Because you've opened the file for appending, you won't see any data clobbering (i.e. data lost/overwritten) but you will see data overlapping and lines split. i.e. each process will buffer its data into (likely 8k) blocks then append that block to the file. The blocks from different processes may be interleaved. So if a line from one process straddles two blocks, the first half of the line will be written, then possibly 8K of lines and half lines from another process, followed by the remainder of that line.
in reply to Safe to open+close for every write, without locking?