Add request name output for ioctl() emulation even if that is not the emulated device handle

[dememax]

For v4l2 requests we are using, + those from /usr/include/asm-generic/ioctls.h like TIOCSTI

[dememax]

What else could be treated?

Beyond V4L2 and standard TTY controls, several other families of ioctl() requests are well-known and widely used for controlling different Linux kernel subsystems.

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## 🌐 Networking (SIOC*)

These requests, typically prefixed with SIOC (Socket IOCTL), are fundamental for network interface configuration. They are defined in headers like <sys/sockio.h> and <net/if.h>.

• SIOCGIFCONF: Gets a list of all network interfaces on the system.
• SIOCGIFADDR / SIOCSIFADDR: Gets or sets an interface's IP address.
• SIOCSIFFLAGS: Sets interface flags (e.g., bringing an interface UP or DOWN).

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## 💾 Block Devices (BLK*)

This family, prefixed with BLK, is used to manage block devices like hard drives and SSDs. They are defined in <linux/fs.h>.

• BLKGETSIZE64: The standard way to get the total size of a block device in bytes.
• BLKFLSBUF: Flushes the device's buffer cache, forcing all buffered data to be written to the disk.
• HDIO_GET_IDENTITY: A classic ioctl (though now often superseded) to get detailed ATA identification data from an IDE/SATA drive.

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## 🖱️ Input Devices (EVIOC*)

The modern way to handle raw input from keyboards, mice, joysticks, and touchscreens on Linux is through the evdev interface. These ioctls, prefixed with EVIOC (Event IOCTL), are defined in <linux/input.h>.

• EVIOCGNAME: Gets the human-readable name of the input device (e.g., "Logitech MX Master 3S").
• EVIOCGBIT: Queries the types of events (e.g., relative motion, key presses, absolute position) and specific codes (e.g., which keys or buttons) the device supports.

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## 🖥️ Graphics and Display (DRM/KMS)

The Direct Rendering Manager (DRM) subsystem is the core of the modern Linux graphics stack. It uses a family of ioctls, prefixed with DRM_IOCTL_, for everything from setting display modes (Kernel Mode Setting or KMS) to submitting rendering commands to the GPU. They are defined in <drm/drm.h>.

• DRM_IOCTL_MODE_GETRESOURCES: The starting point for display management; it enumerates the available connectors (HDMI, DisplayPort), encoders, and framebuffers.
• DRM_IOCTL_MODE_ADDFB: Creates a framebuffer object from user-provided pixel data, which can then be displayed.

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## 🔊 Sound (ALSA)

The Advanced Linux Sound Architecture (ALSA) relies heavily on ioctls for configuring and controlling audio devices. These are prefixed with SNDRV_PCM_IOCTL_ and are used via the ALSA user-space library (libasound).

• SNDRV_PCM_IOCTL_HW_PARAMS: Configures the hardware parameters for an audio stream, such as the sample rate, format (e.g., 16-bit signed), and number of channels.
• SNDRV_PCM_IOCTL_PREPARE: Prepares the audio device for playback or capture to begin.

[dememax]

Other projects who decode ioctl request codes

The primary and most well-known tool for this is strace. It automatically decodes ioctl request codes and many other system call arguments into their human-readable names from the system's header files.

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## strace: The Standard ioctl Decoder

strace intercepts system calls made by a program and prints them to the console. It has a large, built-in database of ioctl command numbers and their corresponding names (like VIDIOC_QBUF or TIOCSTI).

When strace sees an ioctl() call, it doesn't just print the raw number; it looks up that number and prints the symbolic name. Even better, for many known ioctls, it also decodes the third argument, printing the contents of the structure being passed to or from the kernel.

Practical Example

Without strace, in gdb you might see a raw, unhelpful call:

ioctl(3, 3226490394, 0x7ffc1234abcd)

With strace, you get a fully decoded, human-readable line:

# The '-e ioctl' flag filters the output to only show ioctl calls
$ strace -e ioctl /path/to/your/program

ioctl(3, VIDIOC_ENUMINPUT, {index=0, name="", ...}) = 0

As you can see, strace converted 3226490394 to its correct name, VIDIOC_ENUMINPUT, and even started decoding the contents of the struct v4l2_input pointer.

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## Other Tools

While strace is the standard for live tracing, other tools exist for offline decoding:

• ioctl-decoder Scripts: You can find various standalone scripts (often in Python or Perl) online that take an ioctl number as a command-line argument and print its name. These scripts work by parsing the kernel header files on your system.
• Advanced Tracers (bpftrace): For more complex or high-performance tracing, tools like bpftrace can be used. You can write a small script to attach to the ioctl syscall entry point and print the decoded command, but this is far more complex than simply using strace.

For day-to-day development and debugging of ioctl calls, strace is the best and most direct tool for the job. 🕵️‍♂️