How Can I Safely Use ioctl() in Go Without Memory Leaks?

Proper Ioctl Usage in Go

In Go, the ioctl() call is made via the unsafe package's Syscall() function. This brings a considerable level of complexity to the task, compared to C programming.

The Go equivalent of the C code provided, calling ioctl() on file descriptor f with operation IOCTL_MBOX_PROPERTY and buffer buf:

func mBoxProperty(f *os.File, buf [256]int64) {
        err := Ioctl(f.Fd(), IOWR(100, 0, 8), uintptr(unsafe.Pointer(&buf[0])))

        if err != nil {
                log.Fatalln("mBoxProperty() : ", err)
        }

}

As such, Ioctl() is a function that wraps Syscall() with a predefined interface that simplifies the task of making ioctl() calls within Go. Errors encountered during the ioctl() call are returned as syscall.Errno.

It's important to prevent the garbage collector from releasing the memory pointed to by buf before the ioctl() call is complete. This can be done by ensuring that buf remains in scope until the ioctl() call, which can be achieved by declaring buf as a local variable within the function. Alternatively, runtime.KeepAlive(buf) can be used to prevent the garbage collector from releasing the buffer.

In cases where the kernel expects a small memory buffer to be handed over, it's crucial to exercise caution. The garbage collector may free memory objects it believes are not in use, or even move them in memory. These actions remain invisible to the kernel, which retains the old pointer and continues using it.

To mitigate this issue, a small C extension written using cgo can be used to allocate a suitable buffer via malloc(), which is not subject to garbage collection. This buffer can then be passed to ioctl(). By tracking the old pointer value and freeing it after use, memory leaks can be avoided.

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