glfwSetKeyCallback() returning NULL but apparently succeeding

support
1

I ran into this issue with a project where I was trying to extensively check for errors and was (seemingly) erroneously getting an error when setting callbacks despite the callbacks seemingly being registered and getting called appropriately. Here’s a simple C program I made which reproduces the issue:

#include <GLFW/glfw3.h>
#include <stdio.h>

void handleKey(GLFWwindow* window, int key, int scancode, int action, int mods) {
    if (action == GLFW_PRESS) {
        printf("Key pressed: %d\n", key);
    } else if (action == GLFW_RELEASE) {
        printf("Key released: %d\n", key);
    }
}

int main() {
    if (!glfwInit()) {
        printf("Failed to initialize GLFW\n");
        return 1;
    }

    GLFWwindow* window = glfwCreateWindow(800, 600, "Test", NULL, NULL);
    if (window == NULL) {
        printf("Failed to create window\n");
        glfwTerminate();
        return 1;
    }

    printf("Expected key handler: %p\n", handleKey);
    printf("Handler set: %p\n", glfwSetKeyCallback(window, handleKey));

    glfwMakeContextCurrent(window);
    while (!glfwWindowShouldClose(window)) {
        glfwSwapBuffers(window);
        glfwPollEvents();
    }

    glfwMakeContextCurrent(NULL);
    glfwTerminate();
    return 0;
}

Sample output:
Expected key handler: 00007FF665411000
Handler set: 0000000000000000
Key pressed: 65
Key pressed: 68
Key released: 68
Key released: 65
Key pressed: 83
Key pressed: 76
Key released: 76
Key released: 83

I’m using the GLFW 3.4 release on Windows and compiling with

cl /MD /IC:\glfw\include test.c C:\glfw\lib\glfw3.lib User32.Lib Gdi32.Lib Shell32.Lib
2

glfwSetKeyCallback returns previously set callback. If you have not set anything before, NULL is expected.

See GLFW: Input reference

Returns
The previously set callback, or NULL if no callback was set or the library had not been initialized.

3

Ohhh, mb. I didn’t catch that somehow, ty.