Enable `_posixshmem` stdlib module (POSIX shared memory)

[ppenna]

Summary

Enable the _posixshmem C extension module, which provides POSIX shared memory support for multiprocessing.shared_memory.

Current State

Commented out in Modules/Setup.stdlib:

#_posixshmem _multiprocessing/posixshmem.c

Why Enable It

• Required for multiprocessing.shared_memory.SharedMemory
• Enables zero-copy IPC between processes via shared memory segments
• Used by data processing and scientific computing workloads

Work Required

1. Uncomment in Modules/Setup.stdlib:

   _posixshmem -I$(srcdir)/Modules/_multiprocessing _multiprocessing/posixshmem.c -lrt
   

2. Nanvix POSIX API requirements:
   • shm_open(name, oflag, mode) — create/open shared memory object
   • shm_unlink(name) — remove shared memory object
   • ftruncate(fd, length) — set shared memory size
   • mmap() / munmap() — map shared memory into address space
3. Required headers: <sys/mman.h>, <fcntl.h>
4. Link flags: -lrt (may need to verify availability in Nanvix)
5. Dependency: Requires mmap() support (see mmap module issue)
6. Build and test: python -c "from multiprocessing import shared_memory; sm = shared_memory.SharedMemory(create=True, size=1024); sm.close(); sm.unlink()"

POSIX API Dependencies

API	Purpose
shm_open()	Create/open shared memory object
shm_unlink()	Remove shared memory object
ftruncate()	Set size of shared memory
mmap()	Map into address space
munmap()	Unmap from address space

Complexity

High — requires POSIX shared memory and mmap() support in Nanvix.

Source Files

• Modules/_multiprocessing/posixshmem.c