Calling DRM_IOCTL_MODE_GETRESOURCES from ioctl returns zero connector count

My goal is to figure out how to do fast screen capture. One way to do that is utilize the GPU to grab the frame (and then later encode it quickly on the GPU).

I am trying to do this on Linux and using low-level APIs like DRM/KMS to achieve this.

Currently, I am not getting back any connectors.

Running: Ubuntu 18.04.2 LTS (GNU/Linux 4.15.0-1044-aws x86_64)

C++ Code:

#include <stdio.h>
#include <fcntl.h>
#include <drm/drm.h>
#include <drm/drm_mode.h>
#include <sys/ioctl.h>
#include <cstdint>
#include <stdint.h>

using namespace std;

int main() {

    int dri_fd = open("/dev/dri/card0", O_RDWR);
    printf("fd: %d\n", dri_fd);

    if (dri_fd == -1) {
        printf("Could not open card0\n");
        return -1;
    }

    ioctl(dri_fd, DRM_IOCTL_SET_MASTER, 0); // We only need to be "master" to do the actual KMS mode setting
    drm_mode_card_res res = {0};
    ioctl(dri_fd, DRM_IOCTL_MODE_GETRESOURCES, &res);
    printf("FB Count: %d\n", res.count_fbs);
    printf("Connector Count: %d\n", res.count_connectors);
    printf("CRTC Count: %d\n", res.count_crtcs);
    printf("Encoder Count: %d\n", res.count_encoders);

    uint64_t fb_buf[10] = {0};
    uint64_t crtc_buf[10] = {0};
    uint64_t conn_buf[10] = {0};
    uint64_t enc_buf[10] = {0};

    res.fb_id_ptr = (uint64_t)fb_buf; // Frame buffer
    res.crtc_id_ptr = (uint64_t)crtc_buf; // A CRTC represents the overall display pipeline.
    res.connector_id_ptr = (uint64_t)conn_buf; // In DRM connectors are the general abstraction for display sinks, and include als fixed panels or anything else that can display pixels in some form.
    res.encoder_id_ptr = (uint64_t)enc_buf; // An encoder takes pixel data from a CRTC and converts it to a format suitable for any attached connector.

    // Get resource IDs
    ioctl(dri_fd, DRM_IOCTL_MODE_GETRESOURCES, &res);

    return 0;
}

Output:

:~$ sudo ./capture
fd: 3
FB Count: 0
Connector Count: 0
CRTC Count: 0
Encoder Count: 0

My expectation would be that Connector count or FB count would return something non-zero since my M60 NVIDIA GPU appears connected to X11/Gnome:

ubuntu:~$ nvidia-smi
Fri Aug  2 00:59:01 2019
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 430.30       Driver Version: 430.30       CUDA Version: 10.2     |
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|===============================+======================+======================|
|   0  Tesla M60           Off  | 00000000:00:1E.0 Off |                    0 |
| N/A   28C    P8    14W / 150W |    141MiB /  7618MiB |      0%      Default |
+-------------------------------+----------------------+----------------------+

+-----------------------------------------------------------------------------+
| Processes:                                                       GPU Memory |
|  GPU       PID   Type   Process name                             Usage      |
|=============================================================================|
|    0      3255      G   /usr/lib/xorg/Xorg                            57MiB |
|    0      3286      G   /usr/bin/gnome-shell                          82MiB |
+-----------------------------------------------------------------------------+

Other relevant diagnostic information:

ubuntu:~$ cat /etc/modprobe.d/nvidia.conf
options nvidia-drm modeset=1


ubuntu:~$ cat /etc/modprobe.d/blacklist.conf
# This file lists those modules which we don't want to be loaded by
# alias expansion, usually so some other driver will be loaded for the
# device instead.

# evbug is a debug tool that should be loaded explicitly
blacklist evbug

# these drivers are very simple, the HID drivers are usually preferred
blacklist usbmouse
blacklist usbkbd

# replaced by e100
blacklist eepro100

# replaced by tulip
blacklist de4x5

# causes no end of confusion by creating unexpected network interfaces
blacklist eth1394

# snd_intel8x0m can interfere with snd_intel8x0, doesn't seem to support much
# hardware on its own (Ubuntu bug #2011, #6810)
blacklist snd_intel8x0m

# Conflicts with dvb driver (which is better for handling this device)
blacklist snd_aw2

# Causes trackpads to stop working on Lenovo 11e 2nd gen (Ubuntu: #1802135)
# and Lenovo x240 to hang on boot (Ubuntu: #1802689)
blacklist i2c_i801

# replaced by p54pci
blacklist prism54

# replaced by b43 and ssb.
blacklist bcm43xx

# most apps now use garmin usb driver directly (Ubuntu: #114565)
blacklist garmin_gps

# replaced by asus-laptop (Ubuntu: #184721)
blacklist asus_acpi

# low-quality, just noise when being used for sound playback, causes
# hangs at desktop session start (Ubuntu: #246969)
blacklist snd_pcsp

# ugly and loud noise, getting on everyone's nerves; this should be done by a
# nice pulseaudio bing (Ubuntu: #77010)
blacklist pcspkr

# EDAC driver for amd76x clashes with the agp driver preventing the aperture
# from being initialised (Ubuntu: #297750). Blacklist so that the driver
# continues to build and is installable for the few cases where its
# really needed.
blacklist amd76x_edac
blacklist vga16fb
blacklist nouveau
blacklist rivafb
blacklist nvidiafb
blacklist rivatv


ubuntu:~$ lsmod
Module                  Size  Used by
nvidia_drm             45056  3
nvidia_modeset       1110016  3 nvidia_drm
nvidia              18796544  218 nvidia_modeset
drm_kms_helper        167936  1 nvidia_drm
drm                   393216  6 drm_kms_helper,nvidia_drm
i2c_core               73728  3 drm_kms_helper,nvidia,drm
fb_sys_fops            16384  1 drm_kms_helper
syscopyarea            16384  1 drm_kms_helper
sysfillrect            16384  1 drm_kms_helper
sysimgblt              16384  1 drm_kms_helper
ipmi_devintf           20480  0
ipmi_msghandler        53248  2 ipmi_devintf,nvidia
serio_raw              16384  0
sch_fq_codel           20480  9
ib_iser                49152  0
rdma_cm                61440  1 ib_iser
iw_cm                  45056  1 rdma_cm
ib_cm                  53248  1 rdma_cm
ib_core               225280  4 rdma_cm,iw_cm,ib_iser,ib_cm
iscsi_tcp              20480  0
libiscsi_tcp           20480  1 iscsi_tcp
libiscsi               53248  3 libiscsi_tcp,iscsi_tcp,ib_iser
scsi_transport_iscsi    98304  3 iscsi_tcp,ib_iser,libiscsi
parport_pc             36864  0
ppdev                  20480  0
lp                     20480  0
parport                49152  3 parport_pc,lp,ppdev
ip_tables              28672  0
x_tables               40960  1 ip_tables
autofs4                40960  2
btrfs                1126400  0
zstd_compress         163840  1 btrfs
raid10                 53248  0
raid456               143360  0
async_raid6_recov      20480  1 raid456
async_memcpy           16384  2 raid456,async_raid6_recov
async_pq               16384  2 raid456,async_raid6_recov
async_xor              16384  3 async_pq,raid456,async_raid6_recov
async_tx               16384  5 async_pq,async_memcpy,async_xor,raid456,async_raid6_recov
xor                    24576  2 async_xor,btrfs
raid6_pq              114688  4 async_pq,btrfs,raid456,async_raid6_recov
libcrc32c              16384  1 raid456
raid1                  40960  0
raid0                  20480  0
multipath              16384  0
linear                 16384  0
crct10dif_pclmul       16384  0
crc32_pclmul           16384  0
ghash_clmulni_intel    16384  0
pcbc                   16384  0
aesni_intel           188416  0
aes_x86_64             20480  1 aesni_intel
crypto_simd            16384  1 aesni_intel
glue_helper            16384  1 aesni_intel
cryptd                 24576  3 crypto_simd,ghash_clmulni_intel,aesni_intel
ena                    94208  0


ubuntu:~$ dmesg | grep -i NV
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-4.15.0-1044-aws root=UUID=8abb4f2d-c7e0-4daf-80b5-24e1c814cf55 ro rdblacklist=nouveau console=tty1 console=ttyS0 nvme_core.io_timeout=4294967295
[    0.000000] Kernel command line: BOOT_IMAGE=/boot/vmlinuz-4.15.0-1044-aws root=UUID=8abb4f2d-c7e0-4daf-80b5-24e1c814cf55 ro rdblacklist=nouveau console=tty1 console=ttyS0 nvme_core.io_timeout=4294967295
[    0.616004] ACPI: Added _OSI(Linux-Lenovo-NV-HDMI-Audio)
[    2.324353] rtc_cmos 00:02: alarms up to one day, 114 bytes nvram, hpet irqs
[    7.467294] nvidia: loading out-of-tree module taints kernel.
[    7.467301] nvidia: module license 'NVIDIA' taints kernel.
[    7.476833] nvidia: module verification failed: signature and/or required key missing - tainting kernel
[    7.488128] nvidia-nvlink: Nvlink Core is being initialized, major device number 243
[    7.489962] nvidia 0000:00:1e.0: vgaarb: changed VGA decodes: olddecodes=io+mem,decodes=none:owns=io+mem
[    7.490503] NVRM: loading NVIDIA UNIX x86_64 Kernel Module  430.30  Mon Jun 10 05:04:34 CDT 2019
[    7.511462] nvidia-modeset: Loading NVIDIA Kernel Mode Setting Driver for UNIX platforms  430.30  Mon Jun 10 04:38:05 CDT 2019
[    7.514021] [drm] [nvidia-drm] [GPU ID 0x0000001e] Loading driver
[    8.790628] [drm] Initialized nvidia-drm 0.0.0 20160202 for 0000:00:1e.0 on minor 0
[  227.400569] [drm] [nvidia-drm] [GPU ID 0x0000001e] Unloading driver
[  227.400786] NVRM: Persistence mode is deprecated and will be removed in a future release. Please use nvidia-persistenced instead.
[  227.585668] nvidia-modeset: Unloading
[  227.876830] nvidia-nvlink: Unregistered the Nvlink Core, major device number 243
[  237.950918] nvidia-nvlink: Nvlink Core is being initialized, major device number 243
[  237.952092] nvidia 0000:00:1e.0: vgaarb: changed VGA decodes: olddecodes=none,decodes=none:owns=io+mem
[  237.952418] NVRM: loading NVIDIA UNIX x86_64 Kernel Module  430.30  Mon Jun 10 05:04:34 CDT 2019
[  237.955126] nvidia-modeset: Loading NVIDIA Kernel Mode Setting Driver for UNIX platforms  430.30  Mon Jun 10 04:38:05 CDT 2019
[  237.956384] [drm] [nvidia-drm] [GPU ID 0x0000001e] Loading driver
[  239.061266] [drm] Initialized nvidia-drm 0.0.0 20160202 for 0000:00:1e.0 on minor 0