GDI enables operation of the printer driver by calling the driver's Enable function and directing the driver to initialize a physical device for subsequent graphics output. A physical device is a PDEVICE structure that represents a printer and its current operating state. A printer driver uses the physical-device information to determine how to carry out specific tasks, such as which device-dependent graphics commands to use, and which output port to send the commands. The printer driver initializes the physical device by copying information to the PDEVICE structure.
GDI calls the Enable function whenever an application calls the CreateDC function (GDI.53) to create a device context (DC) for the printer. GDI calls the function twice: once to retrieve a copy of the driver's GDIINFO structure, and a second time to initialize the PDEVICE structure. After the first call, GDI uses the dpDEVICEsize member in the GDIINFO structure to determine the size of the driver's PDEVICE structure. GDI then allocates memory for the structure and calls Enable for the second time, passing a pointer to the structure. With this call, the driver initializes the structure.
To initialize the PDEVICE structure, the driver typically examines the names of the printer model and output device or file passed to Enable by GDI. It may also examine any printer environment passed to Enable. The driver then fills the PDEVICE structure with all the information that the output functions need to generate appropriate graphics commands for the given printer model and to send the commands to the given device or file.
Although only the printer driver initializes and uses the PDEVICE structure, GDI allocates memory for the structure, determines when to pass it to the driver's output functions, and deletes the structure when it is no longer needed. Except for the first 16 bits of the PDEVICE structure, the content and format of the structure depends entirely on the printer driver. The first 16 bits, on the other hand, must be set to a nonzero value. GDI reserves zero to indicate a BITMAP structure. GDI creates and uses a BITMAP structure in place of a PDEVICE structure when an application creates a memory-device context.
A printer driver can allocate additional structures and store their addresses in the PDEVICE structure. Because GDI may direct a printer driver to create a large number of physical devices, the printer driver should not allocate additional structures in the limited space of the driver's automatic data segment, especially if the driver allows multiple device contexts.
GDI disables the physical device and possibly frees the printer driver whenever an application calls the DeleteDC function (GDI.68). GDI disables the physical device by calling the driver's Disable function. It expects the driver to complete any outstanding printing job and free any resources associated with the physical device. After the driver returns from the Disable function, GDI frees the memory it allocated for the PDEVICE structure. If there are no other device contexts for this printer driver, Windows frees the driver, removing any driver code and data from memory.