query window size every frame so we have something that makes a little bit of sense

Makefile

@@ -6,7 +6,7 @@ SRC = glad.c gl_utils.c main.c
 OBJ = $(SRC:.c=.o)
 BIN = fallingCand
 
-all: $(BIN)
+all: clean $(BIN)
 
 $(BIN): $(OBJ)
 	$(CC) $(OBJ) -o $@ $(LDFLAGS)

main.c

@@ -3,16 +3,16 @@
 #include <stdlib.h>
 
 #include "gl_utils.h"
+static int g_fbWidth = 800;
+static int g_fbHeight = 800;
 
 int main(void) {
-    int winW = 800;
+    GLFWwindow* window = init_glfw_glad("Falling Sand - Fullscreen Quad", g_fbWidth, g_fbHeight);
-    int winH = 600;
-
-    GLFWwindow* window = init_glfw_glad("Falling Sand - Fullscreen Quad", winW, winH);
     if (!window) {
         return EXIT_FAILURE;
     }
-
+    glfwGetFramebufferSize(window, &g_fbWidth, &g_fbHeight);
+    glViewport(0, 0, g_fbWidth, g_fbHeight);
     printf("Renderer: %s\n", (const char*)glGetString(GL_RENDERER));
     printf("OpenGL:   %s\n", (const char*)glGetString(GL_VERSION));
     printf("GLSL:     %s\n", (const char*)glGetString(GL_SHADING_LANGUAGE_VERSION));
@@ -22,8 +22,7 @@ int main(void) {
         // x, y
         -1.0f, -1.0f,
         3.0f, -1.0f,
-        -1.0f,  3.0f
+        -1.0f, 3.0f};
-    };
 
     GLuint vao = 0, vbo = 0;
     glGenVertexArrays(1, &vao);
@@ -42,17 +41,18 @@ int main(void) {
         GL_FLOAT,
         GL_FALSE,
         2 * sizeof(float),
-        (void*)0
+        (void*)0);
-    );
 
     glBindVertexArray(0);
 
     // --- Create shader program ---
     GLuint program = create_program_from_files(
         "shaders/fullscreen.vert",
-        "shaders/gradient.frag"
+        "shaders/gradient.frag");
-    );
+    GLint uResLoc = glGetUniformLocation(program, "u_resolution");
-
+    if (uResLoc == -1) {
+        fprintf(stderr, "[warn] u_resolution uniform not found (maybe optimized out?)\n");
+    }
     glUseProgram(program);
 
     glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
@@ -60,10 +60,20 @@ int main(void) {
     // --- Main loop ---
     while (!glfwWindowShouldClose(window)) {
         glfwPollEvents();
-
+        // Re-query framebuffer size each frame (cheap and simple)
+        int fbw, fbh;
+        glfwGetFramebufferSize(window, &fbw, &fbh);
+        if (fbw != g_fbWidth || fbh != g_fbHeight) {
+            g_fbWidth = fbw;
+            g_fbHeight = fbh;
+            glViewport(0, 0, g_fbWidth, g_fbHeight);
+        }
         glClear(GL_COLOR_BUFFER_BIT);
 
         glUseProgram(program);
+        if (uResLoc != -1) {
+            glUniform2f(uResLoc, (float)g_fbWidth, (float)g_fbHeight);
+        }
         glBindVertexArray(vao);
         glDrawArrays(GL_TRIANGLES, 0, 3);
 

shaders/gradient.frag

@@ -1,11 +1,11 @@
 #version 430 core
 
 out vec4 FragColor;
-
+uniform vec2 u_resolution;
 void main()
 {
     // Normalized coordinates in [0,1]
-    vec2 uv = gl_FragCoord.xy / vec2(800.0, 600.0);  // we’ll fix this later with a uniform
+    vec2 uv = gl_FragCoord.xy / u_resolution;
     vec3 color = vec3(uv.x, uv.y, 0.2);
     FragColor = vec4(color, 1.0);
 }