While FDEVICECONTROL performs some of the same device control
functions performed by FCONTROL, the two intrinsics look very different
from each other when you use them in a program. FCONTROL uses just
three parameters, through which you specify a devicefile, define
the control operation to be performed, and, depending on the control
operation, pass or receive specific information that may be required
for that operation.
FDEVICECONTROL requires the use of a greater number of parameter
values. For operations that can be performed by either FCONTROL
or FDEVICECONTROL, it is easier to use FCONTROL. The following is
the syntax for FDEVICECONTROL:
FDEVICECONTROL(filenum,buffer,length,controlcode, |
parm1,parm2,fserrorcode); |
FDEVICECONTROL is used for a variety of file control operations,
each of which is appropriate for controlling a specific type of
devicefile.
The controlcode parameter determines
the control operation that will be performed. The only control operation
which effects asynchronous devicefiles is controlcode
value 192. It sends device control directives to an asynchronous
device.
Refer to Table 8-7 “Valid
FDEVICECONTROL Controlcode Values” for a
list of other valid controlcode values and
the operations they perform. For more information on these operations
refer to the MPE/iX Intrinsics Reference Manuals.
When controlcode is 192, parm1
determines the specific control directive to be sent to the devicefile.
parm1 must contain one of the control directive
values specified in Chapter 4 “Using FDEVICECONTROL”
parm2 determines the Read/Write access
to be imposed on the directive. The Read/Write access setting must
be valid for the control directive specified in parm1.
Set parm2 to 1 for Read access, 2 for Write
access, or 3 for both Read and Write access.
For example, if you were to call FDEVICECONTROL with controlcode
set to 192 and parm1 set to 57, which is
the control directive used to obtain the subsystem break character,
you would need to set parm2 to 1, since only
Read access is allowed for that control directive.
If, however, parm1 were set to 36,
which is the control directive used to define the backspace character,
you could set parm2 to 1, 2, or 3. In this
case, setting parm2 to 1 would return the
current backspace character to your program, while setting it to
2 would actually change the backspace character to whatever new
character you pass in the call (through the buffer
parameter). Setting parm2 to 3 would both
return the previous value and change the backspace character to
the new value passed in the call.
Some control directives do not permit all possible values
of parm2 to be used with them. Table 8-8
lists the allowable parm2 access values for
each of the valid control directives that may be specified in parm1.
The filenum parameter
specifies the file number, as obtained through a previous call to
FOPEN or HPFOPEN, of the devicefile on which the control operation
is to be performed.
The buffer parameter
passes and returns the data associated with the control directive
being performed. For example, if the control directive specified
in parm1 is 3, which sets
the line speed for a device, buffer must
contain the new linespeed setting (and will return the previous
speed setting if parm2
is set to Read/Write access).
The length parameter sets the length
of buffer. Positive values indicate the length
in half words. Negative values indicate the length in bytes. Usually
length will be set to 1, indicating that
buffer is 1 half word long.
The fserrorcode parameter returns error
information to your program. If no error occurs during execution,
fserrorcode contains a 0. If an error occurs,
fserrorcode returns the file system error
code corresponding to that error. You will need to call FCHECK for
more information about the error that occurred.
