X Server（xorg-server-1.20.8）定义了119个接口（0号和120之外的初始的时候都是ProcBadRequest）。 128之外的接口都是给扩展用的（#define EXTENSION_BASE 128）。 但是每个扩展可能实现了多个接口，而ProcVector仅仅为每个扩展准备了一个入口。所以扩展协议如果有多个接口，就需要自行分发。所以这里的MainProc其实就是扩展协议的入口。对于XRender协议，MainProc就是ProcRenderDispatch。
所以XRenderComposite的分发过程是： X服务器将来自X客户端的请求通过ProcVector分发给MainProc ProcRenderDispatch。然后ProcRenderDispatch通过ProcRenderVector将命令分发给ProcRenderComposite。 XRrenderComposite => Xlib => ProcRenderDispatch => XRender => ProcRenderComposite 但是XRender本身并没有具体的实现。具体的实现，是通过glamor或者fb实现的。glamor可以通过OpenGL来实现合成。如果没有OpenGL，那么回退到fb。
int (*ProcVector[256]) (ClientPtr /* client */) = { ProcBadRequest, ProcCreateWindow, ProcChangeWindowAttributes, ProcGetWindowAttributes, ProcDestroyWindow, ProcDestroySubwindows, /* 5 */ ProcChangeSaveSet, ProcReparentWindow, ProcMapWindow, ProcMapSubwindows, ProcUnmapWindow, /* 10 */ ProcUnmapSubwindows, ProcConfigureWindow, ProcCirculateWindow, ProcGetGeometry, ProcQueryTree, /* 15 */ ProcInternAtom, ProcGetAtomName, ProcChangeProperty, ProcDeleteProperty, ProcGetProperty, /* 20 */ ProcListProperties, ProcSetSelectionOwner, ProcGetSelectionOwner, ProcConvertSelection, ProcSendEvent, /* 25 */ ProcGrabPointer, ProcUngrabPointer, ProcGrabButton, ProcUngrabButton, ProcChangeActivePointerGrab, /* 30 */ ProcGrabKeyboard, ProcUngrabKeyboard, ProcGrabKey, ProcUngrabKey, ProcAllowEvents, /* 35 */ ProcGrabServer, ProcUngrabServer, ProcQueryPointer, ProcGetMotionEvents, ProcTranslateCoords, /* 40 */ ProcWarpPointer, ProcSetInputFocus, ProcGetInputFocus, ProcQueryKeymap, ProcOpenFont, /* 45 */ ProcCloseFont, ProcQueryFont, ProcQueryTextExtents, ProcListFonts, ProcListFontsWithInfo, /* 50 */ ProcSetFontPath, ProcGetFontPath, ProcCreatePixmap, ProcFreePixmap, ProcCreateGC, /* 55 */ ProcChangeGC, ProcCopyGC, ProcSetDashes, ProcSetClipRectangles, ProcFreeGC, /* 60 */ ProcClearToBackground, ProcCopyArea, ProcCopyPlane, ProcPolyPoint, ProcPolyLine, /* 65 */ ProcPolySegment, ProcPolyRectangle, ProcPolyArc, ProcFillPoly, ProcPolyFillRectangle, /* 70 */ ProcPolyFillArc, ProcPutImage, ProcGetImage, ProcPolyText, ProcPolyText, /* 75 */ ProcImageText8, ProcImageText16, ProcCreateColormap, ProcFreeColormap, ProcCopyColormapAndFree, /* 80 */ ProcInstallColormap, ProcUninstallColormap, ProcListInstalledColormaps, ProcAllocColor, ProcAllocNamedColor, /* 85 */ ProcAllocColorCells, ProcAllocColorPlanes, ProcFreeColors, ProcStoreColors, ProcStoreNamedColor, /* 90 */ ProcQueryColors, ProcLookupColor, ProcCreateCursor, ProcCreateGlyphCursor, ProcFreeCursor, /* 95 */ ProcRecolorCursor, ProcQueryBestSize, ProcQueryExtension, ProcListExtensions, ProcChangeKeyboardMapping, /* 100 */ ProcGetKeyboardMapping, ProcChangeKeyboardControl, ProcGetKeyboardControl, ProcBell, ProcChangePointerControl, /* 105 */ ProcGetPointerControl, ProcSetScreenSaver, ProcGetScreenSaver, ProcChangeHosts, ProcListHosts, /* 110 */ ProcChangeAccessControl, ProcChangeCloseDownMode, ProcKillClient, ProcRotateProperties, ProcForceScreenSaver, /* 115 */ ProcSetPointerMapping, ProcGetPointerMapping, ProcSetModifierMapping, ProcGetModifierMapping, ProcBadRequest, /* 120 */