Introduction to the API

This guide introduces the basic concepts of GLFW and describes initialization, error handling and API guarantees and limitations. For a broad but shallow tutorial, see Getting started instead. For details on a specific function in this category, see the Initialization, version and error reference.

There are also guides for the other areas of GLFW.

Initialization and termination

Before most GLFW functions may be called, the library must be initialized. This initialization checks what features are available on the machine, enumerates monitors, initializes the timer and performs any required platform-specific initialization.

Only the following functions may be called before the library has been successfully initialized, and only from the main thread.

Calling any other function before successful initialization will cause a GLFW_NOT_INITIALIZED error.

Initializing GLFW

The library is initialized with glfwInit, which returns GLFW_FALSE if an error occurred.

if (!glfwInit())
// Handle initialization failure
int glfwInit(void)
Initializes the GLFW library.

If any part of initialization fails, any parts that succeeded are terminated as if glfwTerminate had been called. The library only needs to be initialized once and additional calls to an already initialized library will return GLFW_TRUE immediately.

Once the library has been successfully initialized, it should be terminated before the application exits. Modern systems are very good at freeing resources allocated by programs that exit, but GLFW sometimes has to change global system settings and these might not be restored without termination.

macOS: When the library is initialized the main menu and dock icon are created. These are not desirable for a command-line only program. The creation of the main menu and dock icon can be disabled with the GLFW_COCOA_MENUBAR init hint.

Initialization hints

Initialization hints are set before glfwInit and affect how the library behaves until termination. Hints are set with glfwInitHint.

void glfwInitHint(int hint, int value)
Sets the specified init hint to the desired value.
Joystick hat buttons init hint.
Definition: glfw3.h:1264
#define GLFW_FALSE
Definition: glfw3.h:322

The values you set hints to are never reset by GLFW, but they only take effect during initialization. Once GLFW has been initialized, any values you set will be ignored until the library is terminated and initialized again.

Some hints are platform specific. These may be set on any platform but they will only affect their specific platform. Other platforms will ignore them. Setting these hints requires no platform specific headers or functions.

Shared init hints

GLFW_PLATFORM specifies the platform to use for windowing and input. Possible values are GLFW_ANY_PLATFORM, GLFW_PLATFORM_WIN32, GLFW_PLATFORM_COCOA, GLFW_PLATFORM_X11, GLFW_PLATFORM_WAYLAND and GLFW_PLATFORM_NULL. The default value is GLFW_ANY_PLATFORM, which will choose any platform the library includes support for except for the Null backend.

GLFW_JOYSTICK_HAT_BUTTONS specifies whether to also expose joystick hats as buttons, for compatibility with earlier versions of GLFW that did not have glfwGetJoystickHats. Possible values are GLFW_TRUE and GLFW_FALSE.

GLFW_ANGLE_PLATFORM_TYPE specifies the platform type (rendering backend) to request when using OpenGL ES and EGL via ANGLE. If the requested platform type is unavailable, ANGLE will use its default. Possible values are one of GLFW_ANGLE_PLATFORM_TYPE_NONE, GLFW_ANGLE_PLATFORM_TYPE_OPENGL, GLFW_ANGLE_PLATFORM_TYPE_OPENGLES, GLFW_ANGLE_PLATFORM_TYPE_D3D9, GLFW_ANGLE_PLATFORM_TYPE_D3D11, GLFW_ANGLE_PLATFORM_TYPE_VULKAN and GLFW_ANGLE_PLATFORM_TYPE_METAL.

The ANGLE platform type is specified via the EGL_ANGLE_platform_angle extension. This extension is not used if this hint is GLFW_ANGLE_PLATFORM_TYPE_NONE, which is the default value.

macOS specific init hints

GLFW_COCOA_CHDIR_RESOURCES specifies whether to set the current directory to the application to the Contents/Resources subdirectory of the application's bundle, if present. Possible values are GLFW_TRUE and GLFW_FALSE. This is ignored on other platforms.

GLFW_COCOA_MENUBAR specifies whether to create the menu bar and dock icon when GLFW is initialized. This applies whether the menu bar is created from a nib or manually by GLFW. Possible values are GLFW_TRUE and GLFW_FALSE. This is ignored on other platforms.

X11 specific init hints

GLFW_X11_XCB_VULKAN_SURFACE specifies whether to prefer the VK_KHR_xcb_surface extension for creating Vulkan surfaces, or whether to use the VK_KHR_xlib_surface extension. Possible values are GLFW_TRUE and GLFW_FALSE. This is ignored on other platforms.

Supported and default values

Initialization hint Default value Supported values

Runtime platform selection

GLFW can be compiled for more than one platform (window system) at once. This lets a single library binary support both X11 and Wayland on Linux and other Unix-like systems.

You can control platform selection via the GLFW_PLATFORM initialization hint. By default this is set to GLFW_ANY_PLATFORM, which will look for supported window systems in order of priority and select the first one it finds. It can also be set to any specific platform to have GLFW only look for that one.

Platform selection init hint.
Definition: glfw3.h:1274
Definition: glfw3.h:1302

This mechanism also provides the Null platform, which is always supported but needs to be explicitly requested. This platform is effectively a stub, emulating a window system on a single 1080p monitor, but will not interact with any actual window system.

Definition: glfw3.h:1303

You can test whether a library binary was compiled with support for a specific platform with glfwPlatformSupported.

int glfwPlatformSupported(int platform)
Returns whether the library includes support for the specified platform.
Definition: glfw3.h:1301

Once GLFW has been initialized, you can query which platform was selected with glfwGetPlatform.

int platform = glfwGetPlatform();
int glfwGetPlatform(void)
Returns the currently selected platform.

If you are using any native access functions, especially on Linux and other Unix-like systems, then you may need to check that you are calling the ones matching the selected platform.

Custom heap memory allocator

The heap memory allocator can be customized before initialization with glfwInitAllocator.

GLFWallocator allocator;
allocator.allocate = my_malloc;
allocator.reallocate = my_realloc;
allocator.deallocate = my_free;
allocator.user = NULL;
void glfwInitAllocator(const GLFWallocator *allocator)
Sets the init allocator to the desired value.
Definition: glfw3.h:2069
GLFWallocatefun allocate
Definition: glfw3.h:2070
GLFWdeallocatefun deallocate
Definition: glfw3.h:2072
GLFWreallocatefun reallocate
Definition: glfw3.h:2071
void * user
Definition: glfw3.h:2073

The allocator will be picked up at the beginning of initialization and will be used until GLFW has been fully terminated. Any allocator set after initialization will be picked up only at the next initialization.

The allocator will only be used for allocations that would have been made with the C standard library. Memory allocations that must be made with platform specific APIs will still use those.

The allocation function must have a signature matching GLFWallocatefun. It receives the desired size, in bytes, and the user pointer passed to glfwInitAllocator and returns the address to the allocated memory block.

void* my_malloc(size_t size, void* user)

The reallocation function must have a function signature matching GLFWreallocatefun. It receives the memory block to be reallocated, the new desired size, in bytes, and the user pointer passed to glfwInitAllocator and returns the address to the resized memory block.

void* my_realloc(void* block, size_t size, void* user)

The deallocation function must have a function signature matching GLFWdeallocatefun. It receives the memory block to be deallocated and the user pointer passed to glfwInitAllocator.

void my_free(void* block, void* user)

Terminating GLFW

Before your application exits, you should terminate the GLFW library if it has been initialized. This is done with glfwTerminate.

void glfwTerminate(void)
Terminates the GLFW library.

This will destroy any remaining window, monitor and cursor objects, restore any modified gamma ramps, re-enable the screensaver if it had been disabled and free any other resources allocated by GLFW.

Once the library is terminated, it is as if it had never been initialized and you will need to initialize it again before being able to use GLFW. If the library was not initialized or had already been terminated, it return immediately.

Error handling

Some GLFW functions have return values that indicate an error, but this is often not very helpful when trying to figure out what happened or why it occurred. Other functions have no return value reserved for errors, so error notification needs a separate channel. Finally, far from all GLFW functions have return values.

The last error code for the calling thread can be queried at any time with glfwGetError.

int code = glfwGetError(NULL);
if (code != GLFW_NO_ERROR)
No error has occurred.
Definition: glfw3.h:671
int glfwGetError(const char **description)
Returns and clears the last error for the calling thread.

If no error has occurred since the last call, GLFW_NO_ERROR (zero) is returned. The error is cleared before the function returns.

The error code indicates the general category of the error. Some error codes, such as GLFW_NOT_INITIALIZED has only a single meaning, whereas others like GLFW_PLATFORM_ERROR are used for many different errors.

GLFW often has more information about an error than its general category. You can retrieve a UTF-8 encoded human-readable description along with the error code. If no error has occurred since the last call, the description is set to NULL.

const char* description;
int code = glfwGetError(&description);
if (description)
display_error_message(code, description);

The retrieved description string is only valid until the next error occurs. This means you must make a copy of it if you want to keep it.

You can also set an error callback, which will be called each time an error occurs. It is set with glfwSetErrorCallback.

GLFWerrorfun glfwSetErrorCallback(GLFWerrorfun callback)
Sets the error callback.

The error callback receives the same error code and human-readable description returned by glfwGetError.

void error_callback(int code, const char* description)
display_error_message(code, description);

The error callback is called after the error is stored, so calling glfwGetError from within the error callback returns the same values as the callback argument.

The description string passed to the callback is only valid until the error callback returns. This means you must make a copy of it if you want to keep it.

Reported errors are never fatal. As long as GLFW was successfully initialized, it will remain initialized and in a safe state until terminated regardless of how many errors occur. If an error occurs during initialization that causes glfwInit to fail, any part of the library that was initialized will be safely terminated.

Do not rely on a currently invalid call to generate a specific error, as in the future that same call may generate a different error or become valid.

Coordinate systems

GLFW has two primary coordinate systems: the virtual screen and the window content area or content area. Both use the same unit: virtual screen coordinates, or just screen coordinates, which don't necessarily correspond to pixels.

Both the virtual screen and the content area coordinate systems have the X-axis pointing to the right and the Y-axis pointing down.

Window and monitor positions are specified as the position of the upper-left corners of their content areas relative to the virtual screen, while cursor positions are specified relative to a window's content area.

Because the origin of the window's content area coordinate system is also the point from which the window position is specified, you can translate content area coordinates to the virtual screen by adding the window position. The window frame, when present, extends out from the content area but does not affect the window position.

Almost all positions and sizes in GLFW are measured in screen coordinates relative to one of the two origins above. This includes cursor positions, window positions and sizes, window frame sizes, monitor positions and video mode resolutions.

Two exceptions are the monitor physical size, which is measured in millimetres, and framebuffer size, which is measured in pixels.

Pixels and screen coordinates may map 1:1 on your machine, but they won't on every other machine, for example on a Mac with a Retina display. The ratio between screen coordinates and pixels may also change at run-time depending on which monitor the window is currently considered to be on.

Guarantees and limitations

This section describes the conditions under which GLFW can be expected to function, barring bugs in the operating system or drivers. Use of GLFW outside of these limits may work on some platforms, or on some machines, or some of the time, or on some versions of GLFW, but it may break at any time and this will not be considered a bug.

Pointer lifetimes

GLFW will never free any pointer you provide to it and you must never free any pointer it provides to you.

Many GLFW functions return pointers to dynamically allocated structures, strings or arrays, and some callbacks are provided with strings or arrays. These are always managed by GLFW and should never be freed by the application. The lifetime of these pointers is documented for each GLFW function and callback. If you need to keep this data, you must copy it before its lifetime expires.

Many GLFW functions accept pointers to structures or strings allocated by the application. These are never freed by GLFW and are always the responsibility of the application. If GLFW needs to keep the data in these structures or strings, it is copied before the function returns.

Pointer lifetimes are guaranteed not to be shortened in future minor or patch releases.


GLFW event processing and object destruction are not reentrant. This means that the following functions must not be called from any callback function:

These functions may be made reentrant in future minor or patch releases, but functions not on this list will not be made non-reentrant.

Thread safety

Most GLFW functions must only be called from the main thread (the thread that calls main), but some may be called from any thread once the library has been initialized. Before initialization the whole library is thread-unsafe.

The reference documentation for every GLFW function states whether it is limited to the main thread.

Initialization, termination, event processing and the creation and destruction of windows, cursors and OpenGL and OpenGL ES contexts are all restricted to the main thread due to limitations of one or several platforms.

Because event processing must be performed on the main thread, all callbacks except for the error callback will only be called on that thread. The error callback may be called on any thread, as any GLFW function may generate errors.

The error code and description may be queried from any thread.

Empty events may be posted from any thread.

The window user pointer and close flag may be read and written from any thread, but this is not synchronized by GLFW.

These functions for working with OpenGL and OpenGL ES contexts may be called from any thread, but the window object is not synchronized by GLFW.

The raw timer functions may be called from any thread.

The regular timer may be used from any thread, but reading and writing the timer offset is not synchronized by GLFW.

Library version information may be queried from any thread.

Platform information may be queried from any thread.

All Vulkan related functions may be called from any thread.

GLFW uses synchronization objects internally only to manage the per-thread context and error states. Additional synchronization is left to the application.

Functions that may currently be called from any thread will always remain so, but functions that are currently limited to the main thread may be updated to allow calls from any thread in future releases.

Version compatibility

GLFW uses Semantic Versioning. This guarantees source and binary backward compatibility with earlier minor versions of the API. This means that you can drop in a newer version of the library and existing programs will continue to compile and existing binaries will continue to run.

Once a function or constant has been added, the signature of that function or value of that constant will remain unchanged until the next major version of GLFW. No compatibility of any kind is guaranteed between major versions.

Undocumented behavior, i.e. behavior that is not described in the documentation, may change at any time until it is documented.

If the reference documentation and the implementation differ, the reference documentation will almost always take precedence and the implementation will be fixed in the next release. The reference documentation will also take precedence over anything stated in a guide.

Event order

The order of arrival of related events is not guaranteed to be consistent across platforms. The exception is synthetic key and mouse button release events, which are always delivered after the window defocus event.

Version management

GLFW provides mechanisms for identifying what version of GLFW your application was compiled against as well as what version it is currently running against. If you are loading GLFW dynamically (not just linking dynamically), you can use this to verify that the library binary is compatible with your application.

Compile-time version

The compile-time version of GLFW is provided by the GLFW header with the GLFW_VERSION_MAJOR, GLFW_VERSION_MINOR and GLFW_VERSION_REVISION macros.

printf("Compiled against GLFW %i.%i.%i\n",
The major version number of the GLFW header.
Definition: glfw3.h:288
The revision number of the GLFW header.
Definition: glfw3.h:302
The minor version number of the GLFW header.
Definition: glfw3.h:295

Run-time version

The run-time version can be retrieved with glfwGetVersion, a function that may be called regardless of whether GLFW is initialized.

int major, minor, revision;
glfwGetVersion(&major, &minor, &revision);
printf("Running against GLFW %i.%i.%i\n", major, minor, revision);
void glfwGetVersion(int *major, int *minor, int *rev)
Retrieves the version of the GLFW library.

Version string

GLFW 3 also provides a compile-time generated version string that describes the version, platform, compiler and any platform-specific compile-time options. This is primarily intended for submitting bug reports, to allow developers to see which code paths are enabled in a binary.

The version string is returned by glfwGetVersionString, a function that may be called regardless of whether GLFW is initialized.

Do not use the version string to parse the GLFW library version. The glfwGetVersion function already provides the version of the running library binary.

Do not use the version string to parse what platforms are supported. The glfwPlatformSupported function lets you query platform support.

GLFW 3.4: The format of this string was changed to support the addition of runtime platform selection.

The format of the string is as follows:

  • The version of GLFW
  • For each supported platform:
    • The name of the window system API
    • The name of the window system specific context creation API, if applicable
  • The names of the always supported context creation APIs EGL and OSMesa
  • Any additional compile-time options, APIs and (on Windows) what compiler was used

For example, GLFW 3.4 compiled as a DLL for Windows with MinGW may have a version string like this:

3.4.0 Win32 WGL Null EGL OSMesa MinGW DLL

While GLFW compiled as as static library for Linux with both Wayland and X11 enabled may have a version string like this:

3.4.0 Wayland X11 GLX Null EGL OSMesa monotonic