer to sleep, so other process will get chance to write, but regular file seems that doesn't have to have such limitation, am i right?
What I'm doing is several processes generate log to a file. Of course, with O_APPEND set.

A:

Quote from http://pubs.opengroup.org/onlinepubs/9699919799/toc.htm (Single UNIX Specification, Version 4, 2010 Edition):

This volume of POSIX.1-2008 does not specify behavior of concurrent writes to a file from multiple processes. Applications should use some form of concurrency control.

The specification does address how semantics of writes regarding writes occur in case of multiple readers, but as you can see from above, the behaviour for multiple, concurrent writers is not defined by the specification.
Note above talks about files. For pipes and FIFOs the PIPE_MAX semantics apply, that concurrent writes are guaranteed to be non-divisible up to PIPE_MAX bytes.

Write requests to a pipe or FIFO shall be handled in the same way as a regular file with the following exceptions:
Write requests of {PIPE_BUF} bytes or less shall not be interleaved with data from other processes doing writes on the same pipe. Writes of greater than {PIPE_BUF} bytes may have data interleaved, on arbitrary boundaries, with writes by other processes, whether or not the O_NONBLOCK flag of the file status flags is set.

For real file systems the situation is complex. Some local file systems may enforce atomic writes up to arbitrary sizes (memory limit) by locking a file handle during writing, some might not (I tried to look at ext4 logic, but lost track somewhere around http://lxr.linux.no/linux+v3.5.3/fs/jbd2/transaction.c#L147). 
For non-local file systems the result is more or less for grabs. Just don't try concurrent writing on a networked file system without some form of explicit locking (or you're positively absolutely sure about the semantics of the network file system you're using).
BTW, O_APPEND guarantees that all writes by different processes go to the end of the file. However as SUS above notes, if the writes are really concurrent (occuring at the same time), then the behavior is undefined. On earlier uniprocess and non-pre-emptive UNIXes this didn't really matter, as a call to write(2) completed before someone else got a chance to write...
This question could be answered definitely for specific combination of operating system (Linux?) and file system (ext4?). A general answer? As SUS reads -- "undefined behavior".

