procfs

procfs (or the proc filesystem) is a special filesystem in UNIX-like operating systems that presents information about processes and other system information in a hierarchical file-like structure, providing a more convenient and standardized method for dynamically accessing process data held in the kernel than traditional tracing methods or direct access to kernel memory. Typically, it is mapped to a mount point named /proc at boot time.

Operating systems that support the proc filesystem include, but are not limited to:

• Solaris
• IRIX
• Tru64 UNIX
• BSD
• Linux (which extends it to non-process-related data)
• IBM AIX (which bases its implementation on Linux to improve compatibility)
• QNX
• Plan 9 from Bell Labs

The proc filesystem helps to move functionality from kernel space to user space. For example, the GNU version of ps operates entirely in user mode, using the procfs to obtain its data.

History

UNIX 8th Edition

Tom J. Killian implemented the UNIX 8th Edition version of /proc: he presented a paper titled "Processes as Files" at USENIX in June 1984. The design of procfs aimed to replace the ptrace system call used for process tracing. Detailed documentation could be found in the proc(4) manual page.

SVR4

Roger Faulkner and Ron Gomes ported V8 /proc to SVR4, and published a paper called "The Process File System and Process Model in UNIX System V" at USENIX in January 1991. This kind of procfs supported the creation of ps, but the files could only be accessed with functions read(), write(), and ioctl().

Plan 9

Plan 9 implemented a process file system, but went further than V8. V8's process file system implemented a single file per process. Plan 9 created a hierarchy of separate files to provide those functions, and made /proc a real part of the file system.

4.4BSD

4.4BSD cloned its implementation of /proc from Plan 9.^{[citation needed]} Note that in FreeBSD procfs is being gradually phased out.

Solaris

Solaris 2.6's /proc (finished in 1996) also cloned Plan 9.^{[citation needed]}

Linux

The Linux implementation of /proc also clones that of Plan 9.^{[citation needed]} Under Linux, /proc includes a directory for each running process (including kernel processes) at /proc/PID, containing information about that process, notably including:

• /proc/PID/cmdline, which contains the command which originally started the process.
• /proc/PID/cwd, a symlink to the current working directory of the process.
• /proc/PID/environ, a file containing the names and contents of the environment variables that affect the process.
• /proc/PID/exe, a symlink to the original executable file, if it still exists (a process may continue running after its original executable has been deleted or replaced).
• /proc/PID/fd, a directory containing a symbolic link for each open file descriptor.
• /proc/PID/fdinfo, a directory containing files which describe the position and flags for each open file descriptor.
• /proc/PID/root, a symlink to the root path as seen by the process. For most processes this will be a link to / unless the process is running in a chroot jail.
• /proc/PID/status, a file containing basic information about a process including its run state and memory usage.
• /proc/PID/task, a directory containing hard links to any tasks that have been started by this (i.e.: the parent) process.
• /proc/PID/maps, the memory map showing which addresses currently visible to that process are mapped to which regions in RAM or to files.

Obtaining the PID can be done with utilities like pgrep, pidof or ps:

$ ls -l /proc/$(pgrep -n python)/fd        # List all file descriptors of the most recently started `python' process
samtala 0
lrwx------ 1 baldur baldur 64 2011-03-18 12:31 0 -> /dev/pts/3
lrwx------ 1 baldur baldur 64 2011-03-18 12:31 1 -> /dev/pts/3
lrwx------ 1 baldur baldur 64 2011-03-18 12:31 2 -> /dev/pts/3
$ readlink /proc/$(pgrep -n python)/exe    # List executable used to launch the most recently started `python' process  
/usr/bin/python3.1

It also includes non-process-related system information, although in the 2.6 kernel much of that information moved to a separate pseudo-file system, sysfs, mounted under /sys:

• depending on the mode of power management (if at all), either directory, /proc/acpi or /proc/apm, which predate sysfs and contain various bits of information about the state of power management.
• /proc/buddyinfo, information about the buddy algorithm which handles memory fragmentation.^[1]
• /proc/bus, containing directories representing various buses on the computer, such as input/PCI/USB. This has been largely superseded by sysfs under /sys/bus which is far more informative.
• /proc/fb, a list of the available framebuffers
• /proc/cmdline, giving the boot options passed to the kernel
• /proc/cpuinfo, containing information about the CPU, such as its vendor (and CPU family, model and model names which should allow users to identify the CPU) and its speed (CPU clockspeed), cache size, number of siblings, cores, and CPU flags. It contains a value called "bogomips", frequently misunderstood as measure CPU-speed like a benchmark, while it doesn't actually measure any sensible (for end-users) value at all. It occurs as a side-effect of kernel timer calibration and yields highly varying values depending on CPU type, even at equal clock speeds.

On multi-core CPUs, /proc/cpuinfo contains the two fields "siblings" and "cpu cores" whereas the following calculation is applied^[2]:

"siblings" = (HT per CPU package) * (# of cores per CPU package)
"cpu cores" = (# of cores per CPU package)

A CPU package means physical CPU which can have multiple cores (single core for none, dual core for two, quad core for four). This allows to better distinguish between HT and dual-core, i.e. the number of HT per CPU package can be calculated by siblings / CPU cores. If both values for a CPU package are the same, then hyper-threading is not supported.^[3] For instance, a CPU package with siblings=2 and "cpu cores"=2 is a dual-core CPU but doesn't support HT.

• /proc/crypto, a list of available cryptographic modules
• /proc/devices, a list of character and block devices sorted by device ID but giving the major part of the /dev name too
• /proc/diskstats, giving some information (including device numbers) for each of the logical disk devices
• /proc/filesystems, a list of the file systems supported by the kernel at the time of listing
• /proc/interrupts, /proc/iomem, /proc/ioports and the directory /proc/irq, giving some self-explanatory details about the devices (physical or logical) using the various system resources
• /proc/meminfo, containing a summary of how the kernel is managing its memory.
• /proc/modules, one of the most important files in /proc, containing a list of the kernel modules currently loaded . It gives some indication ( not always entirely correct) of dependencies.
• /proc/mounts, a symlink to self/mounts which contains a list of the currently mounted devices and their mount points (and which file system is in use and what mount options are in use).
• /proc/net, a directory containing a lot of really useful information about the network stack, in particular nf_conntrack which lists existing network connections (particularly useful for tracking routing when iptables FORWARD is used to redirect network connections).
• /proc/partitions, a list of the device-numbers, their size and /dev names which the kernel has identified as existing partitions (for example if /dev/sda contains a partition table, then /dev/sda1 and others will appear as available partitions). Note that if a partition isn't listed in this file, then a patched version of losetup is around which can essentially mount the partition and connect /dev/loop[n] devices to the various partitions (though it is not certain if these will then appear in /proc/partitions).
• /proc/scsi, giving information about any devices connected via a SCSI or RAID controller
• a symbolic link to the current (traversing) process at /proc/self (i.e. /proc/PID/ where PID is that of the current process).
• /proc/slabinfo, listing statistics on the caches for frequently-used objects in the Linux kernel
• /proc/swaps, a list of the active swap partitions, their various sizes and priorities
• Access to dynamically-configurable kernel options under /proc/sys. Under /proc/sys appear directories representing the areas of kernel, containing readable and writable virtual files.
  For example, a commonly referenced virtual file is /proc/sys/net/ipv4/ip_forward, because it is necessary for routing firewalls or tunnels. The file contains either a '1' or a '0': if it is 1 then the IPv4 stack will forward packets not meant for the local host, if it is 0 then it will not.
• /proc/sysvipc, containing memory sharing and IPC information.
• /proc/tty, containing information about the current terminals; /proc/tty/driver looks to be a list of the different types of tty available each of which is a list of those of each type
• /proc/uptime, the length of time the kernel has been running since boot and spent in idle mode (both in seconds)
• /proc/version, containing the Linux kernel version, distribution number, gcc version number (used to build the kernel) and any other pertinent information relating to the version of the kernel currently running
• other files depending on various hardware, module configurations, and changes to the kernel.

The basic utilities that use /proc under Linux come in the procps (/proc processes) package, and only function in conjunction with a mounted /proc.

Cobalt

Cobalt Networks added additional functions to /proc for their systems:

• /proc/cobalt, a directory containing Cobalt-specific data such as the system type and serial number.
• /proc/lcd, a file containing the contents of the front-panel LCD screen. Text written to this file would be displayed on the screen.

References

1. ^ http://www.centos.org/docs/5/html/5.2/Deployment_Guide/s2-proc-buddyinfo.html
2. ^ Baron, Jason. "HT vs. dual-core". http://www.redhat.com/archives/nahant-list/2006-January/msg00176.html. 
3. ^ "Understanding Linux /proc/cpuinfo". http://www.richweb.com/cpu_info. 

• Unix 8th Edition proc(2) manual page - Description of the original procfs.
• Plan 9 procfs manual page - Plan 9 greatly expanded the procfs concept, providing a much expanded interface to control and manipulate processes.
• Linux Manual Pages Proc(5) Linux manual documentation for procfs
• Documentation/filesystems/proc.txt Linux kernel documentation for procfs

External links

• A brief history of /proc Eric Schrock's Weblog
• Access the Linux kernel using the Procfs An IBM developerWorks article by M. Tim Jones
• Linux-Filesystem-Hierarchy Linux Documentation Project
• Creating and using proc files - an article from the Real-Time Embedded blog
• Using the proc filesystem An article by Pétur I. Egilsson