Section 10.4. The /proc Filesystem

Unix systems have come a long way with respect to providing uniform interfaces to different parts of the system; as you learned in Chapter 4, hardware is represented in Linux in the form of a special type of file in the /dev directory. We'll have a lot more to say about this directory in "Device Files," later in this chapter. There is, however, a special filesystem called the /proc filesystem that goes even one step further: it unifies files and processes.

From the user's or the system administrator's point of view, the /proc filesystem looks just like any other filesystem; you can navigate around it with the cd command, list directory contents with the ls command, and view file contents with the cat command. However, none of these files and directories occupies any space on your hard disk. The kernel traps accesses to the /proc filesystem and generates directory and file contents on the fly. In other words, whenever you list a directory or view file contents in the /proc filesystem, the kernel dynamically generates the contents you want to see.

To make this less abstract, let's see some examples. The following example displays the list of files in the top-level directory of the /proc filesystem:

    tigger # ls /proc
    .     3759  5538  5679  5750  6137  9            filesystems  net
    ..    3798  5539  5681  5751  6186  966          fs           partitions
    1     3858  5540  5683  5754  6497  acpi         ide          scsi
    10    3868  5541  5686  5757  6498  asound       interrupts   self
    11    3892  5542  5688  5759  6511  bluetooth    iomem        slabinfo
    1138  3898  5556  5689  5761  6582  buddyinfo    ioports      splash
    14    4     5572  5692  5800  6720  bus          irq          stat
    15    4356  5574  5693  5803  6740  cmdline      kallsyms     swaps
    1584  4357  5579  5698  5826  6741  config.gz    kcore        sys
    1585  4368  5580  5701  5827  6817  cpufreq      kmsg         sysrq-trigger
    1586  4715  5592  5705  5829  6818  cpuinfo      loadavg      sysvipc
    16    4905  5593  5706  5941  6819  crypto       locks        tty
    17    5     5619  5707  6     6886  devices      mdstat       uptime
    18    5103  5658  5713  6063  689   diskstats    meminfo 
      version
    19    5193  5661  5715  6086  6892  dma          misc         vmstat
    2     5219  5663  5717  6107  6894  dri          mm
    2466  5222  5666  5740  6115  6912  driver       modules
    2958  5228  5673  5741  6118  7     execdomains  mounts
    3     5537  5677  5748  6130  8     fb           mtrr

The numbers will be different on your system, but the general organization will be the same. All those numbers are directories that represent each of the processes running on your system. For example, let's look at the information about the process with the ID 3759:

    tigger # ls /proc/3759
    .     auxv     delay    fd           mem      oom_score  statm   wchan
    ..    cmdline  environ  mapped_base  mounts   root       status
    attr  cwd      exe      maps         oom_adj  stat       task

(The output can be slightly different if you are using a different version of the Linux kernel.) You see a number of files that each contain information about this process. For example, the cmdline file shows the command line with which this process was started. status gives information about the internal state of the process, and cwd links to the current working directory of this process.

Probably you'll find the hardware information even more interesting than the process information. All the information that the kernel has gathered about your hardware is collected in the /proc filesystem, even though it can be difficult to find the information you are looking for.

Let's start by checking your machine's memory. This is represented by the file /proc/meminfo:

    owl # cat /proc/meminfo
    MemTotal:      1034304 kB
    MemFree:        382396 kB
    Buffers:         51352 kB
    Cached:         312648 kB
    SwapCached:          0 kB
    Active:         448816 kB
    Inactive:       141100 kB
    HighTotal:      131008 kB
    HighFree:          252 kB
    LowTotal:       903296 kB
    LowFree:        382144 kB
    SwapTotal:     1172724 kB
    SwapFree:      1172724 kB
    Dirty:             164 kB
    Writeback:           0 kB
    Mapped:         294868 kB
    Slab:            38788 kB
    Committed_AS:   339916 kB
    PageTables:       2124 kB
    VmallocTotal:   114680 kB
    VmallocUsed:     78848 kB
    VmallocChunk:    35392 kB
    HugePages_Total:     0
    HugePages_Free:      0
    Hugepagesize:     4096 kB

If you then try the command free , you can see that you get exactly the same information, only in a different format. free does nothing more than read /proc/meminfo and rearrange the output a bit.

Most tools on your system that report information about your hardware do it this way. The /proc filesystem is a portable and easy way to get at this information. The information is especially useful if you want to add new hardware to your system. For example, most hardware boards need a few I/O addresses to communicate with the CPU and the operating system. If you configured two boards to use the same I/O addresses, disaster is about to happen. You can avoid this by checking which I/O addresses the kernel has already detected as being in use:

    tigger # more /proc/ioports
    0000-001f : dma1
    0020-0021 : pic1
    0040-005f : timer
    0060-006f : keyboard
    0070-0077 : rtc
    0080-008f : dma page reg
    00a0-00a1 : pic2
    00c0-00df : dma2
    00f0-00ff : fpu
    0170-0177 : ide1
    01f0-01f7 : ide0
    02f8-02ff : serial
    0376-0376 : ide1
    0378-037a : parport0
    03c0-03df : vesafb
    03f6-03f6 : ide0
    03f8-03ff : serial
    0cf8-0cff : PCI conf1
    c000-cfff : PCI Bus #02
    c000-c0ff : 0000:02:04.0
      c000-c00f : advansys
      c400-c43f : 0000:02:09.0
        c400-c43f : e100
    d000-d00f : 0000:00:07.1
      d000-d007 : ide0
      d008-d00f : ide1
    d400-d4ff : 0000:00:07.5
      d400-d4ff : AMD AMD768 - AC'97
    d800-d83f : 0000:00:07.5
      d800-d83f : AMD AMD768 - Controller
    dc00-dcff : 0000:00:09.0
    e000-e003 : 0000:00:00.0

Now you can look for I/O addresses that are free. Of course, the kernel can show I/O addresses only for boards that it has detected and recognized, but in a correctly configured system, this should be the case for all boards.

You can use the /proc filesystem for the other information you might need when configuring new hardware as well: /proc/interrupts lists the occupied interrupt lines (IRQs) and /proc/dma lists the DMA channels in use.