unix: can i write to the same file in parallel without missing entries?

Yes, if different processes independently open and write to the same file, it may result in overlapping writes and missing data. This happens because each process will get its own file pointer, that advances only by local writes.

Instead of locking, a better option might be to open the log file once in an ancestor of all worker processes, have it inherited across fork(), and used by them for logging. This means that there will be a single shared file pointer, that advances when any of the processes writes a new entry.

In a script you should use ">> file" (double greater than) to append output to that file. The interpreter will open the destination in "append" mode. If your program also wants to append, follow the directives below:

Open a text file in "append" mode ("a+") and give preference to printing only full lines (don't do multiple 'print' followed by a final 'println', but print the entire line with a single 'println').

The fopen documentation states this:

DESCRIPTION     The fopen() function opens the file whose  pathname  is  the     string  pointed to by filename, and associates a stream with     it.     The argument mode points to a string beginning with  one  of     the following sequences:     r or rb             Open file for reading.     w or wb             Truncate to  zero  length or create file                         for writing.     a or ab             Append; open or create file for  writing                         at end-of-file.     r+ or rb+ or r+b    Open file for update (reading and  writ-                         ing).     w+ or wb+ or w+b    Truncate to zero length or  create  file                         for update.     a+ or ab+ or a+b    Append; open or create file for  update,                         writing at end-of-file.

The character b has no effect, but is allowed for ISO Cstandard conformance (see standards(5)). Opening a file withread mode (r as the first character in the mode argument)fails if the file does not exist or cannot be read.

Opening a file with append mode (a as the first character inthe mode argument) causes all subsequent writes to the fileto be forced to the then current end-of-file, regardless ofintervening calls to fseek(3C). If two separate processesopen the same file for append, each process may write freelyto the file without fear of destroying output being writtenby the other. The output from the two processes will beintermixed in the file in the order in which it is written.

It is because of this intermixing that you want to give preference tousing only 'println' (or its equivalent).