Using and Programming Proc filesystemfrom internet ...
Content● Whats 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 ex ecution environment, resource utilization etc. as files.● The original design goal of procfs was to make debugger impl ementation easier.● Systems like Linux have exposed a lot of other system interf aces 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 info rmation 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 depen dent 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 i nitialization 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 (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.