On 24/02/20(Mon) 11:29, Lauri Tirkkonen wrote: > On Mon, Feb 24 2020 10:24:53 +0100, Martin Pieuchot wrote: > > On 23/02/20(Sun) 14:48, Lauri Tirkkonen wrote: > > > I was working on a make jobserver implementation that uses POSIX > > > semaphores as job tokens instead of a complicated socket-based approach. > > > Initially I used named semaphores, which work fine, except if child > > > processes with less privileges need to also open the named semaphore > > > (eg. 'make build' as root executing 'su build -c make'). For that reason > > > I wanted to use an unnamed semaphore (sem_init()) which is stored in shm > > > -- that way I could leave the shm fd open and pass it to children. > > > > > > But unfortunately, sem_t is currently just a pointer to the opaque > > > struct __sem, and sem_int() always calloc()s the storage for the struct. > > > > That's by design. > > Ok - could you elaborate what the design is?
If the size of a descriptor change, because some fields are added and/or removed, it doesn't matter for the application because it only manipulates pointers. That means we can change the data types without creating an ABI break. > > > This means the application cannot control where unnamed semaphores are > > > stored, so I can't put it in shm. > > > > Are you trying to use semaphore shared between process? Did you called > > sem_init() with pshared=1? Have you seen that the current implementation > > doesn't support them? > > Yes, that's what I'm trying to do. Yes, I've seen the current > implementation -- that's why I started this thread, in an attempt to > make them supported. :) > > See the followup patch -- sharing the semaphore between processes does > work with it. Well ignoring the `pshared' argument is questionable. Why don't you remove the "#if notyet" and start playing with the existing code and try to figure out if something is missing for your use case?
