## Name

sys - SerenityOS SysFS

## Description

The kernel can expose system (kernel, firmware and hardware) related information in /sys.

### `bus` directory

This directory include a subdirectory for each discovered and registered bus in the system.

Possible busses to be exposed in this directory are:
1. The `pci` subdirectory that includes all discovered PCI devices as subdirectories.
The subdirectories of the PCI devices include files with basic information on the devices.
2. The `usb` subdirectory that includes all discovered USB devices as files.
The files of the USB devices export basic information on the devices.

### `dev` directory

This directory include two subdirectories - `block` and `char`, each for block
and character devices respectively. The files in these subdirectories are not
device files, but merely a file with filename layout of "major:minor", to aid
userspace in generating the appropriate device files.

### `firmware` directory

This directory include two subdirectories - `acpi` and `bios`.
The `bios` subdirectory maintains files of the exposed SMBIOS blobs, if present
by the firmware.
The `acpi` subdirectory maintains files of the exposed ACPI tables, if present
by the firmware.
A file called `power_state` is responsible for power state switching.

### `kernel` directory

This directory includes two subdirectories - `net` and `variables`.
All other files in the directory are global data nodes which provide statistics
and other kernel-related data to userspace.

#### `kernel` directory entries

* **`processes`** - This node exports a list of all processes that currently exist.
* **`cpuinfo`** - This node exports information on the CPU.
* **`df`** - This node exports information on mounted filesystems and basic statistics on
them.
* **`dmesg`** - This node exports information from the kernel log.
* **`interrupts`** - This node exports information on all IRQ handlers and basic statistics on
them.
* **`keymap`** - This node exports information on the currently used keymap.
* **`memstat`** - This node exports statistics on memory allocation in the kernel.
* **`profile`** - This node exports statistics on profiling data.
* **`stats`** - This node exports statistics on scheduler timing data.
* **`uptime`** - This node exports the uptime data.
* **`jails`** - This node exports information about existing jails (only if the current process is not in jail).
* **`power_state`** - This node only responds to write requests on it. A written value of `1` results
in system reboot. A written value of `2` results in system shutdown.

#### `net` directory

* **`adapters`** - This node exports information on all currently-discovered network adapters.
* **`arp`** - This node exports information on the kernel ARP table.
* **`local`** - This node exports information on local (Unix) sockets.
* **`tcp`** - This node exports information on TCP sockets.
* **`udp`** - This node exports information on UDP sockets.

#### `variables` directory

This subdirectory includes global settings of the kernel.

* **`caps_lock_to_ctrl`** - This node controls remapping of of caps lock to the Ctrl key.
* **`kmalloc_stacks`** - This node controls whether to send information about kmalloc to debug log.
* **`ubsan_is_deadly`** - This node controls the deadliness of the kernel undefined behavior
sanitizer errors.

#### `constants` directory

This subdirectory includes global constants of the kernel.
Those nodes are generated during the boot sequence and are completely static,
therefore making them very fast to read (no kmalloc or locking is needed).

* **`load_base`** - This node reveals the loading address of the kernel.
* **`system_mode`** - This node exports the chosen system mode as it was decided based on the kernel commandline or a default value.
* **`cmdline`** - This node exports the kernel boot commandline that was passed from the bootloader.

### Consistency and stability of data across multiple read operations

When opening a data node, the kernel generates the required data so it's prepared
for read operation when requested to. However, in order to ensure that multiple reads
will not create a corrupted data from that data node, a read operation alone will
not inquire the kernel to refresh the data.
To keep data output being refreshed, the userland has to re-open the data node with a 
new file descriptor, or to perform the `lseek` syscall on the open file descriptor to
reset the offset to 0.

## See also

* [`mount`(2))](help://man/2/mount).
