Using and Programming Proc filesystem

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Content

● What's Procfs

● Basic useness of Procfs

● Programming Procfs

● Example source code

● Conclusion

What is Procfs

● In its simplest form procfs provides file-system abstraction for processes, presenting information about processes, their execution environment, resource utilization etc. as files.

● The original design goal of procfs was to make debugger implementation easier.

● Systems like Linux have exposed a lot of other system interfaces and information through procfs

● There are Linux system utilities that essentially cat files in procfs!

What is Procfs (cont.)

● It is a virtual file system

– Is a special file system in the linux kernel.

– It exists only in memory.

– Is a virtual file system that is not associated with a block device but exists only in memory.

– allows programs in the user space to access to certain information from the kernel.

What is Procfs (cont.)

● It is an abstraction allowing

– access information on

– configure the kernel

● It can be mounted by

# mount -t proc proc /mnt

Now procfs is mounted on /mnt, same to /proc

Basic useness of Procfs

● /proc/sys are sysctl files

– They do not belong to procfs

– Managed by different APIs

● Number named Dirs

– PID as name

– Appear and disappear as processes are created, finish

Basic useness of Procfs (cont.)● Structure

– cmdline : argument list, null-separated strings

– cwd : symbolic link to the current working directory

– environ : environment for the process

– exe : a symbolic link containing the actual path name of the executed command

– fd : a subdirectory containing one entry for each file which the process has open (file descriptor), a symbolic link to the actual file)

– maps : currently mapped memory regions and their access permissions

– root : points to the file system root, e.g /

– stat : information about the process (used by ps)

– statm : information about memory status in pages

– status : much of the information in stat and statm in a more human readable/parsable format

Basic useness of Procfs (cont.)● /proc/self: symbolic link to the current process

● /proc/cpuinfo: a collection of CPU and system architecture dependent items, for each supported architecture a different list

● /proc/devices: listing of major numbers and device groups

● /proc/pci: listing of all PCI devices found during kernel initialization and their configuration (lspci command)

● /proc/tty/driver/serial: information on serial ports

● /proc/version: information on the kernel version

– /proc/sys/kernel/ostype

– /proc/sys/kernel/osrelease

– /proc/sys/kernel/version

Basic useness of Procfs (cont.)● /proc/sys/kernel/hostname: the name of the machine

● /proc/sys/kernel/domainname: the name of the domain

● /proc/meminfo: information on memory usage

● /proc/loadavg: load average numbers give information on the workload:

– 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

– PID of the last process being executed

● /proc/uptime: two pieces of information on the uptime of the system (seconds), and the amount of time spent in idle process (seconds)

– Same information as uptime

Programming Procfs● Program as kernel module

● #includeu <linux/proc_fs.h>

● struct proc_dir_entry* proc_mkdir

(const char* name,struct proc_dir_entry* parent);

● struct proc_dir_entry* create_proc_entry

(const char* name, mode_t mode,struct proc_dir_entry* parent);

● struct proc_dir_entry* proc_symlink

(const char* name,struct proc_dir_entry* parent,

const char *dest);

● void remove_proc_entry

(const char *name, struct proc_dir_entry *parent);

Programming Procfs (cont.)● procfs works with call back functionsfor files:

functions that are called when a specific file is being read or written.

● Such functions have to be initialized by setting the file_operations member in the struct proc_dir_entry* that the function create_proc_entry returned.

● For read only proc file, call create_proc_read_entry is enough.

Programming Procfs (cont.)● define read, write, open, release methods for your proc file

see example source code.

● Original procfs can not pass data larger than 1 page. In order to pass large data, you should define a sequence method.

● Other detailed example, see the source code.

Example source code● kernel module frame work.

● read the source code for each method of proc fs.

● build it

● insmod into kernel

● cd /proc, see /proc/foo and /proc/foo/foo

● try read/write permission

● be root user

● try read/write permission

● rmmod the kernel module

Conclusion● Its candidate is sysfs.

● But Procfs is still very useful, because it is simple, and lot of legacy software need procfs.

Thank you !

Q & A