Added return by setters of previous callback.

src/init.c

@@ -190,8 +190,10 @@ GLFWAPI const char* glfwGetVersionString(void)
     return _glfwPlatformGetVersionString();
 }
 
-GLFWAPI void glfwSetErrorCallback(GLFWerrorfun cbfun)
+GLFWAPI GLFWerrorfun glfwSetErrorCallback(GLFWerrorfun cbfun)
 {
+    GLFWerrorfun previous = _glfwErrorCallback;
     _glfwErrorCallback = cbfun;
+    return previous;
 }
 

src/input.c

@@ -337,45 +337,79 @@ GLFWAPI void glfwSetCursorPos(GLFWwindow* handle, double xpos, double ypos)
     _glfwPlatformSetCursorPos(window, xpos, ypos);
 }
 
-GLFWAPI void glfwSetKeyCallback(GLFWwindow* handle, GLFWkeyfun cbfun)
+GLFWAPI GLFWkeyfun glfwSetKeyCallback(GLFWwindow* handle, GLFWkeyfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWkeyfun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.key;
     window->callbacks.key = cbfun;
+    return previous;
 }
 
-GLFWAPI void glfwSetCharCallback(GLFWwindow* handle, GLFWcharfun cbfun)
+GLFWAPI GLFWcharfun glfwSetCharCallback(GLFWwindow* handle, GLFWcharfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWcharfun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.character;
     window->callbacks.character = cbfun;
+    return previous;
 }
 
-GLFWAPI void glfwSetMouseButtonCallback(GLFWwindow* handle, GLFWmousebuttonfun cbfun)
+GLFWAPI GLFWmousebuttonfun glfwSetMouseButtonCallback(GLFWwindow* handle,
+                                                      GLFWmousebuttonfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWmousebuttonfun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.mouseButton;
     window->callbacks.mouseButton = cbfun;
+    return previous;
 }
 
-GLFWAPI void glfwSetCursorPosCallback(GLFWwindow* handle, GLFWcursorposfun cbfun)
+GLFWAPI GLFWcursorposfun glfwSetCursorPosCallback(GLFWwindow* handle,
+                                                  GLFWcursorposfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWcursorposfun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.cursorPos;
     window->callbacks.cursorPos = cbfun;
+    return previous;
 }
 
-GLFWAPI void glfwSetCursorEnterCallback(GLFWwindow* handle, GLFWcursorenterfun cbfun)
+GLFWAPI GLFWcursorenterfun glfwSetCursorEnterCallback(GLFWwindow* handle,
+                                                      GLFWcursorenterfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWcursorenterfun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.cursorEnter;
     window->callbacks.cursorEnter = cbfun;
+    return previous;
 }
 
-GLFWAPI void glfwSetScrollCallback(GLFWwindow* handle, GLFWscrollfun cbfun)
+GLFWAPI GLFWscrollfun glfwSetScrollCallback(GLFWwindow* handle,
+                                            GLFWscrollfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWscrollfun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.scroll;
     window->callbacks.scroll = cbfun;
+    return previous;
 }
 

src/monitor.c

@@ -289,10 +289,14 @@ GLFWAPI const char* glfwGetMonitorName(GLFWmonitor* handle)
     return monitor->name;
 }
 
-GLFWAPI void glfwSetMonitorCallback(GLFWmonitorfun cbfun)
+GLFWAPI GLFWmonitorfun glfwSetMonitorCallback(GLFWmonitorfun cbfun)
 {
-    _GLFW_REQUIRE_INIT();
+    GLFWmonitorfun previous;
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = _glfw.monitorCallback;
     _glfw.monitorCallback = cbfun;
+    return previous;
 }
 
 GLFWAPI const GLFWvidmode* glfwGetVideoModes(GLFWmonitor* handle, int* count)

src/window.c

@@ -582,46 +582,82 @@ GLFWAPI void* glfwGetWindowUserPointer(GLFWwindow* handle)
     return window->userPointer;
 }
 
-GLFWAPI void glfwSetWindowPosCallback(GLFWwindow* handle, GLFWwindowposfun cbfun)
+GLFWAPI GLFWwindowposfun glfwSetWindowPosCallback(GLFWwindow* handle,
+                                                  GLFWwindowposfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWwindowposfun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.pos;
     window->callbacks.pos = cbfun;
+    return previous;
 }
 
-GLFWAPI void glfwSetWindowSizeCallback(GLFWwindow* handle, GLFWwindowsizefun cbfun)
+GLFWAPI GLFWwindowsizefun glfwSetWindowSizeCallback(GLFWwindow* handle,
+                                                    GLFWwindowsizefun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWwindowsizefun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.size;
     window->callbacks.size = cbfun;
+    return previous;
 }
 
-GLFWAPI void glfwSetWindowCloseCallback(GLFWwindow* handle, GLFWwindowclosefun cbfun)
+GLFWAPI GLFWwindowclosefun glfwSetWindowCloseCallback(GLFWwindow* handle,
+                                                      GLFWwindowclosefun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWwindowclosefun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.close;
     window->callbacks.close = cbfun;
+    return previous;
 }
 
-GLFWAPI void glfwSetWindowRefreshCallback(GLFWwindow* handle, GLFWwindowrefreshfun cbfun)
+GLFWAPI GLFWwindowrefreshfun glfwSetWindowRefreshCallback(GLFWwindow* handle,
+                                                          GLFWwindowrefreshfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWwindowrefreshfun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.refresh;
     window->callbacks.refresh = cbfun;
+    return previous;
 }
 
-GLFWAPI void glfwSetWindowFocusCallback(GLFWwindow* handle, GLFWwindowfocusfun cbfun)
+GLFWAPI GLFWwindowfocusfun glfwSetWindowFocusCallback(GLFWwindow* handle,
+                                                      GLFWwindowfocusfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWwindowfocusfun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.focus;
     window->callbacks.focus = cbfun;
+    return previous;
 }
 
-GLFWAPI void glfwSetWindowIconifyCallback(GLFWwindow* handle, GLFWwindowiconifyfun cbfun)
+GLFWAPI GLFWwindowiconifyfun glfwSetWindowIconifyCallback(GLFWwindow* handle,
+                                                          GLFWwindowiconifyfun cbfun)
 {
     _GLFWwindow* window = (_GLFWwindow*) handle;
-    _GLFW_REQUIRE_INIT();
+    GLFWwindowiconifyfun previous;
+
+    _GLFW_REQUIRE_INIT_OR_RETURN(NULL);
+
+    previous = window->callbacks.iconify;
     window->callbacks.iconify = cbfun;
+    return previous;
 }
 
 GLFWAPI void glfwPollEvents(void)