The Process Filesystem

Unlike some of the more esoteric resources that can be referred to by a file descriptor, the entries found in the /proc directory on any Linux system are in fact real files.

However, they are not entirely like other files: they are transient. That is to say, these files are not stored on any long-term storage media, e.g. a hard drive. These files don’t need long term storage because they provide access to information that only exists at runtime.

Instead of reading the directory structure and contents from a storage medium, the kernel creates the files in /proc at runtime and synthesizes their contents on demand.

Specifically, the kernel creates a directory for each running process on the system named after its pid. In addition, the kernel provides a “magic” symlink named self whose target depends on which process is looking. Any process that examines the symlink sees it resolve to the folder that corresponds to the calling process’s pid.

This directory contains information about running processes. For a complete list of the contents, refer to the kernel documentation and the manpage.

Unfortunately, /proc also contains many miscellaneous files that were added before the community developed /sys. They are still present to preserve backwards compatibility.

A /proctical example

In bash, $$ is a special variable that expands to the pid of the bash process.

For example:

	$ echo $$

This means we can use $$ when building a path to reference the /proc subdirectory corresponding to the running bash process. In P1, the systemcall used the get_task_comm kernel macro to find the name of the running program. /proc also provides userspace access to this information. Here is an example:

	$ cat /proc/$$/comm

We can also discover the absolute path of the executable invoked to start the process by traversing another “magic” symlink named exe:

	$ readlink /proc/$$/exe

If we replace $$ with self, we are now referring to the child process the shell created by forking itself and execing the user command:

	$ cat /proc/self/comm

	$ readlink /proc/self/exe

Another useful entry in /proc for a given process is the fd directory, which contains magic symlinks to all file descriptors owned by the process:

	$ ls -l /proc/self/fd
	... 0 -> /dev/pts/0
	... 1 -> /dev/pts/0
	... 2 -> /dev/pts/0
	... 3 -> /proc/128523/fd

As expected, the first three entries are stdin, stdout, and stderr which are connected to our terminal. We can also see how the ls program opens its own subdirectory in /proc by following the “magic” /proc/self symlink.