proc - process information pseudo-filesystem
/proc is a pseudo-filesystem which is used as an interface to kernel
data structures rather than reading and interpreting /dev/kmem. Most
of it is read-only, but some files allow kernel variables to be
changed.
The following outline gives a quick tour through the /proc hierarchy.
[number]
There is a numerical subdirectory for each running process; the
subdirectory is named by the process ID. Each contains the
following pseudo-files and directories.
cmdline
This holds the complete command line for the process,
unless the whole process has been swapped out, or unless
the process is a zombie. In either of these later cases,
there is nothing in this file: i.e. a read on this file
will return as having read 0 characters. This file is
null-terminated, but not newline-terminated.
cwd This is a link to the current working directory of the
process. To find out the cwd of process 20, for
instance, you can do this:
cd /proc/20/cwd; /bin/pwd
Note that the pwd command is often a shell builtin, and might
not work properly in this context.
environ
This file contains the environment for the process. The
entries are separated by null characters, and there may
be a null character at the end. Thus, to print out the
environment of process 1, you would do:
(cat /proc/1/environ; echo) | tr "\000" "\n"
(For a reason why one should want to do this, see lilo(8).)
exe a pointer to the binary which was executed, and appears
as a symbolic link. A readlink(2) call on the exe special
file returns under Linux 2.0 and earlier a string in the
format:
[device]:inode
For example, [0301]:1502 would be inode 1502 on device
major 03 (IDE, MFM, etc. drives) minor 01 (first
partition on the first drive). Under Linux 2.2 the link
contains the actual path name of the command.
Also, the symbolic link can be dereferenced normally -
attempting to open "exe" will open the executable. You
can even type /proc/[number]/exe to run another copy of
the same process as [number].
find(1) with the -inum option can be used to locate the
file.
fd This is a subdirectory containing one entry for each file
which the process has open, named by its file descriptor,
and which is a symbolic link to the actual file (as the
exe entry does). Thus, 0 is standard input, 1 standard
output, 2 standard error, etc.
Programs that will take a filename, but will not take the
standard input, and which write to a file, but will not
send their output to standard output, can be effectively
foiled this way, assuming that -i is the flag designating
an input file and -o is the flag designating an output
file:
foobar -i /proc/self/fd/0 -o /proc/self/fd/1 ...
and you have a working filter. Note that this will not
work for programs that seek on their files, as the files
in the fd directory are not seekable.
/proc/self/fd/N is approximately the same as /dev/fd/N in
some UNIX and UNIX-like systems. Most Linux MAKEDEV
scripts symbolically link /dev/fd to [..]/proc/self/fd,
in fact.
maps A file containing the currently mapped memory regions and
their access permissions.
The format is:
address perms offset dev inode
00000000-0002f000 r-x-- 00000400 03:03 1401
0002f000-00032000 rwx-p 0002f400 03:03 1401
00032000-0005b000 rwx-p 00000000 00:00 0
60000000-60098000 rwx-p 00000400 03:03 215
60098000-600c7000 rwx-p 00000000 00:00 0
bfffa000-c0000000 rwx-p 00000000 00:00 0
where address is the address space in the process that it
occupies, perms is a set of permissions:
r = read
w = write
x = execute
s = shared
p = private (copy on write)
offset is the offset into the file/whatever, dev is the device
(major:minor), and inode is the inode on that device. 0
indicates that no inode is associated with the memory region, as
the case would be with bss.
Under Linux 2.2 there is an additional field giving a pathname
where applicable.
mem This is not the same as the mem (1:1) device, despite the
fact that it has the same device numbers. The /dev/mem
device is the physical memory before any address
translation is done, but the mem file here is the memory
of the process that accesses it. This cannot be mmap(2)
'ed currently, and will not be until a general mmap(2) is
added to the kernel. (This might have happened by the
time you read this.)
mmap Directory of maps by mmap(2) which are symbolic links
like exe, fd/*, etc. Note that maps includes a superset
of this information, so /proc/*/mmap should be considered
obsolete.
"0" is usually libc.so.4.
/proc/*/mmap was removed in Linux kernel version 1.1.40.
(It really was obsolete!)
root Unix and linux support the idea of a per-process root of
the filesystem, set by the chroot(2) system call. Root
points to the file system root, and behaves as exe, fd/*,
etc. do.
stat Status information about the process. This is used by
ps(1).
The fields, in order, with their proper scanf(3) format
specifiers, are:
pid %d The process id.
comm %s
The filename of the executable, in parentheses.
This is visible whether or not the executable is
swapped out.
state %c
One character from the string "RSDZT" where R is
running, S is sleeping in an interruptible wait, D
is sleeping in an uninterruptible wait or
swapping, Z is zombie, and T is traced or stopped
(on a signal).
ppid %d
The PID of the parent.
pgrp %d
The process group ID of the process.
session %d
The session ID of the process.
tty %d The tty the process uses.
tpgid %d
The process group ID of the process which
currently owns the tty that the process is
connected to.
flags %u
The flags of the process. Currently, every flag
has the math bit set, because crt0.s checks for
math emulation, so this is not included in the
output. This is probably a bug, as not every
process is a compiled C program. The math bit
should be a decimal 4, and the traced bit is
decimal 10.
minflt %u
The number of minor faults the process has made,
those which have not required loading a memory
page from disk.
cminflt %u
The number of minor faults that the process and
its children have made.
majflt %u
The number of major faults the process has made,
those which have required loading a memory page
from disk.
cmajflt %u
The number of major faults that the process and
its children have made.
utime %d
The number of jiffies that this process has been
scheduled in user mode.
stime %d
The number of jiffies that this process has been
scheduled in kernel mode.
cutime %d
The number of jiffies that this process and its
children have been scheduled in user mode.
cstime %d
The number of jiffies that this process and its
children have been scheduled in kernel mode.
counter %d
The current maximum size in jiffies of the
process's next timeslice, or what is currently
left of its current timeslice, if it is the
currently running process.
priority %d
The standard nice value, plus fifteen. The value
is never negative in the kernel.
timeout %u
The time in jiffies of the process's next timeout.
itrealvalue %u
The time (in jiffies) before the next SIGALRM is
sent to the process due to an interval timer.
starttime %d
Time the process started in jiffies after system
boot.
vsize %u
Virtual memory size
rss %u Resident Set Size: number of pages the process has
in real memory, minus 3 for administrative
purposes. This is just the pages which count
towards text, data, or stack space. This does not
include pages which have not been demand-loaded
in, or which are swapped out.
rlim %u
Current limit in bytes on the rss of the process
(usually 2,147,483,647).
startcode %u
The address above which program text can run.
endcode %u
The address below which program text can run.
startstack %u
The address of the start of the stack.
kstkesp %u
The current value of esp (32-bit stack pointer),
as found in the kernel stack page for the process.
kstkeip %u
The current EIP (32-bit instruction pointer).
signal %d
The bitmap of pending signals (usually 0).
blocked %d
The bitmap of blocked signals (usually 0, 2 for
shells).
sigignore %d
The bitmap of ignored signals.
sigcatch %d
The bitmap of catched signals.
wchan %u
This is the "channel" in which the process is
waiting. This is the address of a system call,
and can be looked up in a namelist if you need a
textual name. (If you have an up-to-date
/etc/psdatabase, then try ps -l to see the WCHAN
field in action)
cpuinfo
This is a collection of CPU and system architecture dependent
items, for each supported architecture a different list. The
only two common entries are cpu which is (guess what) the CPU
currently in use and BogoMIPS a system constant which is
calculated during kernel initialization.
devices
Text listing of major numbers and device groups. This can be
used by MAKEDEV scripts for consistency with the kernel.
dma This is a list of the registered ISA DMA (direct memory access)
channels in use.
filesystems
A text listing of the filesystems which were compiled into the
kernel. Incidentally, this is used by mount(1) to cycle through
different filesystems when none is specified.
interrupts
This is used to record the number of interrupts per each IRQ on
(at least) the i386 architechure. Very easy to read formatting,
done in ASCII.
ioports
This is a list of currently registered Input-Output port regions
that are in use.
kcore This file represents the physical memory of the system and is
stored in the core file format. With this pseudo-file, and an
unstripped kernel (/usr/src/linux/tools/zSystem) binary, GDB can
be used to examine the current state of any kernel data
structures.
The total length of the file is the size of physical memory
(RAM) plus 4KB.
kmsg This file can be used instead of the syslog(2) system call to
log kernel messages. A process must have superuser privileges
to read this file, and only one process should read this file.
This file should not be read if a syslog process is running
which uses the syslog(2) system call facility to log kernel
messages.
Information in this file is retrieved with the dmesg(8)
program).
ksyms This holds the kernel exported symbol definitions used by the
modules(X) tools to dynamically link and bind loadable modules.
loadavg
The load average numbers give the number of jobs in the run
queue (state R) or waiting for disk I/O (state D) averaged over
1, 5 and 15 minutes. They are the same as the load average
numbers given by uptime(1) and other programs.
locks This file shows current file locks.
locks This file shows current file locks.
malloc This file is only present if CONFIGDEBUGMALLOC was defined
during compilation.
meminfo
This is used by free(1) to report the amount of free and used
memory (both physical and swap) on the system as well as the
shared memory and buffers used by the kernel.
It is in the same format as free(1), except in bytes rather than
KB.
modules
A text list of the modules that have been loaded by the system.
net various net pseudo-files, all of which give the status of some
part of the networking layer. These files contain ASCII
structures, and are therefore readable with cat. However, the
standard netstat(8) suite provides much cleaner access to these
files.
arp This holds an ASCII readable dump of the kernel ARP table
used for address resolutions. It will show both
dynamically learned and pre-programmed ARP entries. The
format is:
IP address HW type Flags HW address
10.11.100.129 0x1 0x6 00:20:8A:00:0C:5A
10.11.100.5 0x1 0x2 00:C0:EA:00:00:4E
44.131.10.6 0x3 0x2 GW4PTS
Where 'IP address' is the IPv4 address of the machine, the 'HW
type' is the hardware type of the address from RFC 826. The
flags are the internal flags of the ARP structure (as defined in
/usr/include/linux/if_arp.h) and the 'HW address' is the
physical layer mapping for that IP address if it is known.
dev The dev pseudo-file contains network device status
information. This gives the number of received and sent
packets, the number of errors and collisions and other
basic statistics. These are used by the ifconfig(8)
program to report device status. The format is:
Inter-| Receive | Transmit
face |packets errs drop fifo frame|packets errs drop fifo colls carrier
lo: 0 0 0 0 0 2353 0 0 0 0 0
eth0: 644324 1 0 0 1 563770 0 0 0 581 0
ipx No information.
ipx_route
No information.
rarp This file uses the same format as the arp file and
contains the current reverse mapping database used to
provide rarp(8) reverse address lookup services. If RARP
is not configured into the kernel this file will not be
present.
raw Holds a dump of the RAW socket table. Much of the
information is not of use apart from debugging. The 'sl'
value is the kernel hash slot for the socket, the 'local
address' is the local address and protocol number
pair."St" is the internal status of the socket. The
"tx_queue" and "rx_queue" are the outgoing and incoming
data queue in terms of kernel memory usage. The "tr",
"tm->when" and "rexmits" fields are not used by RAW. The
uid field holds the creator euid of the socket.
route No information, but looks similar to route(8)
snmp This file holds the ASCII data needed for the IP, ICMP,
TCP and UDP management information bases for an snmp
agent. As of writing the TCP mib is incomplete. It is
hoped to have it completed by 1.2.0.
tcp Holds a dump of the TCP socket table. Much of the
information is not of use apart from debugging. The "sl"
value is the kernel hash slot for the socket, the "local
address" is the local address and port number pair. The
"remote address" is the remote address and port number
pair (if connected). 'St' is the internal status of the
socket. The 'tx_queue' and 'rx_queue' are the outgoing
and incoming data queue in terms of kernel memory usage.
The "tr", "tm->when" and "rexmits" fields hold internal
information of the kernel socket state and are only
useful for debugging. The uid field holds the creator
euid of the socket.
udp Holds a dump of the UDP socket table. Much of the
information is not of use apart from debugging. The "sl"
value is the kernel hash slot for the socket, the "local
address" is the local address and port number pair. The
"remote address" is the remote address and port number
pair (if connected). "St" is the internal status of the
socket. The "tx_queue" and "rx_queue" are the outgoing
and incoming data queue in terms of kernel memory usage.
The "tr", "tm->when" and "rexmits" fields are not used by
UDP. The uid field holds the creator euid of the socket.
The format is:
sl local_address rem_address st tx_queue rx_queue tr rexmits tm->when uid
1: 01642C89:0201 0C642C89:03FF 01 00000000:00000001 01:000071BA 00000000 0
1: 00000000:0801 00000000:0000 0A 00000000:00000000 00:00000000 6F000100 0
1: 00000000:0201 00000000:0000 0A 00000000:00000000 00:00000000 00000000 0
unix Lists the UNIX domain sockets present within the system
and their status. The format is:
Num RefCount Protocol Flags Type St Path
0: 00000002 00000000 00000000 0001 03
1: 00000001 00000000 00010000 0001 01 /dev/printer
Where 'Num' is the kernel table slot number, 'RefCount' is the
number of users of the socket, 'Protocol' is currently always 0,
'Flags' represent the internal kernel flags holding the status
of the socket. Type is always '1' currently (Unix domain
datagram sockets are not yet supported in the kernel). 'St' is
the internal state of the socket and Path is the bound path (if
any) of the socket.
pci This is a listing of all PCI devices found during kernel
initialization and their configuration.
scsi A directory with the scsi midlevel pseudo-file and various SCSI
lowlevel driver directories, which contain a file for each SCSI
host in this system, all of which give the status of some part
of the SCSI IO subsystem. These files contain ASCII structures,
and are therefore readable with cat.
You can also write to some of the files to reconfigure the
subsystem or switch certain features on or off.
scsi This is a listing of all SCSI devices known to the
kernel. The listing is similar to the one seen during
bootup. scsi currently supports only the add-single-
device command which allows root to add a hotplugged
device to the list of known devices.
An echo 'scsi add-single-device 1 0 5 0' >
/proc/scsi/scsi will cause host scsi1 to scan on SCSI
channel 0 for a device on ID 5 LUN 0. If there is already
a device known on this address or the address is invalid
an error will be returned.
drivername
drivername can currently be: NCR53c7xx, aha152x, aha1542,
aha1740, aic7xxx, buslogic, eata_dma, eata_pio, fdomain,
in2000, pas16, qlogic, scsi_debug, seagate, t128,
u15-24f, ultrastore or wd7000. These directories show up
for all drivers which registered at least one SCSI HBA.
Every directory contains one file per registered host.
Every host-file is named after the number the host got
assigned during initilization.
Reading these files will usually show driver and host
configuration, statistics etc.
Writing to these files allows different things on
different hosts. For example with the latency and
nolatency commands root can switch on and off command
latency measurement code in the eata_dma driver. With the
lockup and unlock commands root can control bus lockups
simulated by the scsi_debug driver.
self This directory refers to the process accessing the /proc
filesystem, and is identical to the /proc directory named by the
process ID of the same process.
stat kernel/system statistics
cpu 3357 0 4313 1362393
The number of jiffies (1/100ths of a second) that the
system spent in user mode, user mode with low priority
(nice), system mode, and the idle task, respectively.
The last value should be 100 times the second entry in
the uptime pseudo-file.
disk 0 0 0 0
The four disk entries are not implemented at this time.
I'm not even sure what this should be, since kernel
statistics on other machines usually track both transfer
rate and I/Os per second and this only allows for one
field per drive.
page 5741 1808
The number of pages the system paged in and the number
that were paged out (from disk).
swap 1 0
The number of swap pages that have been brought in and
out.
intr 1462898
The number of interrupts received from the system boot.
ctxt 115315
The number of context switches that the system underwent.
btime 769041601
boot time, in seconds since the epoch (January 1, 1970).
sys This directory (present since 1.3.57) contains a number of files
and subdirectories corresponding to kernel variables. These
variables can be read and sometimes modified using the proc file
system, and using the sysctl(2) system call. Presently, there
are subdirectories kernel, net, vm that each contain more files
and subdirectories.
kernel This contains files domainname, file-max, file-nr,
hostname, inode-max, inode-nr, osrelease, ostype, panic,
real-root-dev, securelevel, version, with function fairly
clear from the name.
The (read-only) file file-nr gives the number of files presently
opened.
The file file-max gives the maximum number of open files the
kernel is willing to handle. If 1024 is not enough for you, try
echo 4096 > /proc/sys/kernel/file-max
Similarly, the files inode-nr and inode-max indicate the present
and the maximum number of inodes.
The files ostype, osrelease, version give substrings of
/proc/version.
The file panic gives r/w access to the kernel variable
panic_timeout. If this is zero, the kernel will loop on a
panic; if nonzero it indicates that the kernel should autoreboot
after this number of seconds.
The file securelevel seems rather meaningless at present - root
is just too powerful.
uptime This file contains two numbers: the uptime of the system
(seconds), and the amount of time spent in idle process
(seconds).
version
This strings identifies the kernel version that is currently
running. For instance:
Linux version 1.0.9 (quinlan@phaze) #1 Sat May 14 01:51:54 EDT 1994
cat(1), find(1), free(1), mount(1), ps(1), tr(1), uptime(1), chroot(2),
mmap(2), readlink(2), syslog(2), slabinfo(5), hier(7), arp(8),
dmesg(8), netstat(8), ifconfig(8), procinfo(8), route(8), and much more
This roughly conforms to a Linux 1.3.11 kernel. Please update this as
necessary!
Last updated for Linux 1.3.11.
Note that many strings (i.e., the environment and command line) are in
the internal format, with sub-fields terminated by NUL bytes, so you
may find that things are more readable if you use od -c or tr "\000"
"\n" to read them.
This manual page is incomplete, possibly inaccurate, and is the kind of
thing that needs to be updated very often.
The /proc file system may introduce security holes into processes
running with chroot(2). For example, if /proc is mounted in the chroot
hierarchy, a chdir(2) to /proc/1/root will return to the original root
of the file system. This may be considered a feature instead of a bug,
since Linux does not yet support the fchroot(2) call.
1996-07-22 PROC(5)
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