char **XListExtensions(display, nextensions_return)
      Display *display;
      int *nextensions_return;

XListExtensions() returns a list of all extensions supported by the server. If the data returned by the server is in the Latin Portable Character Encoding, then the returned strings are in the Host Portable Character Encoding. Otherwise, the result is implementation dependent.

XFreeExtensionList(list)
      char **list;

XFreeExtensionList() frees the memory allocated by XListExtensions().

The XExtCodes structure returns the information from XInitExtension() and is defined in <Xlib.h>:

typedef struct _XExtCodes {  /* public to extension, cannot be changed */
    int extension;           /* extension number */
    int major_opcode;        /* major op-code assigned by server */
    int first_event;         /* first event number for the extension */
    int first_error;         /* first error number for the extension */
} XExtCodes;

XExtCodes *XInitExtension(display, name)
      Display *display;
      char *name;

XInitExtension() determines if the named extension exists. Then it allocates storage for maintaining the information about the extension on the connection, chains this onto the extension list for the connection, and returns the information the stub implementor will need to access the extension. If the extension does not exist, XInitExtension() returns NULL.

If the extension name is not in the Host Portable Character Encoding the result is implementation dependent. Case matters; the strings thing, Thing, and thinG are all considered different names.

The extension number in the XExtCodes structure is needed in the other calls that follow. This extension number is unique only to a single connection.

XExtCodes *XAddExtension(display)
        Display *display;

For local Xlib extensions, XAddExtension() allocates the XExtCodes structure, bumps the extension number count, and chains the extension onto the extension list. (This permits extensions to Xlib without requiring server extensions.)

These functions allow you to define procedures that are to be called when various circumstances occur. The procedures include the creation of a new GC for a connection, the copying of a GC, the freeing of a GC, the creating and freeing of fonts, the conversion of events defined by extensions to and from wire format, and the handling of errors.

All of these functions return the previous routine defined for this extension.

int (*XESetCloseDisplay(display, extension, proc))()
      Display *display;
      int extension;
      int (*proc)();

(*proc)(display, codes)
    Display *display;
    XExtCodes *codes;

int (*XESetCreateGC(display, extension, proc))()
      Display *display;
      int extension;
      int (*proc)();

(*proc)(display, gc, codes)
    Display *display;
    GC gc;
    XExtCodes *codes;

int (*XESetCopyGC(display, extension, proc))()
      Display *display;
      int extension;
      int (*proc)();

(*proc)(display, gc, codes)
    Display *display;
    GC gc;
    XExtCodes *codes;

int (*XESetFreeGC(display, extension, proc))()
      Display *display;
      int extension;
      int (*proc)();

(*proc)(display, gc, codes)
    Display *display;
    GC gc;
    XExtCodes *codes;

int (*XESetCreateFont(display, extension, proc))()
      Display *display;
      int extension;
      int (*proc)();

(*proc)(display, fs, codes)
    Display *display;
    XFontStruct *fs;
    XExtCodes *codes;

int (*XESetFreeFont(display, extension, proc))()
      Display *display;
      int extension;
      int (*proc)();

(*proc)(display, fs, codes)
    Display *display;
    XFontStruct *fs;
    XExtCodes *codes;

The next three functions allow you to define new events to the library. An XEvent structure always has a type code (type int) as the first component. This uniquely identifies what kind of event it is. The second component is always the serial number (type unsigned long) of the last request processed by the server. The third component is always a boolean (type BoolP) indicating whether the event came from a SendEvent protocol request. The fourth component is always a pointer to the display the event was read from. The fifth component is always a resource ID of one kind or another, usually a window, carefully selected to be useful to toolkit dispatchers. The fifth component should always exist, even if the event does not have a natural “destination”; if there is no value from the protocol to put in this component, initialize it to zero.

---

Note: There is an implementation limit such that your host event structure size cannot be bigger than the size of the XEvent union of structures. There also is no way to guarantee that more than 24 elements or 96 characters in the structure will be fully portable between machines.

---

XESetWireToEvent

int (*XESetWireToEvent()(display, event_number, proc))() Display *display; int event_number; Status (*proc)();

display		Specifies the connection to the X server.
event_number		Specifies the event code.
proc		Specifies the routine to call when converting an event.

You use this procedure to define a procedure to be called when an event needs to be converted from wire format (xEvent) to host format (XEvent). The event number defines which protocol event number to install a conversion routine for. This procedure returns any previously defined procedure.

---

Note: You can replace a core event conversion routine with one of your own, although this is not encouraged. It would, however, allow you to intercept a core event and modify it before being placed in the queue or otherwise examined.

---

When Xlib needs to convert an event from wire format to host format, your routine is called with these arguments:

Status (*proc)(display, re, event) Display *display; XEvent *re; xEvent *event;

Your routine must return status to indicate if the conversion succeeded. The re argument is a pointer to where the host format event should be stored, and the event argument is the 32-byte wire event structure. In the XEvent structure you are creating, you must fill in the five required members of the event structure. You should fill in the type member with the type specified for the xEvent structure. You should copy all other members from the xEvent structure (wire format) to the XEvent structure (host format). Your conversion routine should return True if the event should be placed in the queue or False if it should not be placed in the queue.

unsigned long _XSetLastRequestRead(display, rep)
      Display *display;
      xGenericReply *rep;

XESetEventToWire

Status (*XESetEventToWire()(display, event_number, proc))() Display *display; int event_number; int (*proc)();

display		Specifies the connection to the X server.
event_number		Specifies the event code.
proc		Specifies the routine to call when converting an event.

You use this procedure to define a procedure to be called when an event needs to be converted from host format (XEvent) to wire format (xEvent) form. The event number defines which protocol event number to install a conversion routine for. This procedure returns any previously defined procedure. It returns zero if the conversion fails or nonzero otherwise.

---

Note: You can replace a core event conversion routine with one of your own, although this is not encouraged. It would, however, allow you to intercept a core event and modify it before being sent to another client.

---

When Xlib needs to convert an event from host format to wire format, your routine is called with these arguments:

(*proc)(display, re, event) Display *display; XEvent *re; xEvent *event;

The re argument is a pointer to the host format event, and the event argument is a pointer to where the 32-byte wire event structure should be stored. You should fill in the type with the type from the XEvent structure. All other members then should be copied from the host format to the xEvent structure.

Bool (*XESetWireToError()(display, error_number, proc)()
      Display *display;
      int error_number;
      Bool (*proc)();

Bool (*proc)(display, he, we)
    Display *display;
    XErrorEvent *he;
    xError *we;

int (*XESetError()(display, extension, proc))()
      Display *display;
      int extension;
      int (*proc)();

int (*proc)(display, err, codes, ret_code)
    Display *display;
    xError *err;
    XExtCodes *codes;
    int *ret_code;

char  *(*XESetErrorString()(display, extension, proc))()
      Display *display;
      int extension;
      char *(*proc)();

(*proc)(display, code, codes, buffer, nbytes)
    Display *display;
    int code;
    XExtCodes *codes;
    char *buffer;
    int nbytes;

void (*XESetPrintErrorValues()(display, extension, proc))()
      Display *display;
      int extension;
      void (*proc)();

void (*proc)(display, ev, fp)
    Display *display;
    XErrorEvent *ev;
    void *fp;

int (*XESetFlushGC()(display, extension, proc))()
      Display *display;
      int extension;
      int *(*proc)();

Various Xlib data structures have provisions for extension routines to chain extension supplied data onto a list. These structures are GC, Visual, Screen, ScreenFormat, Display, and XFontStruct. Because the list pointer is always the first member in the structure, a single set of routines can be used to manipulate the data on these lists.

The following structure is used in the routines in this section and is defined in <Xlib.h>:

typedef struct _XExtData {
    int number;              /* number returned by XInitExtension() */
    struct _XExtData *next;  /* next item on list of data for structure */
    int (*free_private)();   /* if defined,  called to free private */
    XPointer private_data;   /* data private to this extension. */
} XExtData;

When any of the data structures listed above are freed, the list is walked, and the structure's free routine (if any) is called. If free is NULL, then the library frees both the data pointed to by the private_data member and the structure itself.

union {    Display *display;
    GC gc;
    Visual *visual;
    Screen *screen;
    ScreenFormat *pixmap_format;
    XFontStruct *font } XEDataObject;

XExtData **XEHeadOfExtensionList(object)
   XEDataObject object;

XAddToExtensionList(structure, ext_data)
      XExtData **structure;
      XExtData *ext_data;

XExtData *XFindOnExtensionList()(structure, number)
      struct _XExtData **structure;
      int number;

XAllocID(display)
     Display *display;

The <Xproto.h> file contains three sets of definitions that are of interest to the stub implementor: request names, request structures, and reply structures.

You need to generate a file equivalent to <Xproto.h> for your extension and need to include it in your stub routine. Each stub routine also must include <Xlibint.h>.

The identifiers are deliberately chosen in such a way that, if the request is called X_DoSomething, then its request structure is xDoSomethingReq, and its reply is xDoSomethingReply. The GetReq family of macros, defined in <Xlibint.h>, takes advantage of this naming scheme.

For each X request, there is a definition in <Xproto.h> that looks similar to this:

#define X_DoSomething   42

In your extension header file this will be a minor opcode, rather than a major opcode.

Every request contains an 8-bit major opcode and a 16-bit length field expressed in units of four bytes. Every request consists of four bytes of header (containing the major opcode, the length field, and a data byte) followed by zero or more additional bytes of data. The length field defines the total length of the request, including the header. The length field in a request must equal the minimum length required to contain the request. If the specified length is smaller or larger than the required length, the server should generate a BadLength error. Unused bytes in a request are not required to be zero. Extensions should be designed in such a way that long protocol requests can be split up into smaller requests, if it is possible to exceed the maximum request size of the server. The protocol guarantees the maximum request size to be no smaller than 4096 units (16384 bytes).

Major opcodes 128 through 255 are reserved for extensions. Extensions are intended to contain multiple requests, so extension requests typically have an additional minor opcode encoded in the “spare” data byte in the request header, but the placement and interpretation of this minor opcode as well as all other fields in extension requests are not defined by the core protocol. Every request is implicitly assigned a sequence number (starting with one) used in replies, errors, and events.

To help but not cure portability problems to certain machines, the B16 and B32 macros have been defined so that they can become bitfield specifications on some machines. For example, on a Cray, these should be used for all 16-bit and 32-bit quantities, as discussed below.

Most protocol requests have a corresponding structure typedef in <Xproto.h>, which looks like:

typedef struct _DoSomethingReq {
    CARD8 reqType;        /* X_DoSomething */
    CARD8 someDatum;      /* used differently in different requests */
    CARD16 length B16;    /* total # of bytes in request, divided by 4 */
    ...
    /* request-specific data */
    ...
} xDoSomethingReq;

If a core protocol request has a single 32-bit argument, you need not declare a request structure in your extension header file. Instead, such requests use <Xproto.h>'s xResourceReq structure. This structure is used for any request whose single argument is a Window, Pixmap, Drawable, GContext, Font, Cursor, Colormap, Atom, or VisualID.

typedef struct _ResourceReq {
    CARD8 reqType;     /* the request type, e.g., X_DoSomething */
    BYTE pad;          /* not used */
    CARD16 length B16; /* 2 (= total # of bytes in request, divided by 4) */
    CARD32 id B32;     /* the Window, Drawable, Font, GContext, etc. */
} xResourceReq;

If convenient, you can do something similar in your extension header file.

In both of these structures, the reqType field identifies the type of the request (for example, X_MapWindow or X_CreatePixmap). The length field tells how long the request is in units of 4-byte longwords. This length includes both the request structure itself and any variable length data, such as strings or lists, that follow the request structure. Request structures come in different sizes, but all requests are padded to be multiples of four bytes long.

A few protocol requests take no arguments at all. Instead, they use <Xproto.h>'s xReq structure, which contains only a reqType and a length (and a pad byte).

If the protocol request requires a reply, then <Xproto.h> also contains a reply structure typedef:

typedef struct _DoSomethingReply {
    BYTE type;                 /* always X_Reply */
    BYTE someDatum;            /* used differently in different requests */
    CARD16 sequenceNumber B16; /* # of requests sent so far */
    CARD32 length B32;         /* # of additional bytes, divided by 4 */
    ...
    /* request-specific data */
    ...
} xDoSomethingReply;

Most of these reply structures are 32 bytes long. If there are not that many reply values, then they contain a sufficient number of pad fields to bring them up to 32 bytes. The length field is the total number of bytes in the request minus 32, divided by 4. This length will be nonzero only if:

Only XGetWindowAttributes(), XQueryFont(), XQueryKeymap(), and XGetKeyboardControl have reply structures longer than 32 bytes in the core protocol.

A few protocol requests return replies that contain no data. <Xproto.h> does not define reply structures for these. Instead, they use the xGenericReply structure, which contains only a type, length, and sequence number (and sufficient padding to make it 32 bytes long).

An Xlib stub routine should always start like this:

#include "Xlibint.h"
XDoSomething (arguments, ... )
/* argument declarations */
{
register XDoSomethingReq *req;

If the protocol request has a reply, then the variable declarations should include the reply structure for the request. The following is an example:

xDoSomethingReply rep;

To lock the display structure for systems that want to support multithreaded access to a single display connection, each stub will need to lock its critical section. Generally, this section is the point from just before the appropriate GetReq call until all arguments to the call have been stored into the buffer. The precise instructions needed for this locking depend upon the machine architecture. Two calls, which are generally implemented as macros, have been provided.

LockDisplay(display)
      Display *display;

UnlockDisplay(display)
      Display *display;

After the variable declarations, a stub routine should call one of four macros defined in <Xlibint.h>: GetReq(), GetReqExtra(), GetResReq(), or GetEmptyReq(). All of these macros take, as their first argument, the name of the protocol request as declared in <Xproto.h> except with X_ removed. Each one declares a Display structure pointer, called dpy, and a pointer to a request structure, called req, which is of the appropriate type. The macro then appends the request structure to the output buffer, fills in its type and length field, and sets req to point to it.

If the protocol request has no arguments (for instance, X_GrabServer), then use GetEmptyReq().

GetEmptyReq (DoSomething, req);

If the protocol request has a single 32-bit argument (such as a Pixmap, Window, Drawable, Atom, and so on), then use GetResReq(). The second argument to the macro is the 32-bit object. X_MapWindow is a good example.

GetResReq (DoSomething, rid, req);

The rid argument is the Pixmap, Window, or other resource ID.

If the protocol request takes any other argument list, then call GetReq(). After the GetReq(), you need to set all the other fields in the request structure, usually from arguments to the stub routine.

GetReq (DoSomething, req);
/* fill in arguments here */
req->arg1 = arg1;
req->arg2 = arg2;

A few stub routines (such as XCreateGC() and XCreatePixmap()) return a resource ID to the caller but pass a resource ID as an argument to the protocol request. Such routines use the macro XAllocID to allocate a resource ID from the range of IDs that were assigned to this client when it opened the connection.

rid = req->rid = XAllocID();
return (rid);

Finally, some stub routines transmit a fixed amount of variable length data after the request. Typically, these routines (such as XMoveWindow() and XSetBackground()) are special cases of more general functions like XMoveResizeWindow() and XChangeGC(). These special case routines use GetReqExtra(), which is the same as GetReq except that it takes an additional argument (the number of extra bytes to allocate in the output buffer after the request structure). This number should always be a multiple of four.

Some protocol requests take additional variable length data that follow the xDoSomethingReq structure. The format of this data varies from request to request. Some requests require a sequence of 8-bit bytes, others a sequence of 16-bit or 32-bit entities, and still others a sequence of structures.

It is necessary to add the length of any variable length data to the length field of the request structure. That length field is in units of 32-bit longwords. If the data is a string or other sequence of 8-bit bytes, then you must round the length up and shift it before adding:

req->length += (nbytes+3)>>2;

To transmit variable length data, use the Data macros. If the data fits into the output buffer, then this macro copies it to the buffer. If it does not fit, however, the Data macro calls _XSend(), which transmits first the contents of the buffer and then your data. The Data macros take three arguments: the Display, a pointer to the beginning of the data, and the number of bytes to be sent.

Data(display, (char *) data, nbytes);
Data16(display, (short *) data, nbytes);
Data32(display, (long *) data, nbytes);

Data(), Data16(), and Data32() are macros that may use their last argument more than once, so that argument should be a variable rather than an expression such as “nitems*sizeof(item)”. You should do that kind of computation in a separate statement before calling them. Use the appropriate macro when sending byte, short, or long data.

If the protocol request requires a reply, then call the procedure _XSend instead of the Data macro. _XSend takes the same arguments, but because it sends your data immediately instead of copying it into the output buffer (which would later be flushed anyway by the following call on _XReply()), it is faster.

If the protocol request has a reply, then call _XReply after you have finished dealing with all the fixed and variable length arguments. _XReply flushes the output buffer and waits for an xReply packet to arrive. If any events arrive in the meantime, _XReply places them in the queue for later use.

Status _XReply(display, rep, extra, discard)
      Display *display;
      xReply *rep;
      int extra;
      Bool discard;

_XReply waits for a reply packet and copies its contents into the specified rep. _XReply handles error and event packets that occur before the reply is received. _XReply takes four arguments:

Because most reply structures are 32 bytes long, the third argument is usually 0. The only core protocol exceptions are the replies to XGetWindowAttributes(), XQueryFont(), XQueryKeymap(), and XGetKeyboardControl(), which have longer replies.

The last argument should be False if the reply structure is followed by additional variable length data (such as a list or string). It should be True if there is not any variable length data.

---

Note: This last argument is provided for upward-compatibility reasons to allow a client to communicate properly with a hypothetical later version of the server that sends more data than the client expected. For example, some later version of XGetWindowAttributes might use a larger, but compatible, xGetWindowAttributesReply that contains additional attribute data at the end.

---

_XReply returns True if it received a reply successfully or False if it received any sort of error.

For a request with a reply that is not followed by variable length data, you write something like:

_XReply(display, (xReply *)&rep, 0, True);
*ret1 = rep.ret1;
*ret2 = rep.ret2;
*ret3 = rep.ret3;
UnlockDisplay()(dpy);
SyncHandle();
return (rep.ret4);
}

If there is variable length data after the reply, change the True to False, and use the appropriate _XRead function to read the variable length data.

_XRead(display, data_return, nbytes)
       Display *display;
       char *data_return;
       long nbytes;

_XRead reads the specified number of bytes into data_return.

_XRead16(display, data_return, nbytes)
       Display *display;
       short *data_return;
       long nbytes;

_XRead32(display, data_return, nbytes)
       Display *display;
       long *data_return;
       long nbytes;

_XRead16Pad(display, data_return, nbytes)
       Display *display;
       short *data_return;
       long nbytes;

_XReadPad(display, data_return, nbytes)
       Display *display;
       char *data_return;
       long nbytes;

To ease debugging, each routine should have a call, just before returning to the user, to a routine called SyncHandle(). This routine generally is implemented as a macro. If synchronous mode is enabled (see XSynchronize()), the request is sent immediately. The library, however, waits until any error the routine could generate at the server has been handled.

To support the possible reentry of these routines, you must observe several conventions when allocating and deallocating memory, most often done when returning data to the user from the window system of a size the caller could not know in advance (for example, a list of fonts or a list of extensions). The standard C library routines on many systems are not protected against signals or other multithreaded uses. The following analogies to standard I/O library routines have been defined:

These should be used in place of any calls you would make to the normal C library routines.

If you need a single scratch buffer inside a critical section (for example, to pack and unpack data to and from the wire protocol), the general memory allocators may be too expensive to use (particularly in output routines, which are performance critical). The routine below returns a scratch buffer for your use:

char *_XAllocScratch(display, nbytes)
      Display *display;
      unsigned long nbytes;

This storage must only be used inside of the critical section of your stub.

Many machine architectures, including many of the more recent RISC architectures, do not correctly access data at unaligned locations; their compilers pad out structures to preserve this characteristic. Many other machines capable of unaligned references pad inside of structures as well to preserve alignment, because accessing aligned data is usually much faster. Because the library and the server use structures to access data at arbitrary points in a byte stream, all data in request and reply packets must be naturally aligned; that is, 16-bit data starts on 16-bit boundaries in the request and 32-bit data on 32-bit boundaries. All requests must be a multiple of 32 bits in length to preserve the natural alignment in the data stream. You must pad structures out to 32-bit boundaries. Pad information does not have to be zeroed unless you want to preserve such fields for future use in your protocol requests. Floating point varies radically between machines and should be avoided completely if at all possible.

This code may run on machines with 16-bit ints. So, if any integer argument, variable, or return value either can take only nonnegative values or is declared as a CARD16 in the protocol, be sure to declare it as unsigned int and not as int. (This, of course, does not apply to Booleans or enumerations.)

Similarly, if any integer argument or return value is declared CARD32 in the protocol, declare it as an unsigned long and not as int or long. This also goes for any internal variables that may take on values larger than the maximum 16-bit unsigned int.

The library currently assumes that a char is 8 bits, a short is 16 bits, an int is 16 or 32 bits, and a long is 32 bits. The PackData macro is a half-hearted attempt to deal with the possibility of 32 bit shorts. However, much more work is needed to make this work properly.

The remaining problem a writer of an extension stub routine faces that the core protocol does not face is to map from the call to the proper major and minor opcodes. While there are a number of strategies, the simplest and fastest is outlined below.