Does /proc/ contains tid

procprocess

I have process id 1234. This process contains thread id 1235.

When I use ls -l /proc I see only the pid (1234) but when I open the thread status file using cat /proc/1235/status I can see data.

Why is that?

Can I access with C code directly with tid? /proc/1235/mem without knowing the process id?

Best Answer

Yes, /proc “contains” directory entries for thread identifiers as well as process identifiers, but only the latter are enumerated by getdents, so ls only shows process identifiers. This is described in man 5 proc, in the “Overview” section, since release 5.00 of the man-pages project:

/proc/[pid] subdirectories

The /proc/[pid] subdirectories are visible when iterating through /proc with getdents(2) (and thus are visible when one uses ls(1) to view the contents of /proc).

/proc/[tid] subdirectories

The /proc/[tid] subdirectories are not visible when iterating through /proc with getdents(2) (and thus are not visible when one uses ls(1) to view the contents of /proc).

Why is that?

I suspect it’s to preserve backwards-compatibility (for programs written before threads existed in their current form on Linux), and to limit scalability problems.

Can I access with C code directly with tid? /proc/1235/mem without know the process id?

Yes, if you know the tid you can access /proc/${tid} directly, without going through the pid.

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You can look at field 5th in output of /proc/[pid]/stat.

$ ps -ejH | grep firefox
 3043  2683  2683 ?        00:00:21   firefox

$ < /proc/3043/stat sed -n '$s/.*) [^ ]* [^ ]* \([^ ]*\).*/\1/p'
2683

From man proc:

/proc/[pid]/stat
              Status information about the process.  This is used by ps(1).  It is defined in /usr/src/linux/fs/proc/array.c.

              The fields, in order, with their proper scanf(3) format specifiers, are:

              pid %d      The process ID.

              comm %s     The filename of the executable, in parentheses.  This is visible whether or not the executable is swapped out.

              state %c    One character from the string "RSDZTW" where R is running, S is sleeping in an interruptible wait, D is waiting in
                          uninterruptible disk sleep, Z is zombie, T is traced or stopped (on a signal), and W is paging.

              ppid %d     The PID of the parent.

              pgrp %d     The process group ID of the process.

              session %d  The session ID of the process.

Note that you cannot use:

awk '{print $5}'

Because that file is not a blank separated list. The second field (the process name may contain blanks or even newline characters). For instance, most of the threads of firefox typically have space characters in their name.

So you need to print the 3rd field after the last occurrence of a ) character in there.

Best Answer

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