11-05-2001
The algorithm for allocating process IDs differs by type of OS. Some OS types allocate in sequential order up to 32,000 and then start over at the beginning using free slots. Others, namely AIX, will reuse PIDs as they are freed. Regardless of how the PID is allocated, the PID is actually an index into the Process Table in the Kernel which is an array of structures of type proc (see /usr/include/sys/proc.h). If the process is runnable, active, or in a zombie state, the slot in the process table is marked as full. When the process dies and is successfully cleaned up, the slot is available for allocation by the dispatcher when a new process is started. So, to summarize, PIDs are unique in all types of UNIX.
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LEARN ABOUT DEBIAN
pstree
PSTREE(1) User Commands PSTREE(1)
NAME
pstree - display a tree of processes
SYNOPSIS
pstree [-a|--arguments] [-c|--compact] [-h|--highlight-all|-Hpid|--highlight-pid pid] [-g]--show-pgids] [-l|--long] [-n|--numeric-sort]
[-p|--show-pids] [-s|--show-parents] [-u|--uid-changes] [-Z|--security-context] [-A|--ascii|-G|--vt100|-U|--unicode] [pid|user]
pstree -V|--version
DESCRIPTION
pstree shows running processes as a tree. The tree is rooted at either pid or init if pid is omitted. If a user name is specified, all
process trees rooted at processes owned by that user are shown.
pstree visually merges identical branches by putting them in square brackets and prefixing them with the repetition count, e.g.
init-+-getty
|-getty
|-getty
`-getty
becomes
init---4*[getty]
Child threads of a process are found under the parent process and are shown with the process name in curly braces, e.g.
icecast2---13*[{icecast2}]
If pstree is called as pstree.x11 then it will prompt the user at the end of the line to press return and will not return until that has
happened. This is useful for when pstree is run in a xterminal.
OPTIONS
-a Show command line arguments. If the command line of a process is swapped out, that process is shown in parentheses. -a implicitly
disables compaction for processes but not threads.
-A Use ASCII characters to draw the tree.
-c Disable compaction of identical subtrees. By default, subtrees are compacted whenever possible.
-G Use VT100 line drawing characters.
-h Highlight the current process and its ancestors. This is a no-op if the terminal doesn't support highlighting or if neither the cur-
rent process nor any of its ancestors are in the subtree being shown.
-H Like -h, but highlight the specified process instead. Unlike with -h, pstree fails when using -H if highlighting is not available.
-g Show PGIDs. Process Group IDs are shown as decimal numbers in parentheses after each process name. -p implicitly disables com-
paction. If both PIDs and PGIDs are displayed then PIDs are shown first.
-l Display long lines. By default, lines are truncated to the display width or 132 if output is sent to a non-tty or if the display
width is unknown.
-n Sort processes with the same ancestor by PID instead of by name. (Numeric sort.)
-p Show PIDs. PIDs are shown as decimal numbers in parentheses after each process name. -p implicitly disables compaction.
-s Show parent processes of the specified process.
-u Show uid transitions. Whenever the uid of a process differs from the uid of its parent, the new uid is shown in parentheses after
the process name.
-U Use UTF-8 (Unicode) line drawing characters. Under Linux 1.1-54 and above, UTF-8 mode is entered on the console with echo -e
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