libdrmtap-sys

Raw FFI bindings for libdrmtap — a C library for capturing Linux screen framebuffers via DRM/KMS.

What is libdrmtap?

libdrmtap captures screen contents at the kernel level using DRM/KMS APIs. Unlike PipeWire/portal-based capture, it works:

• At the login screen (GDM, SDDM, LightDM)
• Without user interaction (no "Select screen to share" prompt)
• On Wayland (bypasses the compositor security model)
• Headless (no display server needed)

⚠️ Testing Status

Verified working: virtio_gpu (QEMU/Parallels VMs), Intel Meteor Lake (i915, dual 3840x2160, EGL CCS detiling of the tiled/compressed framebuffer), and NVIDIA Jetson Orin Nano (nvidia-drm, Wayland).

The AMD (amdgpu) backend is implemented but not yet validated on real hardware.

If you test AMD or other configurations, please report results via GitHub Issues.

Requirements

• Linux with DRM/KMS support (kernel 4.20+ for the tiled/modifier framebuffer path, i.e. Ubuntu 20.04+; linear/VM framebuffers work on older kernels)
• A C compiler — the embedded C sources are compiled statically at build time. There is no system libdrmtap install, meson install, or pkg-config lookup of a shared library.
• libdrm development headers (located via pkg-config), plus EGL, OpenGL ES 2, libseccomp, and libcap (and their -dev headers) — the crate links these.

The build also compiles the privileged drmtap-helper binary from the same embedded sources, with exploit-mitigation hardening (stack-protector-strong, FORTIFY, PIE, full RELRO). Its path is exported to downstream build scripts as DEP_DRMTAP_HELPER_BIN so a consumer (e.g. RustDesk) can copy and setcap it. The library captures directly when it already has DRM master / CAP_SYS_ADMIN; otherwise it spawns the helper over a socketpair to read other clients' framebuffers, returning the scanout as a zero-copy DMA-BUF fd via SCM_RIGHTS.

Pixel output

Frames are returned as 8-bit XRGB8888 (BGRA in memory). Tiled and compressed framebuffers (Intel X/Y-tiled + CCS, AMD, Nvidia block-linear, virtio/virgl) are GPU-detiled through an EGL/GLES2 backend, and HDR10 scanouts (PQ / BT.2020 — AR30/XR30 and 16-bit XR48/AR48/XB48/AB48) are tone-mapped to SDR when the connector advertises HDR (HDR_OUTPUT_METADATA): PQ decode, BT.2020 → BT.709 gamut, a highlight-preserving curve, then sRGB. Plain SDR 10-bit gets a straight bit-depth reduction; HLG falls back to that reduction and P010 (overlay-video YUV) is not handled.

Usage

This is a -sys crate with raw FFI bindings. For a safe wrapper, use libdrmtap.

use libdrmtap_sys::*;
use std::ptr;

unsafe {
    let ctx = drmtap_open(ptr::null());
    if !ctx.is_null() {
        let driver = drmtap_gpu_driver(ctx);
        // ... capture frames ...
        drmtap_close(ctx);
    }
}

License

MIT