macOS Passing a string(in user mode) to kernel kext(kext have a dynamic buffer)

I have a Daemon and a kernel kext.
in kext a buffer defined like :

unsigned char* buffer;

this buffer is dynamic.i mean buffer get characters from anywhere and in a certain time a daemon read buffer and write to file.
now i want to pass a string in user mode to kext's buffer that this string write to file with other characters in buffer.

How i can do that?

Thanks