dirty and small example of libdrm

drm_test.c

/* please refer better example: https://github.com/dvdhrm/docs/tree/master/drm-howto/ */
#define _XOPEN_SOURCE 600

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <xf86drm.h>
#include <xf86drmMode.h>

static const char *dri_path = "/dev/dri/card0";

enum {
	DEPTH = 24,
	BPP = 32,
};

struct drm_dev_t {
	uint32_t *buf;
	uint32_t conn_id, enc_id, crtc_id, fb_id;
	uint32_t width, height;
	uint32_t pitch, size, handle;
	drmModeModeInfo mode;
	drmModeCrtc *saved_crtc;
	struct drm_dev_t *next;
};

void fatal(char *str)
{
	fprintf(stderr, "%s\n", str);
	exit(EXIT_FAILURE);
}

void error(char *str)
{
	perror(str);
	exit(EXIT_FAILURE);
}

int eopen(const char *path, int flag)
{
	int fd;

	if ((fd = open(path, flag)) < 0) {
		fprintf(stderr, "cannot open \"%s\"\n", path);
		error("open");
	}
	return fd;
}

void *emmap(int addr, size_t len, int prot, int flag, int fd, off_t offset)
{
	uint32_t *fp;

	if ((fp = (uint32_t *) mmap(0, len, prot, flag, fd, offset)) == MAP_FAILED)
		error("mmap");
	return fp;
}

int drm_open(const char *path)
{
	int fd, flags;
	uint64_t has_dumb;

	fd = eopen(path, O_RDWR);

	/* set FD_CLOEXEC flag */
	if ((flags = fcntl(fd, F_GETFD)) < 0
		|| fcntl(fd, F_SETFD, flags | FD_CLOEXEC) < 0)
		fatal("fcntl FD_CLOEXEC failed");

	/* check capability */
	if (drmGetCap(fd, DRM_CAP_DUMB_BUFFER, &has_dumb) < 0 || has_dumb == 0)
		fatal("drmGetCap DRM_CAP_DUMB_BUFFER failed or doesn't have dumb buffer");

	return fd;
}

struct drm_dev_t *drm_find_dev(int fd)
{
	int i;
	struct drm_dev_t *dev = NULL, *dev_head = NULL;
	drmModeRes *res;
	drmModeConnector *conn;
	drmModeEncoder *enc;

	if ((res = drmModeGetResources(fd)) == NULL)
		fatal("drmModeGetResources() failed");

	/* find all available connectors */
	for (i = 0; i < res->count_connectors; i++) {
		conn = drmModeGetConnector(fd, res->connectors[i]);

		if (conn != NULL && conn->connection == DRM_MODE_CONNECTED && conn->count_modes > 0) {
			dev = (struct drm_dev_t *) malloc(sizeof(struct drm_dev_t));
			memset(dev, 0, sizeof(struct drm_dev_t));

			dev->conn_id = conn->connector_id;
			dev->enc_id = conn->encoder_id;
			dev->next = NULL;

			memcpy(&dev->mode, &conn->modes[0], sizeof(drmModeModeInfo));
			dev->width = conn->modes[0].hdisplay;
			dev->height = conn->modes[0].vdisplay;

			/* FIXME: use default encoder/crtc pair */
			if ((enc = drmModeGetEncoder(fd, dev->enc_id)) == NULL)
				fatal("drmModeGetEncoder() faild");
			dev->crtc_id = enc->crtc_id;
			drmModeFreeEncoder(enc);

			dev->saved_crtc = NULL;

			/* create dev list */
			dev->next = dev_head;
			dev_head = dev;
		}
		drmModeFreeConnector(conn);
	}
	drmModeFreeResources(res);

	return dev_head;
}

void drm_setup_fb(int fd, struct drm_dev_t *dev)
{
	struct drm_mode_create_dumb creq;
	struct drm_mode_map_dumb mreq;

	memset(&creq, 0, sizeof(struct drm_mode_create_dumb));
	creq.width = dev->width;
	creq.height = dev->height;
	creq.bpp = BPP; // hard conding

	if (drmIoctl(fd, DRM_IOCTL_MODE_CREATE_DUMB, &creq) < 0)
		fatal("drmIoctl DRM_IOCTL_MODE_CREATE_DUMB failed");

	dev->pitch = creq.pitch;
	dev->size = creq.size;
	dev->handle = creq.handle;

	if (drmModeAddFB(fd, dev->width, dev->height,
		DEPTH, BPP, dev->pitch, dev->handle, &dev->fb_id))
		fatal("drmModeAddFB failed");

	memset(&mreq, 0, sizeof(struct drm_mode_map_dumb));
	mreq.handle = dev->handle;

	if (drmIoctl(fd, DRM_IOCTL_MODE_MAP_DUMB, &mreq))
		fatal("drmIoctl DRM_IOCTL_MODE_MAP_DUMB failed");

	dev->buf = (uint32_t *) emmap(0, dev->size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, mreq.offset);

	dev->saved_crtc = drmModeGetCrtc(fd, dev->crtc_id); /* must store crtc data */
	if (drmModeSetCrtc(fd, dev->crtc_id, dev->fb_id, 0, 0, &dev->conn_id, 1, &dev->mode))
		fatal("drmModeSetCrtc() failed");
}

void drm_destroy(int fd, struct drm_dev_t *dev_head)
{
	struct drm_dev_t *devp, *devp_tmp;
	struct drm_mode_destroy_dumb dreq;

	for (devp = dev_head; devp != NULL;) {
		if (devp->saved_crtc)
			drmModeSetCrtc(fd, devp->saved_crtc->crtc_id, devp->saved_crtc->buffer_id,
				devp->saved_crtc->x, devp->saved_crtc->y, &devp->conn_id, 1, &devp->saved_crtc->mode);
		drmModeFreeCrtc(devp->saved_crtc);

		munmap(devp->buf, devp->size);

		drmModeRmFB(fd, devp->fb_id);

		memset(&dreq, 0, sizeof(dreq));
		dreq.handle = devp->handle;
		drmIoctl(fd, DRM_IOCTL_MODE_DESTROY_DUMB, &dreq);

		devp_tmp = devp;
		devp = devp->next;
		free(devp_tmp);
	}

	close(fd);
}

int main()
{
	int fd;
	int i, j;
	uint8_t color;
	struct drm_dev_t *dev_head, *dev;

	/* init */
	fd = drm_open(dri_path);
	dev_head = drm_find_dev(fd);

	if (dev_head == NULL) {
		fprintf(stderr, "available drm_dev not found\n");
		return EXIT_FAILURE;
	}

	printf("available connector(s)\n\n");
	for (dev = dev_head; dev != NULL; dev = dev->next) {
		printf("connector id:%d\n", dev->conn_id);
		printf("\tencoder id:%d crtc id:%d fb id:%d\n", dev->enc_id, dev->crtc_id, dev->fb_id);
		printf("\twidth:%d height:%d\n", dev->width, dev->height);
	}

	/* FIXME: use first drm_dev */
	dev = dev_head;
	drm_setup_fb(fd, dev);

	/* draw something */
	for (i = 0; i < dev->height; i++)
		for (j = 0; j < dev->width; j++) {
			color = (double) (i * j) / (dev->height * dev->width) * 0xFF;
			*(dev->buf + i * dev->width + j) = (uint32_t) 0xFFFFFF & (0x00 << 16 | color << 8 | color);
		}

	sleep(3);

	/* destroy */
	drm_destroy(fd, dev_head);

	return 0;
}

Nice. Almost certainly you need to fix the /* draw something */ body with something this:

*(dev->buf + i * (dev->pitch/(BPP/8)) + j) = (uint32_t) 0xFFFFFF & (0x00 << 16 | color << 8 | color);

Sometimes pitch != width !