11.3 Dangers When Using Archives

It is important to be careful when using parallel execution (the -j switch; see section Parallel Execution) and archives. If multiple ar commands run at the same time on the same archive file, they will not know about each other and can corrupt the file.

Possibly a future version of make will provide a mechanism to circumvent this problem by serializing all commands that operate on the same archive file. But for the time being, you must either write your makefiles to avoid this problem in some other way, or not use -j.