UNIX Filesystems: Evolution, Design, and Implementation

The /proc Filesystem

The /proc filesystem was first introduced in 8th Edition UNIX and was described in Tom Killian's 1984 Usenix paper “Processes as Files” [KILL84].

The /proc filesystem was to replace the ptrace() system call, with the advantage that the full process address space was visible and could be manipulated with read() and write() system calls. This contrasts with the interfaces offered by ptrace(), the system call traditionally used by debuggers, that only provides a word-at-a-time interface.

Roger Faulkner and Ron Gomes ported the research version of /proc to SVR4 and presented their work in another USENIX paper: “The Process File System and Process Model in UNIX System V” [FAUL91]. At that time, Faulkner was with Sun Microsystems and Gomes with AT&T Bell Laboratories. As described in the paper, future work was intended to restructure /proc from a flat file system into a directory hierarchy describing a process. That work was undertaken at both Sun and USL and will be described later.

In the early /proc implementation, whose name is derived from the directory on which it is mounted, there is an entry in the directory for each process in the system. The name of the file displayed corresponds to the process ID, while the size of the file represents the size of the process address space. The file permissions correspond to the user who owns the process.

Figure 11.1 shows at a high level how the /proc filesystem is implemented. Standard file-related system calls such as ...