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Window guide

Table of Contents

This guide introduces the window related functions of GLFW. There are also guides for the other areas of GLFW.

Window objects

The GLFWwindow object encapsulates both a window and a context. They are created with glfwCreateWindow and destroyed with glfwDestroyWindow, or glfwTerminate, if any remain. As the window and context are inseparably linked, the object pointer is used as both a context and window handle.

To see the event stream provided to the various window related callbacks, run the events test program.

Window creation

A window and its OpenGL or OpenGL ES context are created with glfwCreateWindow, which returns a handle to the created window object. For example, this creates a 640 by 480 windowed mode window:

GLFWwindow* window = glfwCreateWindow(640, 480, "My Title", NULL, NULL);

If window creation fails, NULL will be returned, so it is necessary to check the return value.

The window handle is passed to all window related functions and is provided to along with all input events, so event handlers can tell which window received the event.

Full screen windows

To create a full screen window, you need to specify which monitor the window should use. In most cases, the user's primary monitor is a good choice. For more information about retrieving monitors, see Retrieving monitors.

GLFWwindow* window = glfwCreateWindow(640, 480, "My Title", glfwGetPrimaryMonitor(), NULL);

Full screen windows cover the entire display area of a monitor, have no border or decorations.

Each field of the GLFWvidmode structure corresponds to a function parameter or window hint and combine to form the desired video mode for that window. The supported video mode most closely matching the desired video mode will be set for the chosen monitor as long as the window has input focus. For more information about retrieving video modes, see Video modes.

Video mode field Corresponds to
GLFWvidmode.width width parameter
GLFWvidmode.height height parameter
GLFWvidmode.redBits GLFW_RED_BITS hint
GLFWvidmode.greenBits GLFW_GREEN_BITS hint
GLFWvidmode.blueBits GLFW_BLUE_BITS hint
GLFWvidmode.refreshRate GLFW_REFRESH_RATE hint

Once you have a full screen window, you can change its resolution with glfwSetWindowSize. The new video mode will be selected and set the same way as the video mode chosen by glfwCreateWindow.

By default, the original video mode of the monitor will be restored and the window iconified if it loses input focus, to allow the user to switch back to the desktop. This behavior can be disabled with the GLFW_AUTO_ICONIFY window hint, for example if you wish to simultaneously cover multiple windows with full screen windows.

"Windowed full screen" windows

If the closest match for the desired video mode is the current one, the video mode will not be changed, making window creation faster and application switching much smoother. This is sometimes called windowed full screen or borderless full screen window and counts as a full screen window. To create such a window, simply request the current video mode.

Window destruction

When a window is no longer needed, destroy it with glfwDestroyWindow.

Window destruction always succeeds. Before the actual destruction, all callbacks are removed so no further events will be delivered for the window. All windows remaining when glfwTerminate is called are destroyed as well.

When a full screen window is destroyed, the original video mode of its monitor is restored, but the gamma ramp is left untouched.

Window creation hints

There are a number of hints that can be set before the creation of a window and context. Some affect the window itself, others affect the framebuffer or context. These hints are set to their default values each time the library is initialized with glfwInit, can be set individually with glfwWindowHint and reset all at once to their defaults with glfwDefaultWindowHints.

Note that hints need to be set before the creation of the window and context you wish to have the specified attributes.

Hard and soft constraints

Some window hints are hard constraints. These must match the available capabilities exactly for window and context creation to succeed. Hints that are not hard constraints are matched as closely as possible, but the resulting context and framebuffer may differ from what these hints requested.

The following hints are always hard constraints:

The following additional hints are hard constraints when requesting an OpenGL context, but are ignored when requesting an OpenGL ES context:

Window related hints

GLFW_RESIZABLE specifies whether the windowed mode window will be resizable by the user. The window will still be resizable using the glfwSetWindowSize function. This hint is ignored for full screen windows.

GLFW_VISIBLE specifies whether the windowed mode window will be initially visible. This hint is ignored for full screen windows.

GLFW_DECORATED specifies whether the windowed mode window will have window decorations such as a border, a close widget, etc. An undecorated window may still allow the user to generate close events on some platforms. This hint is ignored for full screen windows.

GLFW_FOCUSED specifies whether the windowed mode window will be given input focus when created. This hint is ignored for full screen and initially hidden windows.

GLFW_AUTO_ICONIFY specifies whether the full screen window will automatically iconify and restore the previous video mode on input focus loss. This hint is ignored for windowed mode windows.

GLFW_FLOATING specifies whether the windowed mode window will be floating above other regular windows, also called topmost or always-on-top. This is intended primarily for debugging purposes and cannot be used to implement proper full screen windows. This hint is ignored for full screen windows.

Framebuffer related hints

GLFW_RED_BITS, GLFW_GREEN_BITS, GLFW_BLUE_BITS, GLFW_ALPHA_BITS, GLFW_DEPTH_BITS and GLFW_STENCIL_BITS specify the desired bit depths of the various components of the default framebuffer. GLFW_DONT_CARE means the application has no preference.

GLFW_ACCUM_RED_BITS, GLFW_ACCUM_GREEN_BITS, GLFW_ACCUM_BLUE_BITS and GLFW_ACCUM_ALPHA_BITS specify the desired bit depths of the various components of the accumulation buffer. GLFW_DONT_CARE means the application has no preference.

Accumulation buffers are a legacy OpenGL feature and should not be used in new code.

GLFW_AUX_BUFFERS specifies the desired number of auxiliary buffers. GLFW_DONT_CARE means the application has no preference.

Auxiliary buffers are a legacy OpenGL feature and should not be used in new code.

GLFW_STEREO specifies whether to use stereoscopic rendering. This is a hard constraint.

GLFW_SAMPLES specifies the desired number of samples to use for multisampling. Zero disables multisampling. GLFW_DONT_CARE means the application has no preference.

GLFW_SRGB_CAPABLE specifies whether the framebuffer should be sRGB capable.

GLFW_DOUBLEBUFFER specifies whether the framebuffer should be double buffered. You nearly always want to use double buffering. This is a hard constraint.

Monitor related hints

GLFW_REFRESH_RATE specifies the desired refresh rate for full screen windows. If set to GLFW_DONT_CARE, the highest available refresh rate will be used. This hint is ignored for windowed mode windows.

Context related hints

GLFW_CLIENT_API specifies which client API to create the context for. Possible values are GLFW_OPENGL_API and GLFW_OPENGL_ES_API. This is a hard constraint.

GLFW_CONTEXT_VERSION_MAJOR and GLFW_CONTEXT_VERSION_MINOR specify the client API version that the created context must be compatible with. The exact behavior of these hints depend on the requested client API.

OpenGL: GLFW_CONTEXT_VERSION_MAJOR and GLFW_CONTEXT_VERSION_MINOR are not hard constraints, but creation will fail if the OpenGL version of the created context is less than the one requested. It is therefore perfectly safe to use the default of version 1.0 for legacy code and you may still get backwards-compatible contexts of version 3.0 and above when available.
While there is no way to ask the driver for a context of the highest supported version, GLFW will attempt to provide this when you ask for a version 1.0 context, which is the default for these hints.
OpenGL ES: GLFW_CONTEXT_VERSION_MAJOR and GLFW_CONTEXT_VERSION_MINOR are not hard constraints, but creation will fail if the OpenGL ES version of the created context is less than the one requested. Additionally, OpenGL ES 1.x cannot be returned if 2.0 or later was requested, and vice versa. This is because OpenGL ES 3.x is backward compatible with 2.0, but OpenGL ES 2.0 is not backward compatible with 1.x.

GLFW_OPENGL_FORWARD_COMPAT specifies whether the OpenGL context should be forward-compatible, i.e. one where all functionality deprecated in the requested version of OpenGL is removed. This may only be used if the requested OpenGL version is 3.0 or above. If OpenGL ES is requested, this hint is ignored.

Forward-compatibility is described in detail in the OpenGL Reference Manual.

GLFW_OPENGL_DEBUG_CONTEXT specifies whether to create a debug OpenGL context, which may have additional error and performance issue reporting functionality. If OpenGL ES is requested, this hint is ignored.

GLFW_OPENGL_PROFILE specifies which OpenGL profile to create the context for. Possible values are one of GLFW_OPENGL_CORE_PROFILE or GLFW_OPENGL_COMPAT_PROFILE, or GLFW_OPENGL_ANY_PROFILE to not request a specific profile. If requesting an OpenGL version below 3.2, GLFW_OPENGL_ANY_PROFILE must be used. If OpenGL ES is requested, this hint is ignored.

OpenGL profiles are described in detail in the OpenGL Reference Manual.

GLFW_CONTEXT_ROBUSTNESS specifies the robustness strategy to be used by the context. This can be one of GLFW_NO_RESET_NOTIFICATION or GLFW_LOSE_CONTEXT_ON_RESET, or GLFW_NO_ROBUSTNESS to not request a robustness strategy.

GLFW_CONTEXT_RELEASE_BEHAVIOR specifies the release behavior to be used by the context. Possible values are one of GLFW_ANY_RELEASE_BEHAVIOR, GLFW_RELEASE_BEHAVIOR_FLUSH or GLFW_RELEASE_BEHAVIOR_NONE. If the behavior is GLFW_ANY_RELEASE_BEHAVIOR, the default behavior of the context creation API will be used. If the behavior is GLFW_RELEASE_BEHAVIOR_FLUSH, the pipeline will be flushed whenever the context is released from being the current one. If the behavior is GLFW_RELEASE_BEHAVIOR_NONE, the pipeline will not be flushed on release.

Context release behaviors are described in detail by the GL_KHR_context_flush_control extension.

Supported and default values

Window hint Default value Supported values
GLFW_RESIZABLE GL_TRUE GL_TRUE or GL_FALSE
GLFW_VISIBLE GL_TRUE GL_TRUE or GL_FALSE
GLFW_DECORATED GL_TRUE GL_TRUE or GL_FALSE
GLFW_FOCUSED GL_TRUE GL_TRUE or GL_FALSE
GLFW_AUTO_ICONIFY GL_TRUE GL_TRUE or GL_FALSE
GLFW_FLOATING GL_FALSE GL_TRUE or GL_FALSE
GLFW_RED_BITS 8 0 to INT_MAX or GLFW_DONT_CARE
GLFW_GREEN_BITS 8 0 to INT_MAX or GLFW_DONT_CARE
GLFW_BLUE_BITS 8 0 to INT_MAX or GLFW_DONT_CARE
GLFW_ALPHA_BITS 8 0 to INT_MAX or GLFW_DONT_CARE
GLFW_DEPTH_BITS 24 0 to INT_MAX or GLFW_DONT_CARE
GLFW_STENCIL_BITS 8 0 to INT_MAX or GLFW_DONT_CARE
GLFW_ACCUM_RED_BITS 0 0 to INT_MAX or GLFW_DONT_CARE
GLFW_ACCUM_GREEN_BITS 0 0 to INT_MAX or GLFW_DONT_CARE
GLFW_ACCUM_BLUE_BITS 0 0 to INT_MAX or GLFW_DONT_CARE
GLFW_ACCUM_ALPHA_BITS 0 0 to INT_MAX or GLFW_DONT_CARE
GLFW_AUX_BUFFERS 0 0 to INT_MAX or GLFW_DONT_CARE
GLFW_SAMPLES 0 0 to INT_MAX or GLFW_DONT_CARE
GLFW_REFRESH_RATE GLFW_DONT_CARE 0 to INT_MAX or GLFW_DONT_CARE
GLFW_STEREO GL_FALSE GL_TRUE or GL_FALSE
GLFW_SRGB_CAPABLE GL_FALSE GL_TRUE or GL_FALSE
GLFW_DOUBLEBUFFER GL_TRUE GL_TRUE or GL_FALSE
GLFW_CLIENT_API GLFW_OPENGL_API GLFW_OPENGL_API or GLFW_OPENGL_ES_API
GLFW_CONTEXT_VERSION_MAJOR 1 Any valid major version number of the chosen client API
GLFW_CONTEXT_VERSION_MINOR 0 Any valid minor version number of the chosen client API
GLFW_CONTEXT_ROBUSTNESS GLFW_NO_ROBUSTNESS GLFW_NO_ROBUSTNESS, GLFW_NO_RESET_NOTIFICATION or GLFW_LOSE_CONTEXT_ON_RESET
GLFW_CONTEXT_RELEASE_BEHAVIOR GLFW_ANY_RELEASE_BEHAVIOR GLFW_ANY_RELEASE_BEHAVIOR, GLFW_RELEASE_BEHAVIOR_FLUSH or GLFW_RELEASE_BEHAVIOR_NONE
GLFW_OPENGL_FORWARD_COMPAT GL_FALSE GL_TRUE or GL_FALSE
GLFW_OPENGL_DEBUG_CONTEXT GL_FALSE GL_TRUE or GL_FALSE
GLFW_OPENGL_PROFILE GLFW_OPENGL_ANY_PROFILE GLFW_OPENGL_ANY_PROFILE, GLFW_OPENGL_COMPAT_PROFILE or GLFW_OPENGL_CORE_PROFILE

Window event processing

See Event processing.

Window properties and events

User pointer

Each window has a user pointer that can be set with glfwSetWindowUserPointer and fetched with glfwGetWindowUserPointer. This can be used for any purpose you need and will not be modified by GLFW throughout the life-time of the window.

The initial value of the pointer is NULL.

Window closing and close flag

When the user attempts to close the window, for example by clicking the close widget or using a key chord like Alt+F4, the close flag of the window is set. The window is however not actually destroyed and, unless you watch for this state change, nothing further happens.

The current state of the close flag is returned by glfwWindowShouldClose and can be set or cleared directly with glfwSetWindowShouldClose. A common pattern is to use the close flag as a main loop condition.

while (!glfwWindowShouldClose(window))
{
render(window);
glfwSwapBuffers(window);
}

If you wish to be notified when the user attempts to close a window, set a close callback.

glfwSetWindowCloseCallback(window, window_close_callback);

The callback function is called directly after the close flag has been set. It can be used for example to filter close requests and clear the close flag again unless certain conditions are met.

void window_close_callback(GLFWwindow* window)
{
if (!time_to_close)
glfwSetWindowShouldClose(window, GL_FALSE);
}

Window size

The size of a window can be changed with glfwSetWindowSize. For windowed mode windows, this sets the size, in screen coordinates of the client area or content area of the window. The window system may impose limits on window size.

glfwSetWindowSize(window, 640, 480);

For full screen windows, the specified size becomes the new resolution of the window's desired video mode. The video mode most closely matching the new desired video mode is set immediately. The window is resized to fit the resolution of the set video mode.

If you wish to be notified when a window is resized, whether by the user or the system, set a size callback.

glfwSetWindowSizeCallback(window, window_size_callback);

The callback function receives the new size, in screen coordinates, of the client area of the window when it is resized.

void window_size_callback(GLFWwindow* window, int width, int height)
{
}

There is also glfwGetWindowSize for directly retrieving the current size of a window.

int width, height;
glfwGetWindowSize(window, &width, &height);
Note
Do not pass the window size to glViewport or other pixel-based OpenGL calls. The window size is in screen coordinates, not pixels. Use the framebuffer size, which is in pixels, for pixel-based calls.

The above functions work with the size of the client area, but decorated windows typically have title bars and window frames around this rectangle. You can retrieve the extents of these with glfwGetWindowFrameSize.

int left, top, right, bottom;
glfwGetWindowFrameSize(window, &left, &top, &right, &bottom);

The returned values are the distances, in screen coordinates, from the edges of the client area to the corresponding edges of the full window. As they are distances and not coordinates, they are always zero or positive.

Framebuffer size

While the size of a window is measured in screen coordinates, OpenGL works with pixels. The size you pass into glViewport, for example, should be in pixels. On some machines screen coordinates and pixels are the same, but on others they will not be. There is a second set of functions to retrieve the size, in pixels, of the framebuffer of a window.

If you wish to be notified when the framebuffer of a window is resized, whether by the user or the system, set a size callback.

glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

The callback function receives the new size of the framebuffer when it is resized, which can for example be used to update the OpenGL viewport.

void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
glViewport(0, 0, width, height);
}

There is also glfwGetFramebufferSize for directly retrieving the current size of the framebuffer of a window.

int width, height;
glfwGetFramebufferSize(window, &width, &height);
glViewport(0, 0, width, height);

The size of a framebuffer may change independently of the size of a window, for example if the window is dragged between a regular monitor and a high-DPI one.

Window position

The position of a windowed-mode window can be changed with glfwSetWindowPos. This moves the window so that the upper-left corner of its client area has the specified screen coordinates. The window system may put limitations on window placement.

glfwSetWindowPos(window, 100, 100);

If you wish to be notified when a window is moved, whether by the user, system or your own code, set a position callback.

glfwSetWindowPosCallback(window, window_pos_callback);

The callback function receives the new position of the upper-left corner of the client area when the window is moved.

void window_pos_callback(GLFWwindow* window, int xpos, int ypos)
{
}

There is also glfwGetWindowPos for directly retrieving the current position of the client area of the window.

int xpos, ypos;
glfwGetWindowPos(window, &xpos, &ypos);

Window title

All GLFW windows have a title, although undecorated or full screen windows may not display it or only display it in a task bar or similar interface. You can set a UTF-8 encoded window title with glfwSetWindowTitle.

glfwSetWindowTitle(window, "My Window");

The specified string is copied before the function returns, so there is no need to keep it around.

As long as your source file is encoded as UTF-8, you can use any Unicode characters directly in the source.

glfwSetWindowTitle(window, "プラネテス");

Window monitor

Full screen windows are associated with a specific monitor. You can get the handle for this monitor with glfwGetWindowMonitor.

GLFWmonitor* monitor = glfwGetWindowMonitor(window);

This monitor handle is one of those returned by glfwGetMonitors.

For windowed mode windows, this function returns NULL. This is the recommended way to tell full screen windows from windowed mode windows.

Window iconification

Windows can be iconified (i.e. minimized) with glfwIconifyWindow.

When a full screen window is iconified, the original video mode of its monitor is restored until the user or application restores the window.

Iconified windows can be restored with glfwRestoreWindow.

When a full screen window is restored, the desired video mode is restored to its monitor as well.

If you wish to be notified when a window is iconified or restored, whether by the user, system or your own code, set a iconify callback.

glfwSetWindowIconifyCallback(window, window_iconify_callback);

The callback function receives changes in the iconification state of the window.

void window_iconify_callback(GLFWwindow* window, int iconified)
{
if (iconified)
{
// The window was iconified
}
else
{
// The window was restored
}
}

You can also get the current iconification state with glfwGetWindowAttrib.

int iconified = glfwGetWindowAttrib(window, GLFW_ICONIFIED);

Window visibility

Windowed mode windows can be hidden with glfwHideWindow.

This makes the window completely invisible to the user, including removing it from the task bar, dock or window list. Full screen windows cannot be hidden and calling glfwHideWindow on a full screen window does nothing.

Hidden windows can be shown with glfwShowWindow.

Windowed mode windows can be created initially hidden with the GLFW_VISIBLE window hint. Windows created hidden are completely invisible to the user until shown. This can be useful if you need to set up your window further before showing it, for example moving it to a specific location.

You can also get the current visibility state with glfwGetWindowAttrib.

int visible = glfwGetWindowAttrib(window, GLFW_VISIBLE);

Window input focus

If you wish to be notified when a window gains or loses input focus, whether by the user, system or your own code, set a focus callback.

glfwSetWindowFocusCallback(window, window_focus_callback);

The callback function receives changes in the input focus state of the window.

void window_focus_callback(GLFWwindow* window, int focused)
{
if (focused)
{
// The window gained input focus
}
else
{
// The window lost input focus
}
}

You can also get the current input focus state with glfwGetWindowAttrib.

int focused = glfwGetWindowAttrib(window, GLFW_FOCUSED);

Window damage and refresh

If you wish to be notified when the contents of a window is damaged and needs to be refreshed, set a window refresh callback.

glfwSetWindowRefreshCallback(m_handle, window_refresh_callback);

The callback function is called when the contents of the window needs to be refreshed.

void window_refresh_callback(GLFWwindow* window)
{
draw_editor_ui(window);
glfwSwapBuffers(window);
}
Note
On compositing window systems such as Aero, Compiz or Aqua, where the window contents are saved off-screen, this callback might only be called when the window or framebuffer is resized.

Window attributes

Windows have a number of attributes that can be returned using glfwGetWindowAttrib. Some reflect state that may change during the lifetime of the window, while others reflect the corresponding hints and are fixed at the time of creation. Some are related to the actual window and others to its context.

{
// window has input focus
}

Window related attributes

GLFW_FOCUSED indicates whether the specified window has input focus. Initial input focus is controlled by the window hint with the same name.

GLFW_ICONIFIED indicates whether the specified window is iconified, whether by the user or with glfwIconifyWindow.

GLFW_VISIBLE indicates whether the specified window is visible. Window visibility can be controlled with glfwShowWindow and glfwHideWindow and initial visibility is controlled by the window hint with the same name.

GLFW_RESIZABLE indicates whether the specified window is resizable by the user. This is set on creation with the window hint with the same name.

GLFW_DECORATED indicates whether the specified window has decorations such as a border, a close widget, etc. This is set on creation with the window hint with the same name.

GLFW_FLOATING indicates whether the specified window is floating, also called topmost or always-on-top. This is controlled by the window hint with the same name.

Context related attributes

GLFW_CLIENT_API indicates the client API provided by the window's context; either GLFW_OPENGL_API or GLFW_OPENGL_ES_API.

GLFW_CONTEXT_VERSION_MAJOR, GLFW_CONTEXT_VERSION_MINOR and GLFW_CONTEXT_REVISION indicate the client API version of the window's context.

GLFW_OPENGL_FORWARD_COMPAT is GL_TRUE if the window's context is an OpenGL forward-compatible one, or GL_FALSE otherwise.

GLFW_OPENGL_DEBUG_CONTEXT is GL_TRUE if the window's context is an OpenGL debug context, or GL_FALSE otherwise.

GLFW_OPENGL_PROFILE indicates the OpenGL profile used by the context. This is GLFW_OPENGL_CORE_PROFILE or GLFW_OPENGL_COMPAT_PROFILE if the context uses a known profile, or GLFW_OPENGL_ANY_PROFILE if the OpenGL profile is unknown or the context is an OpenGL ES context. Note that the returned profile may not match the profile bits of the context flags, as GLFW will try other means of detecting the profile when no bits are set.

GLFW_CONTEXT_ROBUSTNESS indicates the robustness strategy used by the context. This is GLFW_LOSE_CONTEXT_ON_RESET or GLFW_NO_RESET_NOTIFICATION if the window's context supports robustness, or GLFW_NO_ROBUSTNESS otherwise.

Framebuffer related attributes

The attributes of the default framebuffer (i.e. the framebuffer attached to the window) are not provided by this function but can be queried with both OpenGL and OpenGL ES.

If you are using version 3.0 or later of OpenGL or OpenGL ES, the glGetFramebufferAttachmentParameteriv function can be used to retrieve the number of bits for the red, green, blue, alpha, depth and stencil buffer channels. Otherwise, the glGetIntegerv function can be used.

The number of MSAA samples are always retrieved with glGetIntegerv. For contexts supporting framebuffer objects, the number of samples of the currently bound framebuffer is returned.

Attribute glGetIntegerv glGetFramebufferAttachmentParameteriv
Red bits GL_RED_BITS GL_FRAMEBUFFER_ATTACHMENT_RED_SIZE
Green bits GL_GREEN_BITS GL_FRAMEBUFFER_ATTACHMENT_GREEN_SIZE
Blue bits GL_BLUE_BITS GL_FRAMEBUFFER_ATTACHMENT_BLUE_SIZE
Alpha bits GL_ALPHA_BITS GL_FRAMEBUFFER_ATTACHMENT_ALPHA_SIZE
Depth bits GL_DEPTH_BITS GL_FRAMEBUFFER_ATTACHMENT_DEPTH_SIZE
Stencil bits GL_STENCIL_BITS GL_FRAMEBUFFER_ATTACHMENT_STENCIL_SIZE
MSAA samples GL_SAMPLES Not provided by this function

When calling glGetFramebufferAttachmentParameteriv, the red, green, blue and alpha sizes can be queried from the GL_BACK_LEFT, while the depth and stencil sizes can be queried from the GL_DEPTH and GL_STENCIL attachments, respectively.

Buffer swapping

GLFW windows are by default double buffered. That means that you have two rendering buffers; a front buffer and a back buffer. The front buffer is the one being displayed and the back buffer the one you render to.

When the entire frame has been rendered, it is time to swap the back and the front buffers in order to display what has been rendered and begin rendering a new frame. This is done with glfwSwapBuffers.

Sometimes it can be useful to select when the buffer swap will occur. With the function glfwSwapInterval it is possible to select the minimum number of monitor refreshes the driver wait should from the time glfwSwapBuffers was called before swapping the buffers:

If the interval is zero, the swap will take place immediately when glfwSwapBuffers is called without waiting for a refresh. Otherwise at least interval retraces will pass between each buffer swap. Using a swap interval of zero can be useful for benchmarking purposes, when it is not desirable to measure the time it takes to wait for the vertical retrace. However, a swap interval of one lets you avoid tearing.

Note that this may not work on all machines, as some drivers have user-controlled settings that override any swap interval the application requests. It is also by default disabled on Windows Vista and later when using DWM (Aero), as using it there sometimes leads to severe jitter. You can forcibly enable it for machines using DWM using Windows specific CMake options.