Casio IT-2000W - Manual (Page 133)

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UngetCommChar
int UngetCommChar(idComDev, chUnget)
int idComDev; /* Communication device */
char chUnget; /* Character to be placed in the queue */
The UngetCommChar function replaces the specified character in the reception queue. At the next
read operation, this character will be read first.
< Parameter >
idComDev : Specifies the communication device which receives the characters. The OpenComm
function returns this value.
chUnget : Specifies the characters to be placed in the reception queue.
< Return value >
Returns zero if the function is terminated normally. Otherwise it returns a value less than zero.
Note:
The UngetCommChar function cannot be called repeatedly. To make it possible to call this function
again, it is necessary to read out characters in the queue.
Note on IrDA:
There are no particular use restrictions.