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diff --git a/docs/ctty.htm b/docs/ctty.htm new file mode 100644 index 000000000..26d2c7956 --- /dev/null +++ b/docs/ctty.htm @@ -0,0 +1,474 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> +<html><head> + <!-- saved from http://www.win.tue.nl/~aeb/linux/lk/lk-10.html --> + <meta name="GENERATOR" content="SGML-Tools 1.0.9"><title>The Linux kernel: Processes</title> +</head> +<body> +<hr> +<h2><a name="s10">10. Processes</a></h2> + +<p>Before looking at the Linux implementation, first a general Unix +description of threads, processes, process groups and sessions. +</p><p>A session contains a number of process groups, and a process group +contains a number of processes, and a process contains a number +of threads. +</p><p>A session can have a controlling tty. +At most one process group in a session can be a foreground process group. +An interrupt character typed on a tty ("Teletype", i.e., terminal) +causes a signal to be sent to all members of the foreground process group +in the session (if any) that has that tty as controlling tty. +</p><p>All these objects have numbers, and we have thread IDs, process IDs, +process group IDs and session IDs. +</p><p> +</p><h2><a name="ss10.1">10.1 Processes</a> +</h2> + +<p> +</p><h3>Creation</h3> + +<p>A new process is traditionally started using the <code>fork()</code> +system call: +</p><blockquote> +<pre>pid_t p; + +p = fork(); +if (p == (pid_t) -1) + /* ERROR */ +else if (p == 0) + /* CHILD */ +else + /* PARENT */ +</pre> +</blockquote> +<p>This creates a child as a duplicate of its parent. +Parent and child are identical in almost all respects. +In the code they are distinguished by the fact that the parent +learns the process ID of its child, while <code>fork()</code> +returns 0 in the child. (It can find the process ID of its +parent using the <code>getppid()</code> system call.) +</p><p> +</p><h3>Termination</h3> + +<p>Normal termination is when the process does +</p><blockquote> +<pre>exit(n); +</pre> +</blockquote> + +or +<blockquote> +<pre>return n; +</pre> +</blockquote> + +from its <code>main()</code> procedure. It returns the single byte <code>n</code> +to its parent. +<p>Abnormal termination is usually caused by a signal. +</p><p> +</p><h3>Collecting the exit code. Zombies</h3> + +<p>The parent does +</p><blockquote> +<pre>pid_t p; +int status; + +p = wait(&status); +</pre> +</blockquote> + +and collects two bytes: +<p> +<figure> +<eps file="absent"> +<img src="ctty_files/exit_status.png"> +</eps> +</figure></p><p>A process that has terminated but has not yet been waited for +is a <i>zombie</i>. It need only store these two bytes: +exit code and reason for termination. +</p><p>On the other hand, if the parent dies first, <code>init</code> (process 1) +inherits the child and becomes its parent. +</p><p> +</p><h3>Signals</h3> + +<p> +</p><h3>Stopping</h3> + +<p>Some signals cause a process to stop: +<code>SIGSTOP</code> (stop!), +<code>SIGTSTP</code> (stop from tty: probably ^Z was typed), +<code>SIGTTIN</code> (tty input asked by background process), +<code>SIGTTOU</code> (tty output sent by background process, and this was +disallowed by <code>stty tostop</code>). +</p><p>Apart from ^Z there also is ^Y. The former stops the process +when it is typed, the latter stops it when it is read. +</p><p>Signals generated by typing the corresponding character on some tty +are sent to all processes that are in the foreground process group +of the session that has that tty as controlling tty. (Details below.) +</p><p>If a process is being traced, every signal will stop it. +</p><p> +</p><h3>Continuing</h3> + +<p><code>SIGCONT</code>: continue a stopped process. +</p><p> +</p><h3>Terminating</h3> + +<p><code>SIGKILL</code> (die! now!), +<code>SIGTERM</code> (please, go away), +<code>SIGHUP</code> (modem hangup), +<code>SIGINT</code> (^C), +<code>SIGQUIT</code> (^\), etc. +Many signals have as default action to kill the target. +(Sometimes with an additional core dump, when such is +allowed by rlimit.) +The signals <code>SIGCHLD</code> and <code>SIGWINCH</code> +are ignored by default. +All except <code>SIGKILL</code> and <code>SIGSTOP</code> can be +caught or ignored or blocked. +For details, see <code>signal(7)</code>. +</p><p> +</p><h2><a name="ss10.2">10.2 Process groups</a> +</h2> + +<p>Every process is member of a unique <i>process group</i>, +identified by its <i>process group ID</i>. +(When the process is created, it becomes a member of the process group +of its parent.) +By convention, the process group ID of a process group +equals the process ID of the first member of the process group, +called the <i>process group leader</i>. +A process finds the ID of its process group using the system call +<code>getpgrp()</code>, or, equivalently, <code>getpgid(0)</code>. +One finds the process group ID of process <code>p</code> using +<code>getpgid(p)</code>. +</p><p>One may use the command <code>ps j</code> to see PPID (parent process ID), +PID (process ID), PGID (process group ID) and SID (session ID) +of processes. With a shell that does not know about job control, +like <code>ash</code>, each of its children will be in the same session +and have the same process group as the shell. With a shell that knows +about job control, like <code>bash</code>, the processes of one pipeline. like +</p><blockquote> +<pre>% cat paper | ideal | pic | tbl | eqn | ditroff > out +</pre> +</blockquote> + +form a single process group. +<p> +</p><h3>Creation</h3> + +<p>A process <code>pid</code> is put into the process group <code>pgid</code> by +</p><blockquote> +<pre>setpgid(pid, pgid); +</pre> +</blockquote> + +If <code>pgid == pid</code> or <code>pgid == 0</code> then this creates +a new process group with process group leader <code>pid</code>. +Otherwise, this puts <code>pid</code> into the already existing +process group <code>pgid</code>. +A zero <code>pid</code> refers to the current process. +The call <code>setpgrp()</code> is equivalent to <code>setpgid(0,0)</code>. +<p> +</p><h3>Restrictions on setpgid()</h3> + +<p>The calling process must be <code>pid</code> itself, or its parent, +and the parent can only do this before <code>pid</code> has done +<code>exec()</code>, and only when both belong to the same session. +It is an error if process <code>pid</code> is a session leader +(and this call would change its <code>pgid</code>). +</p><p> +</p><h3>Typical sequence</h3> + +<p> +</p><blockquote> +<pre>p = fork(); +if (p == (pid_t) -1) { + /* ERROR */ +} else if (p == 0) { /* CHILD */ + setpgid(0, pgid); + ... +} else { /* PARENT */ + setpgid(p, pgid); + ... +} +</pre> +</blockquote> + +This ensures that regardless of whether parent or child is scheduled +first, the process group setting is as expected by both. +<p> +</p><h3>Signalling and waiting</h3> + +<p>One can signal all members of a process group: +</p><blockquote> +<pre>killpg(pgrp, sig); +</pre> +</blockquote> +<p>One can wait for children in ones own process group: +</p><blockquote> +<pre>waitpid(0, &status, ...); +</pre> +</blockquote> + +or in a specified process group: +<blockquote> +<pre>waitpid(-pgrp, &status, ...); +</pre> +</blockquote> +<p> +</p><h3>Foreground process group</h3> + +<p>Among the process groups in a session at most one can be +the <i>foreground process group</i> of that session. +The tty input and tty signals (signals generated by ^C, ^Z, etc.) +go to processes in this foreground process group. +</p><p>A process can determine the foreground process group in its session +using <code>tcgetpgrp(fd)</code>, where <code>fd</code> refers to its +controlling tty. If there is none, this returns a random value +larger than 1 that is not a process group ID. +</p><p>A process can set the foreground process group in its session +using <code>tcsetpgrp(fd,pgrp)</code>, where <code>fd</code> refers to its +controlling tty, and <code>pgrp</code> is a process group in the +its session, and this session still is associated to the controlling +tty of the calling process. +</p><p>How does one get <code>fd</code>? By definition, <code>/dev/tty</code> +refers to the controlling tty, entirely independent of redirects +of standard input and output. (There is also the function +<code>ctermid()</code> to get the name of the controlling terminal. +On a POSIX standard system it will return <code>/dev/tty</code>.) +Opening the name of the +controlling tty gives a file descriptor <code>fd</code>. +</p><p> +</p><h3>Background process groups</h3> + +<p>All process groups in a session that are not foreground +process group are <i>background process groups</i>. +Since the user at the keyboard is interacting with foreground +processes, background processes should stay away from it. +When a background process reads from the terminal it gets +a SIGTTIN signal. Normally, that will stop it, the job control shell +notices and tells the user, who can say <code>fg</code> to continue +this background process as a foreground process, and then this +process can read from the terminal. But if the background process +ignores or blocks the SIGTTIN signal, or if its process group +is orphaned (see below), then the read() returns an EIO error, +and no signal is sent. (Indeed, the idea is to tell the process +that reading from the terminal is not allowed right now. +If it wouldn't see the signal, then it will see the error return.) +</p><p>When a background process writes to the terminal, it may get +a SIGTTOU signal. May: namely, when the flag that this must happen +is set (it is off by default). One can set the flag by +</p><blockquote> +<pre>% stty tostop +</pre> +</blockquote> + +and clear it again by +<blockquote> +<pre>% stty -tostop +</pre> +</blockquote> + +and inspect it by +<blockquote> +<pre>% stty -a +</pre> +</blockquote> + +Again, if TOSTOP is set but the background process ignores or blocks +the SIGTTOU signal, or if its process group is orphaned (see below), +then the write() returns an EIO error, and no signal is sent. +<p> +</p><h3>Orphaned process groups</h3> + +<p>The process group leader is the first member of the process group. +It may terminate before the others, and then the process group is +without leader. +</p><p>A process group is called <i>orphaned</i> when <i>the +parent of every member is either in the process group +or outside the session</i>. +In particular, the process group of the session leader +is always orphaned. +</p><p>If termination of a process causes a process group to become +orphaned, and some member is stopped, then all are sent first SIGHUP +and then SIGCONT. +</p><p>The idea is that perhaps the parent of the process group leader +is a job control shell. (In the same session but a different +process group.) As long as this parent is alive, it can +handle the stopping and starting of members in the process group. +When it dies, there may be nobody to continue stopped processes. +Therefore, these stopped processes are sent SIGHUP, so that they +die unless they catch or ignore it, and then SIGCONT to continue them. +</p><p>Note that the process group of the session leader is already +orphaned, so no signals are sent when the session leader dies. +</p><p>Note also that a process group can become orphaned in two ways +by termination of a process: either it was a parent and not itself +in the process group, or it was the last element of the process group +with a parent outside but in the same session. +Furthermore, that a process group can become orphaned +other than by termination of a process, namely when some +member is moved to a different process group. +</p><p> +</p><h2><a name="ss10.3">10.3 Sessions</a> +</h2> + +<p>Every process group is in a unique <i>session</i>. +(When the process is created, it becomes a member of the session +of its parent.) +By convention, the session ID of a session +equals the process ID of the first member of the session, +called the <i>session leader</i>. +A process finds the ID of its session using the system call +<code>getsid()</code>. +</p><p>Every session may have a <i>controlling tty</i>, +that then also is called the controlling tty of each of +its member processes. +A file descriptor for the controlling tty is obtained by +opening <code>/dev/tty</code>. (And when that fails, there was no +controlling tty.) Given a file descriptor for the controlling tty, +one may obtain the SID using <code>tcgetsid(fd)</code>. +</p><p>A session is often set up by a login process. The terminal +on which one is logged in then becomes the controlling tty +of the session. All processes that are descendants of the +login process will in general be members of the session. +</p><p> +</p><h3>Creation</h3> + +<p>A new session is created by +</p><blockquote> +<pre>pid = setsid(); +</pre> +</blockquote> + +This is allowed only when the current process is not a process group leader. +In order to be sure of that we fork first: +<blockquote> +<pre>p = fork(); +if (p) exit(0); +pid = setsid(); +</pre> +</blockquote> + +The result is that the current process (with process ID <code>pid</code>) +becomes session leader of a new session with session ID <code>pid</code>. +Moreover, it becomes process group leader of a new process group. +Both session and process group contain only the single process <code>pid</code>. +Furthermore, this process has no controlling tty. +<p>The restriction that the current process must not be a process group leader +is needed: otherwise its PID serves as PGID of some existing process group +and cannot be used as the PGID of a new process group. +</p><p> +</p><h3>Getting a controlling tty</h3> + +<p>How does one get a controlling terminal? Nobody knows, +this is a great mystery. +</p><p>The System V approach is that the first tty opened by the process +becomes its controlling tty. +</p><p>The BSD approach is that one has to explicitly call +</p><blockquote> +<pre>ioctl(fd, TIOCSCTTY, ...); +</pre> +</blockquote> + +to get a controlling tty. +<p>Linux tries to be compatible with both, as always, and this +results in a very obscure complex of conditions. Roughly: +</p><p>The <code>TIOCSCTTY</code> ioctl will give us a controlling tty, +provided that (i) the current process is a session leader, +and (ii) it does not yet have a controlling tty, and +(iii) maybe the tty should not already control some other session; +if it does it is an error if we aren't root, or we steal the tty +if we are all-powerful. +</p><p>Opening some terminal will give us a controlling tty, +provided that (i) the current process is a session leader, and +(ii) it does not yet have a controlling tty, and +(iii) the tty does not already control some other session, and +(iv) the open did not have the <code>O_NOCTTY</code> flag, and +(v) the tty is not the foreground VT, and +(vi) the tty is not the console, and +(vii) maybe the tty should not be master or slave pty. +</p><p> +</p><h3>Getting rid of a controlling tty</h3> + +<p>If a process wants to continue as a daemon, it must detach itself +from its controlling tty. Above we saw that <code>setsid()</code> +will remove the controlling tty. Also the ioctl TIOCNOTTY does this. +Moreover, in order not to get a controlling tty again as soon as it +opens a tty, the process has to fork once more, to assure that it +is not a session leader. Typical code fragment: +</p><p> +</p><pre> if ((fork()) != 0) + exit(0); + setsid(); + if ((fork()) != 0) + exit(0); +</pre> +<p>See also <code>daemon(3)</code>. +</p><p> +</p><h3>Disconnect</h3> + +<p>If the terminal goes away by modem hangup, and the line was not local, +then a SIGHUP is sent to the session leader. +Any further reads from the gone terminal return EOF. +(Or possibly -1 with <code>errno</code> set to EIO.) +</p><p>If the terminal is the slave side of a pseudotty, and the master side +is closed (for the last time), then a SIGHUP is sent to the foreground +process group of the slave side. +</p><p>When the session leader dies, a SIGHUP is sent to all processes +in the foreground process group. Moreover, the terminal stops being +the controlling terminal of this session (so that it can become +the controlling terminal of another session). +</p><p>Thus, if the terminal goes away and the session leader is +a job control shell, then it can handle things for its descendants, +e.g. by sending them again a SIGHUP. +If on the other hand the session leader is an innocent process +that does not catch SIGHUP, it will die, and all foreground processes +get a SIGHUP. +</p><p> +</p><h2><a name="ss10.4">10.4 Threads</a> +</h2> + +<p>A process can have several threads. New threads (with the same PID +as the parent thread) are started using the <code>clone</code> system +call using the <code>CLONE_THREAD</code> flag. Threads are distinguished +by a <i>thread ID</i> (TID). An ordinary process has a single thread +with TID equal to PID. The system call <code>gettid()</code> returns the +TID. The system call <code>tkill()</code> sends a signal to a single thread. +</p><p>Example: a process with two threads. Both only print PID and TID and exit. +(Linux 2.4.19 or later.) +</p><pre>% cat << EOF > gettid-demo.c +#include <unistd.h> +#include <sys/types.h> +#define CLONE_SIGHAND 0x00000800 +#define CLONE_THREAD 0x00010000 +#include <linux/unistd.h> +#include <errno.h> +_syscall0(pid_t,gettid) + +int thread(void *p) { + printf("thread: %d %d\n", gettid(), getpid()); +} + +main() { + unsigned char stack[4096]; + int i; + + i = clone(thread, stack+2048, CLONE_THREAD | CLONE_SIGHAND, NULL); + if (i == -1) + perror("clone"); + else + printf("clone returns %d\n", i); + printf("parent: %d %d\n", gettid(), getpid()); +} +EOF +% cc -o gettid-demo gettid-demo.c +% ./gettid-demo +clone returns 21826 +parent: 21825 21825 +thread: 21826 21825 +% +</pre> +<p> +</p><p> +</p><hr> + +</body></html> |