libtk/doc/CrtSelHdlr.3

#include <tk.h>
Tk_CreateSelHandler(tkwin, selection, target, proc, clientData, format)
Tk_DeleteSelHandler(tkwin, selection, target)
 ARGUMENTS
.AS Tk_SelectionProc clientData
.AP Tk_Window tkwin in
Window for which proc will provide selection information.
.AP Atom selection in
The name of the selection for which proc will provide
selection information.
.AP Atom target in
Form in which proc can provide the selection (e.g. STRING
or FILE_NAME). Corresponds to type arguments in selection
commands.
.AP Tk_SelectionProc *proc in
Procedure to invoke whenever the selection is owned by tkwin
and the selection contents are requested in the format given by
target.
.AP ClientData clientData in
Arbitrary one-word value to pass to proc.
.AP Atom format in
If the selection requestor isn't in this process, format determines
the representation used to transmit the selection to its
requestor.
E
 DESCRIPTION

Tk_CreateSelHandler arranges for a particular procedure
(proc) to be called whenever selection is owned by
tkwin and the selection contents are requested in the
form given by target.
Target should be one of
the entries defined in the left column of Table 2 of the
X Inter-Client Communication Conventions Manual (ICCCM) or
any other form in which an application is willing to present
the selection. The most common form is STRING.

Proc should have arguments and result that match the
type Tk_SelectionProc:
.CS
typedef int Tk_SelectionProc(
 ClientData clientData,
 int offset,
 char *buffer,
 int maxBytes);
.CE
The clientData parameter to proc is a copy of the
clientData argument given to Tk_CreateSelHandler.
Typically, clientData points to a data
structure containing application-specific information that is
needed to retrieve the selection. Offset specifies an
offset position into the selection, buffer specifies a
location at which to copy information about the selection, and
maxBytes specifies the amount of space available at
buffer. Proc should place a NULL-terminated string
at buffer containing maxBytes or fewer characters
(not including the terminating NULL), and it should return a
count of the number of non-NULL characters stored at
buffer. If the selection no longer exists (e.g. it once
existed but the user deleted the range of characters containing
it), then proc should return -1.

When transferring large selections, Tk will break them up into
smaller pieces (typically a few thousand bytes each) for more
efficient transmission. It will do this by calling proc
one or more times, using successively higher values of offset
to retrieve successive portions of the selection. If proc
returns a count less than maxBytes it means that the entire
remainder of the selection has been returned. If proc's return
value is maxBytes it means there may be additional information
in the selection, so Tk must make another call to proc to
retrieve the next portion.

Proc always returns selection information in the form of a
character string. However, the ICCCM allows for information to
be transmitted from the selection owner to the selection requestor
in any of several formats, such as a string, an array of atoms, an
array of integers, etc. The format argument to
Tk_CreateSelHandler indicates what format should be used to
transmit the selection to its requestor (see the middle column of
Table 2 of the ICCCM for examples). If format is not
STRING, then Tk will take the value returned by proc and divided
it into fields separated by white space. If format is ATOM,
then Tk will return the selection as an array of atoms, with each
field in proc's result treated as the name of one atom. For
any other value of format, Tk will return the selection as an
array of 32-bit values where each field of proc's result is
treated as a number and translated to a 32-bit value. In any event,
the format atom is returned to the selection requestor along
with the contents of the selection.

If Tk_CreateSelHandler is called when there already exists a
handler for selection and target on tkwin, then the
existing handler is replaced with a new one.

Tk_DeleteSelHandler removes the handler given by tkwin,
selection, and target, if such a handler exists.
If there is no such handler then it has no effect.

 KEYWORDS
format, handler, selection, target