Section 10.4. The proc Filesystem


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.



Running Linux
Running Linux
ISBN: 0596007604
EAN: 2147483647
Year: 2004
Pages: 220

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