MIT Shared Memory Extension

15

This chapter describes the functions that a ddx handler may call to enable full functionality of the MIT Shared Memory (MIT_SHM) extension. This extension is an MIT standard that is distributed with X11 Release 5 (X11R5).

The MIT_SHM extension is a version of the ximage interface where the actual image data is stored in a shared memory segment. This extension can yield a significant increase in performance for large images.

The following document is part of the MIT_SHM extension, and is online in the doc/hardcopy/Xext directory.

MIT_SHM - The MIT Shared Memory Extension, Jonathan Corbet, formatted and edited for release 5 by Keith Packard, MIT X Consortium.

MIT Shared Memory Interface

All ddx handlers may use the following functions to implement the MIT Shared Memory extension.

Table 15-1

Function Name	Description
`ShmRegisterFbFuncs`	Registers the cfb-compatible functions. The ddx handler must accept fake pixmaps. Fake pixmaps are pixmaps with `devPrivates` initialized to NULL and an internal format compatible with cfb. Note that `ShmRegisterFbFuncs` is called in `mpgScreenInit` and `miScreenInit`, so if your handler calls either of these functions, no other work is required.
`ShmRegisterFuncs`	Registers the specified shared memory function vectors. Note that to enable creation of shared memory pixmaps, you must use `ShmRegisterFuncs` or `ShmRegisterFbFuncs`.
`ShmSetPixmapFormat`	Registers the specified pixmap format.

ShmRegisterFbFuncs


  void ShmRegisterFbFuncs(ScreenPtr pScreen)

*Purpose*	This function is provided to register the predefined shared memory functions. The predefined `ShmFuncs` record is registered as follows:
`ShmFuncs fbFuncs={fbShmCreatePixmap, fbShmPutImage};`
*Called by*	This function is called during device screen initialization.
*Results*	This function initializes the `shmFuncs` array indexed by the specified screen number with the `ShmFuncs` record.
*Returns*	None.

ShmRegisterFuncs


  void ShmRegisterFuncs(ScreenPtr pScreen, ShmFuncsPtr funcs)

*Purpose*	This function is provided to register the shared memory functions. The ShmFuncsPtr has been defined as follows:
`typedef struct _ShmFuncs {`
*Called by*	This function is called during device screen initialization.
*Results*	This function initializes the `shmFuncs` array indexed by the specified screen number with `funcs.`
*ReturnsShmSetPixmapFormat*	None.
`void ShmSetPixmapFormat(ScreenPtr pScreen, int format)`	None.
*Purpose*	This function is provided to register the shared memory pixmap format. The valid pixmap formats are XYPixmap, XYBitmap or ZPixmap.
*Called by*	This function is called during device screen initialization.
*Results*	This function initializes the `shmPixFormat` array indexed by the specified screen number with `format`.
*Returns*	None.