# ITC

> [HTML Version](itc_sbr.htm)

**xcall ITC, func, type, mesg, retv**

(Unix only) ITC allows programs to send and receive messages between jobs.

**Parameters**

MAP1 FUNC,<any numeric type>     \! opcode

MAP1 TYPE,<any numeric type>     \! message type or process ID

MAP1 RETV,B,1                    \! return code (0=ok)

MAP1 MESG                        \! message

MAP2 MTYPE,B,4                \! internal code

MAP2 MESGX,X,<any size>       \! message contents



_func_

indicates the operation to perform, as described in the following table. 

| **Value** | **Meaning** |
|------|------|
| 0 | Send message to cooperating receiver |
| 1 | Receive message (do not wait if not available) |
| 2 | Receive message (wait until available) |
| 3 | Send message and signal receiver  |




_type_

identifies the type of message, or the process ID of the target job to signal. In the case of _func_ 3, it must be the process ID of the target job to signal. (In this operation, the contents of the message may not be important; since the signal itself could be used to notify the target job to look for subsequent messages.) For the other operations, it must be a value that both sender and receiver agree on, because the receive operation will ignore messages not sent with the specified _type_. (The idea here is to allow multiple message formats or priorities to be used simultaneously.) The exception is that the receiver may set _type_=0 to receive the first message in the queue, regardless of its type. To avoid confusion between types that are process IDs and types that are message types, it is recommended that you adopt a standard of using message _type _values higher than the range of process IDs (which might extend as high as 2\*\*24 on very large systems. _Type_ should also be below 2\*\*31 to avoid confusing them with negative _type_ numbers, which are used to request to read any message whose _mtype_ is less than or equal to the absolute value of the specified _type_.

_mesg_

contains the message to send or receive. Obviously, the communicating programs need to agree in advance on the layout corresponding to each message type. It must begin, as shown above, with a 4 byte field which is reserved for internal use by the subroutine. (Actually, it plugs the _type_ value into _mtype_ for outgoing messages, and ignores any incoming messages whose _mtype_ does not match the _type_ specified.)

_retv_

returns a 0 or 1 indicating the success of the operation (0 = success).

**Comments**

Unix systems do not allow one user to signal another unless they are both share the same effective user ID or if the sender is root. For this reason, some sites set the "setuid" flag on the ashell executable to make all A-Shell users effectively the same user, or else they rely on the system administrator being able to switch to root in order to use operations which require signaling other users. Such operations include ITC FUNC=3, KILL.LIT, CHAT.LIT, SEND.LIT, FORCE.LIT, and JSTAT.LIT.