Tk_GetGC manual page - Library Procedures

Tcl 8.7/Tk8.7a5 Documentation > Tk C API > GetGC

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NAME

Tk_GetGC, Tk_FreeGC — maintain database of read-only graphics contexts

SYNOPSIS

#include <tk.h>
GC
Tk_GetGC(tkwin, valueMask, valuePtr)
Tk_FreeGC(display, gc)

ARGUMENTS

Tk_Window tkwin (in): Token for window in which the graphics context will be used.
unsigned long valueMask (in): Mask of bits (such as GCForeground or GCStipple) indicating which fields of *valuePtr are valid.
XGCValues *valuePtr (in): Pointer to structure describing the desired values for the graphics context.
Display *display (in): Display for which gc was allocated.
GC gc (in): X identifier for graphics context that is no longer needed. Must have been allocated by Tk_GetGC.

Tk_GetGC and Tk_FreeGC manage a collection of graphics contexts being used by an application. The procedures allow graphics contexts to be shared, thereby avoiding the server overhead that would be incurred if a separate GC were created for each use. Tk_GetGC takes arguments describing the desired graphics context and returns an X identifier for a GC that fits the description. The graphics context that is returned will have default values in all of the fields not specified explicitly by valueMask and valuePtr.

Tk_GetGC maintains a database of all the graphics contexts it has created. Whenever possible, a call to Tk_GetGC will return an existing graphics context rather than creating a new one. This approach can substantially reduce server overhead, so Tk_GetGC should generally be used in preference to the Xlib procedure XCreateGC, which creates a new graphics context on each call.

Since the return values of Tk_GetGC are shared, callers should never modify the graphics contexts returned by Tk_GetGC. If a graphics context must be modified dynamically, then it should be created by calling XCreateGC instead of Tk_GetGC.

When a graphics context is no longer needed, Tk_FreeGC should be called to release it. There should be exactly one call to Tk_FreeGC for each call to Tk_GetGC. When a graphics context is no longer in use anywhere (i.e. it has been freed as many times as it has been gotten) Tk_FreeGC will release it to the X server and delete it from the database.

KEYWORDS