POSIX Thread's locks failed

Hello all.

Sorry for my english.

I use libpcap and libc_r under FreeBSD 5.2.1-p9,
and i has faced a problem, that sometimes pthread_mutex
doesn't work properly. 
I'm getting packets from bpf with pcap_next function in one
thread and processing them in another. Both threads
are using the same pthread_mutex to synchronize.
Everything works perfectly, but sometimes i get errors
could arise only if synchronization failed.

regards,
Nik