This module exports:

void HandleMsgSend( Thread *, int )	explained below
int HandleMsgRecv( Thread *, Message * )	explained below
void DeliverMsg( Thread *src, Thread *dest, int payload )	explained below

Look at the header file: syscall.e

Module functions:

void HandleMsgSend( Thread *dest, int payload )	Sends a message whose value is payload to dest. If the recipient thread is dead or dest is NULL the sender will be killed.
int HandleMsgRecv( Thread *src, Message *reply )	Checks whether the current thread has a message from src, or from any thread if src is NULL: if so it fills the reply value with the one from the message and leaves the calling thread in the ready queue; otherwise the caller is placed in the wait queue. The caller will be terminated if an invalid reply pointer is used.
void DeliverMsg( Thread *src, Thread *dest, int payload )

Look at the code: syscall.cc