Unable to read data from share memory

Biswa

Hii all,
I am try to read data from share memory and it is showing
"Unable to attach to shared memory segment: "QSharedMemory::handle:: UNIX key file doesn't exist" "
hear is my source code for read data from share memory```
code_text

#include <QSharedMemory>
#include <QDebug>

int main(int argc, char *argv[]) {
    QCoreApplication a(argc, argv);

    // Name of the shared memory segment
    QString shmKey = "my_memory";

    // Create a shared memory object
    QSharedMemory sharedMemory(shmKey);

    // Attach to the shared memory segment
    if (!sharedMemory.attach()) {
        qDebug() << "Unable to attach to shared memory segment:" << sharedMemory.errorString();
        return 1;
    }

    // Read data from the shared memory
    char *from = static_cast<char*>(sharedMemory.data());
    QString message = QString::fromUtf8(from);

    qDebug() << "Data read from shared memory:" << message;

    return a.exec();
}

JonB

@Biswa
As https://doc.qt.io/qt-6/qsharedmemory.html#details says:

One QSharedMemory object must create() the segment and this call specifies the size of the segment. All other processes simply attach() to the segment that must already exist.

So where have you, either in this process or another process, called bool QSharedMemory::create(qsizetype size, QSharedMemory::AccessMode mode = ReadWrite)? I imagine that is what error message UNIX key file doesn't exist indicates. Tell me, does the message really include the word UNIX in it??

Biswa

@JonB
I want to write data in Shared Memory using python language and that data i want to read from Shared Memory.

Bonnie

@Biswa Then how do you create shared memory in python? Can you share the code? Are you sure they are using the same mechanism?
One thing need to be noticed is that, when you create a QSharedMemory object by "QSharedMemory sharedMemory(shmKey);", shmKey will not be the actual key used by the native API.

Biswa

@Bonnie @Bonnie
for writing data in shared memory am using pythoncode

import posix_ipc
import mmap
import os
import struct

SHARED_MEMORY_NAME = "/my_memory123"
MEMORY_SIZE = 1024

# Create or open shared memory segment
shared_memory = posix_ipc.SharedMemory(SHARED_MEMORY_NAME, posix_ipc.O_CREAT, size=MEMORY_SIZE)

# Map the shared memory segment to memory
mapfile = mmap.mmap(shared_memory.fd, MEMORY_SIZE)

# Example integers to write
integers = [0x02, 0x03, 0x04, 0x05]  # List of integers
# Convert integers to bytes
data = struct.pack(f'{len(integers)}I', *integers)

# Write integer data to shared memory
mapfile.seek(0)  # Move to the start of the shared memory segment
mapfile.write(data)

print("Integer data written to shared memory.")

# Clean up
mapfile.close()
os.close(shared_memory.fd)

Bonnie

@Biswa Seems this posix_ipc and QSharedMemory both use shm_open API under POSIX, I suppose this could work.
As I said, here we need to let QSharedMemory use "/my_memory123" as the native API key (to call shm_open).
So instead of creating a QSharedMemory with key directly, try to use

QSharedMemory sharedMemory;
sharedMemory.setNativeKey("/my_memory123");

P.S. There's some helpful information in Qt's documentation about this kind of case.
For Qt5, read the Warning part of https://doc.qt.io/qt-5/qsharedmemory.html#details
For Qt6, read https://doc.qt.io/qt-6/native-ipc-keys.html#interoperability-with-non-qt-applications