PROCFS(5) FreeBSD File Formats Manual PROCFS(5)


procfsprocess file system


proc  /proc procfs rw 0 0


The process file system, or procfs, implements a view of the system process table inside the file system. It is normally mounted on /proc, and is required for the complete operation of programs such as ps(1) and w(1).

The procfs provides a two-level view of process space, unlike the previous FreeBSD 1.1 procfs implementation. At the highest level, processes themselves are named, according to their process ids in decimal, with no leading zeros. There is also a special node called curproc which always refers to the process making the lookup request.

Each node is a directory which contains the following entries:

Each directory contains several files:

a write-only file which supports a variety of control operations. Control commands are written as strings to the ctl file. The control commands are:
stops the target process and arranges for the sending process to become the debug control process.
continue execution of the target process and remove it from control by the debug process (which need not be the sending process).
continue running the target process until a signal is delivered, a breakpoint is hit, or the target process exits.
single step the target process, with no signal delivery.
wait for the target process to come to a steady state ready for debugging. The target process must be in this state before any of the other commands are allowed.

The string can also be the name of a signal, lower case and without the SIG prefix, in which case that signal is delivered to the process (see sigaction(2)).

The procctl(8) utility can be used to clear tracepoints in a stuck process.

The debug registers as defined by struct dbregs in < machine/reg.h>. dbregs is currently only implemented on the i386 architecture.
The type of the executable referenced by the file entry.
A symbolic link to the file from which the process text was read. This can be used to gain access to the process' symbol table, or to start another copy of the process. If the file cannot be found, the link target is ‘ unknown’.
The floating point registers as defined by struct fpregs in < machine/reg.h>. fpregs is only implemented on machines which have distinct general purpose and floating point register sets.
A map of the process' virtual memory.
The complete virtual memory image of the process. Only those address which exist in the process can be accessed. Reads and writes to this file modify the process. Writes to the text segment remain private to the process.
Used for sending signals to the process. Not implemented.
Used for sending signal to the process group. Not implemented.
Allows read and write of the kernel osrel value assigned to the process. It affects the compatibility shims that are turned on and off depending on the value. Initial process value is read from the ABI note tag in the executed ELF image, and is zero if the tag not supported by binary format or was not found.
Allows read and write access to the process' register set. This file contains a binary data structure struct regs defined in < machine/reg.h>. regs can only be written when the process is stopped.
This is a read-only file containing the process current and maximum limits. Each line is of the format rlimit current max, with -1 indicating infinity.
The process status. This file is read-only and returns a single line containing multiple space-separated fields as follows:

  • command name
  • process id
  • parent process id
  • process group id
  • session id
  • device name of the controlling terminal, or a minus sign (“-”) if there is no controlling terminal.
  • a list of process flags: ctty if there is a controlling terminal, sldr if the process is a session leader, noflags if neither of the other two flags are set.
  • the process start time in seconds and microseconds, comma separated.
  • the user time in seconds and microseconds, comma separated.
  • the system time in seconds and microseconds, comma separated.
  • the wait channel message
  • the process credentials consisting of the effective user id and the list of groups (whose first member is the effective group id) all comma separated.
  • the hostname of the jail in which the process runs, or ‘ -’ to indicate that the process is not running within a jail.

In a normal debugging environment, where the target is fork/exec'd by the debugger, the debugger should fork and the child should stop itself (with a self-inflicted SIGSTOP for example). The parent should issue a wait and then an attach command via the appropriate ctl file. The child process will receive a SIGTRAP immediately after the call to exec (see execve(2)).

Each node is owned by the process's user, and belongs to that user's primary group, except for the mem node, which belongs to the kmem group.


normal mount point for the procfs.
directory containing process information for process pid.
directory containing process information for the current process
the process executable name
used to send control messages to the process
executable type
executable image
the process floating point register set
virtual memory map of the process
the complete virtual address space of the process
used for signaling the process
used for signaling the process group
the process osrel value
the process register set
the process current and maximum rlimit
the process' current status


To mount a procfs file system on /proc:

mount -t procfs proc /proc


This manual page written by Garrett Wollman, based on the description provided by Jan-Simon Pendry, and revamped later by Mike Pritchard.
April 22, 2013 FreeBSD