Proc should have arguments and result that match the type Tk_SelectionProc:
typedef Tcl_Size Tk_SelectionProc(
void *clientData,
Tcl_Size offset,
char *buffer,
Tcl_Size maxBytes);
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.