On a Unix and equivalent systems if the file you are writing to was opened with the O_APPEND flag, writes are guaranteed to be atomic regardless of size, unless the file is a pipe or FIFO, in which case atomicity is guaranteed only if the write size is PIPE_BUF or fewer bytes in length. So sayeth the Single Unix Specification on the write system call:

If the O_APPEND flag of the file status flags is set, the file offset shall be set to the end of the file prior to each write and no intervening file modification operation shall occur between changing the file offset and the write operation....

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.

On most Linux systems, PIPE_BUF is 4096.

See the write(2) man page on your system and the Single Unix Specification for more.

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