When POSIX semaphores are disabled (e.g. by unmounting /dev/shm on a Linux system), then _multiprocessing module builds with undefined symbol _PyMp_sem_unlink:$ ./configure ......checking whether POSIX semaphores are enabled... no...$ make...building '_multiprocessing' extensioncreating build/temp.linux-x86_64-3.5/tmp/cpython/Modules/_multiprocessingx86_64-pc-linux-gnu-gcc -pthread -fPIC -Wno-unused-result -Werror=declaration-after-statement -DNDEBUG -march=core2 -O2 -fno-ident -pipe -ggdb3 -Wall -Wpointer-sign -IModules/_multiprocessing -I./Include -I. -IInclude -I/usr/local/include -I/tmp/cpython/Include -I/tmp/cpython -c /tmp/cpython/Modules/_multiprocessing/multiprocessing.c -o build/temp.linux-x86_64-3.5/tmp/cpython/Modules/_multiprocessing/multiprocessing.ox86_64-pc-linux-gnu-gcc -pthread -shared -Wl,-O1 -Wl,--as-needed -Wl,--gc-sections -Wl,--hash-style=gnu -Wl,--sort-common build/temp.linux-x86_64-3.5/tmp/cpython/Modules/_multiprocessing/multiprocessing.o -L. -L/usr/local/lib -lpython3.5m -o build/lib.linux-x86_64-3.5/_multiprocessing.cpython-35m.so*** WARNING: renaming "_multiprocessing" since importing it failed: build/lib.linux-x86_64-3.5/_multiprocessing.cpython-35m.so: undefined symbol: _PyMp_sem_unlink...Following modules built successfully but were removed because they could not be imported:_multiprocessingThis problem was introduced in Python 3.4. This problem is absent in older versions of Python.Potential fix:---Modules/_multiprocessing/multiprocessing.c+++Modules/_multiprocessing/multiprocessing.c@@ -129,5 +129,7 @@     {"send", multiprocessing_send, METH_VARARGS, ""}, #endif+#ifndef POSIX_SEMAPHORES_NOT_ENABLED     {"sem_unlink", _PyMp_sem_unlink, METH_VARARGS, ""},+#endif     {NULL} };

New changesetf618f6739200 by Richard Oudkerk in branch '3.4':Issue#21704: Fix build error for _multiprocessing when semaphoreshttp://hg.python.org/cpython/rev/f618f6739200New changeseta9637b29954d by Richard Oudkerk in branch 'default':Issue#21704: Merge.http://hg.python.org/cpython/rev/a9637b29954d

New changeset1a00be3d79bc by Ezio Melotti in branch '3.4':#21704: remove duplicate name inMisc/ACKS.http://hg.python.org/cpython/rev/1a00be3d79bcNew changeset723e0a7c4914 by Ezio Melotti in branch 'default':#21704: merge with 3.4.http://hg.python.org/cpython/rev/723e0a7c4914

For reference and breadcrumbs, this also occurs when:checking for sem_open... yeschecking for sem_timedwait... yeschecking for sem_getvalue... yeschecking for sem_unlink... yes<snip>checking whether POSIX semaphores are enabled... nochecking for broken sem_getvalue... yes<snip>*** WARNING: renaming "_multiprocessing" since importing it failed: build/lib.freebsd-8.4-RELEASE-i386-3.4/_multiprocessing.so: Undefined symbol "_PyMp_sem_unlink"This can occur on FreeBSD jails, among other things, when host & jail versions differ.Attaching full QA log from RedPorts on FreeBSD 8.4-RELEASE