Constant SYS_IPC_CALL 

pub const SYS_IPC_CALL: usize = 203;

Synchronous RPC: send a message and wait for a reply.

• port_handle: capability handle of the target port
• msg_ptr: pointer to the request message
• msg_len: length of the request

Blocks until the server replies. Returns the number of bytes in the reply. Equivalent to SYS_IPC_SEND + SYS_IPC_RECV with automatic reply routing.