Inside Macintosh: QuickDraw GX Printing Extensions and Drivers /

Chapter 4 - Printing Messages / Printing Messages Reference

Printing Messages / Device Communications Messages

GXWriteData

QuickDraw GX sends the GXWriteData message to send data directly to the device, without buffering. You can override the GXWriteData message to provide your own I/O mechanism. Your override of the GXWriteData message must match the following formal declaration:

OSErr MyWriteData (Ptr data, long length);

data

A pointer to the data to send to device.

length

The number of bytes of data.

function result

An error code. The value noErr indicates that the operation was successful.

DESCRIPTION

The GXWriteData message sends unbuffered data to a device and waits for I/O to complete.

The default implementation of this message calls the asynchronous I/O functions of the Macintosh system software to send the data to the device. The default implementation then flushes the buffers, sends the resulting data through, and waits for the I/O operation to complete.

You can override this message as part of implementing your own I/O and buffering mechanism or changing how I/O is performed with your device.

SPECIAL CONSIDERATIONS

You can use the GXWriteData message yourself to send data to your device without buffering or to force the buffers to be flushed.

If you are implementing your own I/O mechanism, you need to totally override the GXWriteData message. If you are using the standard QuickDraw GX buffering and Macintosh I/O system, you must forward this message.

RESULT CODES

gxSegmentLoadFailedErr	A required code segment could not be found, or there was not enough memory to load it.
gxPrUserAbortErr	The user has canceled printing.

The default implementation of the GXWriteData message can also return the communications errors that are listed in Table 4-2 on page 4-42.

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