FreeBSD and procfs

Many Unix systems have support for proc file system (process file system).Procfs filesystem type is pseudo. FreeBSD is one of that Unix systems. Unlike Linux, which has information other than processes, FreeBSD procfs support is only about the processes on the system. FreeBSD doesn't mount the procfs on boot by default. You need to manually add it to fstab for auto mount on boot or mount it by command for temporarily usage.Common mount point for procfs is /proc on Unix systems.

echo "none            /proc        procfs  rw   0 0" >> /etc/fsab

mount -t procfs none /proc

Every process is presented as directories named by it's pid number on the /proc mount point.
Procfs gives information on running processes on the system like memory mapping,command line arguments of running process, process resource limits and many other.Following is a sample procfs directory structure on a FreeBSD machine.



As you can see, every pid is represented as a directory in procfs. Every directory contains following files. Some of the files are write only or read only where you read information or send information to process.

- status  (read-only) : returns process status
- mem     (read/write): virtual memory image of the process
- file    (depends)   : symbolic link to running process
- regs    (read/write): process registers
- ctl     (write-only): used to send signal to process or 
                        attach/deattach it for debugging
- cmdline (read-only) : command line arguments of running process
- rlimits (read-only) : current resource limits of running process
- map     (read-only) : memory mappings of the running process.
- etype   (read-only) : type of the executable (eg. FreeBSD ELF32)
- fpregs  (read/write): floating point registers 

Some of the information provided by these files are in binary format.For example "regs" and "fpregs" files are in binary format. They depend on the architecture of the underlying machine (i386, amd64,sparc64,etc..). Following is the format of the "regs" file on the i386 machine.

struct reg {
        unsigned int    r_fs;
        unsigned int    r_es;
        unsigned int    r_ds;
        unsigned int    r_edi;
        unsigned int    r_esi;
        unsigned int    r_ebp;
        unsigned int    r_isp;
        unsigned int    r_ebx;
        unsigned int    r_edx;
        unsigned int    r_ecx;
        unsigned int    r_eax;
        unsigned int    r_trapno;
        unsigned int    r_err;
        unsigned int    r_eip;
        unsigned int    r_cs;
        unsigned int    r_eflags;
        unsigned int    r_esp;
        unsigned int    r_ss;
        unsigned int    r_gs;
};

You can use "cat" command to read information provided by procfs for text based information unlike the ones I mentioned above in binary format like "regs","fpregs" and "mem".

[root@freebsd ]# cat cmdline 
/usr/sbin/moused-p/dev/ums0-tauto-I/var/run/moused.ums0.pid

You can check a running process's resource limits by looking into rlimit file.

[root@freebsd ]# cat rlimit 
cpu -1 -1
fsize -1 -1
data 536870912 536870912
stack 67108864 67108864
core -1 -1
rss -1 -1
memlock -1 -1
nproc 5547 5547
nofile 11095 11095
sbsize -1 -1
vmem -1 -1

First digits is minimum and last one is maximum values of the given resource name. "-1" means infinite. For examle, nofile (open file descriptor) limit for this process is 11095 as minimum and maximum.

"status" file gives information about process status as follows.

- command name
- pid
- parent pid
- process group id
- session id
- major/minor of the terminal, "-" if no terminal is in action
- process flags
- process start time in seconds and microseconds separated by comma
- user time in seconds and microseconds separated by comma
- system time in seconds and microseconds separated by comma
- wait channel name
- effective userid and group lists separated by comma

Following is "cat status" result of a process.

[root@freebsd ]# cat status 
svscan 28246 1 28245 0 ttyp0 noflags 1242921860,839572 0,318052 3,263576 nanslp 0 0 0,0,0,5 -