Data unchanged when passing data to Python in multiprocessing shared memory

[Barry]

> On 1 Feb 2022, at 20:26, Jen Kris via Python-list <python-list at python.org> wrote:
> 
> I am using multiprocesssing.shared_memory to pass data between NASM and Python.  The shared memory is created in NASM before Python is called.  Python connects to the shm:  shm_00 = shared_memory.SharedMemory(name='shm_object_00',create=False).  
> 
> I have used shared memory at other points in this project to pass text data from Python back to NASM with no problems.  But now this time I need to pass a 32-bit integer (specifically 32,894) from NASM to Python. 
> 
> First I convert the integer to bytes in a C program linked into NASM:
> 
>     unsigned char bytes[4]
>     unsigned long int_to_convert = 32894;
> 
>     bytes[0] = (int_to_convert >> 24) & 0xFF;
>     bytes[1] = (int_to_convert >> 16) & 0xFF;
>     bytes[2] = (int_to_convert >> 8) & 0xFF;
>     bytes[3] = int_to_convert & 0xFF;
>     memcpy(outbuf, bytes, 4);
> 
> where outbuf is a pointer to the shared memory.  On return from C to NASM, I verify that the first four bytes of the shared memory contain what I want, and they are 0, 0, -128, 126 which is binary 00000000 00000000 10000000 01111110, and that's correct (32,894). 
> 
> Next I send a message to Python through a FIFO to read the data from shared memory.  Python uses the following code to read the first four bytes of the shared memory:
> 
>         byte_val = shm_00.buf[:4]
>         print(shm_00.buf[0])
>         print(shm_00.buf[1])
>         print(shm_00.buf[2])
>         print(shm_00.buf[3])
> 
> But the bytes show as 40 39 96 96, which is exactly what the first four bytes of this shared memory contained before I called C to overwrite them with the bytes 0, 0, -128, 126.  So Python does not see the updated bytes, and naturally int.from_bytes(byte_val, "little") does not return the result I want. 
> 
> I know that Python refers to shm00.buf, using the buffer protocol.  Is that the reason that Python can't see the data that has been updated by another language? 
> 
> So my question is, how can I alter the data in shared memory in a non-Python language to pass back to Python? 

Maybe you need to use a memory barrier to force the data to be seen by another cpu?
Maybe use shm lock operation to sync both sides?
Googling I see people talking about using stdatomic.h for this.

But I am far from clear what you would need to do.

Barry

> 
> Thanks,
> 
> Jen
> 
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