If multiple processes simultaneously write to the same pipe, data from one process can be interleaved with data from another process, if modules are pushed on the pipe or the write is greater than PIPE_BUF. The order of data that is written is not necessarily the order of data that is read. To ensure that writes of less than PIPE_BUF bytes are not interleaved with data written by other processes, any modules pushed on the pipe should have a maximum packet size of at least PIPE_BUF.
PIPE_BUF is an implementation-specific constant that specifies the maximum number of bytes that are atomic when writing to a pipe. When writing to a pipe, write requests of PIPE_BUF or fewer bytes are not interleaved with data from other processes doing writes to the same pipe. However, write requests of more than PIPE_BUF bytes may have data interleaved on arbitrary byte boundaries with writes by other processes whether or not the O_NONBLOCK or O_NDELAY flag is set.
If the module packet size is at least the size of PIPE_BUF, the stream head packages the data in such a way that the first message is at least PIPE_BUF bytes. The remaining data may be packaged into smaller or equal-sized blocks depending on buffer availability. If the first module on the stream cannot support a packet of PIPE_BUF, atomic writes on the pipe cannot be guaranteed.