job.texi

一个C源代码分析器

@node Job Control
@chapter Job Control

@cindex process groups
@cindex job control
@cindex job
@cindex session
@dfn{Job control} refers to the protocol for allowing a user to move
between multiple @dfn{process groups} (or @dfn{jobs}) within a single
@dfn{login session}.  The job control facilities are set up so that
appropriate behavior for most programs happens automatically and they
need not do anything special about job control.  So you can probably
ignore the material in this chapter unless you are writing a shell or
login program.

You need to be familiar with concepts relating to process creation
(@pxref{Process Creation Concepts}) and signal handling (@pxref{Signal
Handling}) in order to understand this material presented in this
chapter.

@menu
* Concepts of Job Control::     Jobs can be controlled by a shell.
* Job Control is Optional::     Not all POSIX systems support job control.
* Controlling Terminal::        How a process gets its controlling terminal.
* Access to the Terminal::      How processes share the controlling terminal.
* Orphaned Process Groups::     Jobs left after the user logs out.
* Implementing a Shell::        What a shell must do to implement job control.
* Functions for Job Control::   Functions to control process groups.
@end menu

@node Concepts of Job Control, Job Control is Optional,  , Job Control
@section Concepts of Job Control

@cindex shell
The fundamental purpose of an interactive shell is to read
commands from the user's terminal and create processes to execute the
programs specified by those commands.  It can do this using the
@code{fork} (@pxref{Creating a Process}) and @code{exec}
(@pxref{Executing a File}) functions.

A single command may run just one process---but often one command uses
several processes.  If you use the @samp{|} operator in a shell command,
you explicitly request several programs in their own processes.  But
even if you run just one program, it can use multiple processes
internally.  For example, a single compilation command such as @samp{cc
-c foo.c} typically uses four processes (though normally only two at any
given time).  If you run @code{make}, its job is to run other programs
in separate processes.

The processes belonging to a single command are called a @dfn{process
group} or @dfn{job}.  This is so that you can operate on all of them at
once.  For example, typing @kbd{C-c} sends the signal @code{SIGINT} to
terminate all the processes in the foreground process group.

@cindex session
A @dfn{session} is a larger group of processes.  Normally all the
proccesses that stem from a single login belong to the same session.

Every process belongs to a process group.  When a process is created, it
becomes a member of the same process group and session as its parent
process.  You can put it in another process group using the
@code{setpgid} function, provided the process group belongs to the same
session.

@cindex session leader
The only way to put a process in a different session is to make it the
initial process of a new session, or a @dfn{session leader}, using the
@code{setsid} function.  This also puts the session leader into a new
process group, and you can't move it out of that process group again.

Usually, new sessions are created by the system login program, and the
session leader is the process running the user's login shell.

@cindex controlling terminal
A shell that supports job control must arrange to control which job can
use the terminal at any time.  Otherwise there might be multiple jobs
trying to read from the terminal at once, and confusion about which
process should receive the input typed by the user.  To prevent this,
the shell must cooperate with the terminal driver using the protocol
described in this chapter.

@cindex foreground job
@cindex background job
The shell can give unlimited access to the controlling terminal to only
one process group at a time.  This is called the @dfn{foreground job} on
that controlling terminal.  Other process groups managed by the shell
that are executing without such access to the terminal are called
@dfn{background jobs}.

@cindex stopped job
If a background job needs to read from its controlling
terminal, it is @dfn{stopped} by the terminal driver; if the
@code{TOSTOP} mode is set, likewise for writing.  The user can stop
a foreground job by typing the SUSP character (@pxref{Special
Characters}) and a program can stop any job by sending it a
@code{SIGSTOP} signal.  It's the responsibility of the shell to notice
when jobs stop, to notify the user about them, and to provide mechanisms
for allowing the user to interactively continue stopped jobs and switch
jobs between foreground and background.

@xref{Access to the Terminal}, for more information about I/O to the
controlling terminal,

@node Job Control is Optional, Controlling Terminal, Concepts of Job Control , Job Control
@section Job Control is Optional
@cindex job control is optional

Not all operating systems support job control.  The GNU system does
support job control, but if you are using the GNU library on some other
system, that system may not support job control itself.

You can use the @code{_POSIX_JOB_CONTROL} macro to test at compile-time
whether the system supports job control.  @xref{System Options}.

If job control is not supported, then there can be only one process
group per session, which behaves as if it were always in the foreground.
The functions for creating additional process groups simply fail with
the error code @code{ENOSYS}.

The macros naming the various job control signals (@pxref{Job Control
Signals}) are defined even if job control is not supported.  However,
the system never generates these signals, and attempts to send a job
control signal or examine or specify their actions report errors or do
nothing.


@node Controlling Terminal, Access to the Terminal, Job Control is Optional, Job Control
@section Controlling Terminal of a Process

One of the attributes of a process is its controlling terminal.  Child
processes created with @code{fork} inherit the controlling terminal from
their parent process.  In this way, all the processes in a session
inherit the controlling terminal from the session leader.  A session
leader that has control of a terminal is called the @dfn{controlling
process} of that terminal.

@cindex controlling process
You generally do not need to worry about the exact mechanism used to
allocate a controlling terminal to a session, since it is done for you
by the system when you log in.
@c ??? How does GNU system let a process get a ctl terminal.

An individual process disconnects from its controlling terminal when it
calls @code{setsid} to become the leader of a new session.
@xref{Process Group Functions}.

@c !!! explain how it gets a new one (by opening any terminal)
@c ??? How you get a controlling terminal is system-dependent.
@c We should document how this will work in the GNU system when it is decided.
@c What Unix does is not clean and I don't think GNU should use that.

@node Access to the Terminal, Orphaned Process Groups, Controlling Terminal, Job Control
@section Access to the Controlling Terminal
@cindex controlling terminal, access to

Processes in the foreground job of a controlling terminal have
unrestricted access to that terminal; background proesses do not.  This
section describes in more detail what happens when a process in a
background job tries to access its controlling terminal.

@cindex @code{SIGTTIN}, from background job
When a process in a background job tries to read from its controlling
terminal, the process group is usually sent a @code{SIGTTIN} signal.
This normally causes all of the processes in that group to stop (unless
they handle the signal and don't stop themselves).  However, if the
reading process is ignoring or blocking this signal, then @code{read}
fails with an @code{EIO} error instead.

@cindex @code{SIGTTOU}, from background job
Similarly, when a process in a background job tries to write to its
controlling terminal, the default behavior is to send a @code{SIGTTOU}
signal to the process group.  However, the behavior is modified by the
@code{TOSTOP} bit of the local modes flags (@pxref{Local Modes}).  If
this bit is not set (which is the default), then writing to the
controlling terminal is always permitted without sending a signal.
Writing is also permitted if the @code{SIGTTOU} signal is being ignored
or blocked by the writing process.

Most other terminal operations that a program can do are treated as
reading or as writing.  (The description of each operation should say
which.)

For more information about the primitive @code{read} and @code{write}
functions, see @ref{I/O Primitives}.


@node Orphaned Process Groups, Implementing a Shell, Access to the Terminal, Job Control
@section Orphaned Process Groups
@cindex orphaned process group

When a controlling process terminates, its terminal becomes free and a
new session can be established on it.  (In fact, another user could log
in on the terminal.)  This could cause a problem if any processes from
the old session are still trying to use that terminal.

To prevent problems, process groups that continue running even after the
session leader has terminated are marked as @dfn{orphaned process
groups}.

When a process group becomes an orphan, its processes are sent a
@code{SIGHUP} signal.  Ordinarily, this causes the processes to
terminate.  However, if a program ignores this signal or establishes a
handler for it (@pxref{Signal Handling}), it can continue running as in
the orphan process group even after its controlling process terminates;
but it still cannot access the terminal any more.

@node Implementing a Shell, Functions for Job Control, Orphaned Process Groups, Job Control
@section Implementing a Job Control Shell

This section describes what a shell must do to implement job control, by
presenting an extensive sample program to illustrate the concepts
involved.

@iftex
@itemize @bullet
@item 
@ref{Data Structures}, introduces the example and presents
its primary data structures.

@item
@ref{Initializing the Shell}, discusses actions which the shell must
perform to prepare for job control.

@item
@ref{Launching Jobs}, includes information about how to create jobs
to execute commands.

@item
@ref{Foreground and Background}, discusses what the shell should
do differently when launching a job in the foreground as opposed to
a background job.

@item
@ref{Stopped and Terminated Jobs}, discusses reporting of job status
back to the shell.

@item
@ref{Continuing Stopped Jobs}, tells you how to continue jobs that
have been stopped.

@item
@ref{Missing Pieces}, discusses other parts of the shell.
@end itemize
@end iftex

@menu
* Data Structures::             Introduction to the sample shell.
* Initializing the Shell::      What the shell must do to take
				 responsibility for job control.
* Launching Jobs::              Creating jobs to execute commands.
* Foreground and Background::   Putting a job in foreground of background.
* Stopped and Terminated Jobs::  Reporting job status.
* Continuing Stopped Jobs::     How to continue a stopped job in
				 the foreground or background.
* Missing Pieces::              Other parts of the shell.
@end menu

@node Data Structures, Initializing the Shell,  , Implementing a Shell
@subsection Data Structures for the Shell

All of the program examples included in this chapter are part of
a simple shell program.  This section presents data structures
and utility functions which are used throughout the example.

The sample shell deals mainly with two data structures.  The
@code{job} type contains information about a job, which is a
set of subprocesses linked together with pipes.  The @code{process} type
holds information about a single subprocess.  Here are the relevant
data structure declarations:

@smallexample
@group
/* @r{A process is a single process.}  */
typedef struct process
@{
  struct process *next;       /* @r{next process in pipeline} */
  char **argv;                /* @r{for exec} */
  pid_t pid;                  /* @r{process ID} */
  char completed;             /* @r{true if process has completed} */
  char stopped;               /* @r{true if process has stopped} */
  int status;                 /* @r{reported status value} */
@} process;
@end group

@group
/* @r{A job is a pipeline of processes.}  */
typedef struct job
@{
  struct job *next;           /* @r{next active job} */
  char *command;              /* @r{command line, used for messages} */
  process *first_process;     /* @r{list of processes in this job} */
  pid_t pgid;                 /* @r{process group ID} */
  char notified;              /* @r{true if user told about stopped job} */
  struct termios tmodes;      /* @r{saved terminal modes} */
  int stdin, stdout, stderr;  /* @r{standard i/o channels} */
@} job;

/* @r{The active jobs are linked into a list.  This is its head.}   */
job *first_job = NULL;
@end group
@end smallexample

Here are some utility functions that are used for operating on @code{job}
objects.

@smallexample
@group
/* @r{Find the active job with the indicated @var{pgid}.}  */
job *
find_job (pid_t pgid)
@{
  job *j;
  
  for (j = first_job; j; j = j->next)
    if (j->pgid == pgid)
      return j;
  return NULL;
@}
@end group

@group
/* @r{Return true if all processes in the job have stopped or completed.}  */
int
job_is_stopped (job *j)
@{
  process *p;
  
  for (p = j->first_process; p; p = p->next)
    if (!p->completed && !p->stopped)
      return 0;
  return 1;
@}
@end group

@group
/* @r{Return true if all processes in the job have completed.}  */
int
job_is_completed (job *j)
@{
  process *p;
  
  for (p = j->first_process; p; p = p->next)
    if (!p->completed)
      return 0;
  return 1;
@}
@end group
@end smallexample


@node Initializing the Shell, Launching Jobs, Data Structures, Implementing a Shell
@subsection Initializing the Shell
@cindex job control, enabling
@cindex subshell

When a shell program that normally performs job control is started, it
has to be careful in case it has been invoked from another shell that is
already doing its own job control.  

A subshell that runs interactively has to ensure that it has been placed
in the foreground by its parent shell before it can enable job control
itself.  It does this by getting its initial process group ID with the
@code{getpgrp} function, and comparing it to the process group ID of the
current foreground job associated with its controlling terminal (which
can be retrieved using the @code{tcgetpgrp} function).

If the subshell is not running as a foreground job, it must stop itself
by sending a @code{SIGTTIN} signal to its own process group.  It may not
arbitrarily put itself into the foreground; it must wait for the user to
tell the parent shell to do this.  If the subshell is continued again,
it should repeat the check and stop itself again if it is still not in
the foreground.

@cindex job control, enabling
Once the subshell has been placed into the foreground by its parent
shell, it can enable its own job control.  It does this by calling
@code{setpgid} to put itself into its own process group, and then
calling @code{tcsetpgrp} to place this process group into the
foreground.

When a shell enables job control, it should set itself to ignore all the
job control stop signals so that it doesn't accidentally stop itself.
You can do this by setting the action for all the stop signals to
@code{SIG_IGN}.

A subshell that runs non-interactively cannot and should not support job
control.  It must leave all processes it creates in the same process
group as the shell itself; this allows the non-interactive shell and its
child processes to be treated as a single job by the parent shell.  This
is easy to do---just don't use any of the job control primitives---but
you must remember to make the shell do it.


Here is the initialization code for the sample shell that shows how to
do all of this.

@smallexample
/* @r{Keep track of attributes of the shell.}  */

#include <sys/types.h>
#include <termios.h>
#include <unistd.h>

pid_t shell_pgid;
struct termios shell_tmodes;
int shell_terminal;
int shell_is_interactive;


/* @r{Make sure the shell is running interactively as the foreground job}
   @r{before proceeding.} */

void
init_shell ()
@{
  
  /* @r{See if we are running interactively.}  */