Set up drawing to framebuffer

.gitignore

@@ -0,0 +1 @@
+asteroids

build.sh

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+#!/bin/sh
+
+cc="gcc"
+warn="-std=c23 -pedantic -Wall -Wextra"
+opt="-O2 -march=native"
+libs="-I/usr/include/libdrm -ldrm"
+
+$cc $flags $libs main.c -o asteroids

main.c

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+#define _POSIX_C_SOURCE 200809L
+
+#include <assert.h>
+#include <fcntl.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include <xf86drm.h>
+#include <xf86drmMode.h>
+
+int main()
+{
+	int err;
+
+	const int fd = open("/dev/dri/card1", O_RDWR | O_CLOEXEC);
+	assert(fd != -1);
+
+	drmModeRes *res = drmModeGetResources(fd);
+	assert(res != nullptr);
+	drmModeConnector *conn = drmModeGetConnector(fd, res->connectors[0]);
+	assert(conn != nullptr);
+	drmModeModeInfo mode = conn->modes[0];
+	uint32_t conn_id = conn->connector_id;
+
+	struct drm_mode_create_dumb create_req = {
+		.width = mode.hdisplay,
+		.height = mode.vdisplay,
+		.bpp = 32,
+	};
+	err = ioctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &create_req);
+	assert(err != -1);
+
+	uint32_t fb_id;
+	err = drmModeAddFB(
+	    fd, create_req.width, create_req.height, 24, 32, create_req.pitch,
+	    create_req.handle, &fb_id);
+	assert(err == 0);
+
+	err = drmModeSetCrtc(fd, res->crtcs[0], fb_id, 0, 0, &conn_id, 1, &mode);
+	assert(err == 0);
+	drmModeCrtc *crtc = drmModeGetCrtc(fd, res->crtcs[0]);
+	assert(crtc != nullptr);
+
+	struct drm_mode_map_dumb map_req = { .handle = create_req.handle };
+	err = ioctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &map_req);
+	assert(err != -1);
+
+	uint32_t *const fb = mmap(
+	    0, create_req.size, PROT_READ | PROT_WRITE, MAP_SHARED, fd,
+	    map_req.offset);
+	assert(fb != MAP_FAILED);
+
+	for (int y = 0; y < mode.vdisplay; ++y) {
+		for (int x = 0; x < mode.hdisplay; ++x) {
+			const uint8_t r = (x * 255) / mode.hdisplay;
+			const uint8_t g = (y * 255) / mode.vdisplay;
+			const uint8_t b = 128;
+			fb[y * (create_req.pitch / 4) + x] = (r << 16) | (g << 8) | b;
+		}
+	}
+
+	getchar();
+
+	munmap(fb, create_req.size);
+	struct drm_mode_destroy_dumb delete_req
+	    = { .handle = create_req.handle };
+	ioctl(fd, DRM_IOCTL_MODE_DESTROY_DUMB, &delete_req);
+
+	drmModeSetCrtc(
+	    fd, crtc->crtc_id, crtc->buffer_id, crtc->x, crtc->y, &conn_id, 1,
+	    &crtc->mode);
+	drmModeFreeCrtc(crtc);
+	drmModeFreeConnector(conn);
+	drmModeFreeResources(res);
+	close(fd);
+
+	return 0;
+}