How to share pthreads primitives across processes.
The POSIX threads library has some useful primitives for locking between multiple threads, primarily mutexes and condition variables.
Typically these are only effective to lock between threads within the same
process. However, pthreads defines a
for both of these primitives which can be used to specify that they should
also be used between processes.
This attribute simply indicates that the primitives be used in a way which
is compatible with shared access, however — application code must still arrange
to store them in shared memory. This is fairly easily arranged with an
shmat() if your processes don’t have a parent/child relationship.
The use of shared memory makes things a little more complicated, which is disappointing, but it still seems to me that using these primitives to synchronise processes is probably still a little more elegant than using pipes or something similar (even if that’s probably a little more portable).
I’ve added a code example to my wiki which illustrates this.
I’ve used an anonymous
mmap() for shared memory — the previous revision of
the page used System V shared memory, but since this isn’t cleaned up
automatically on application exit then the
mmap() approach is safer.