freebsd-dev/share/doc/psd/04.uprog/p5

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.\"	@(#)p5	8.1 (Berkeley) 6/8/93
.\"
.NH
PROCESSES
.PP
It is often easier to use a program written
by someone else than to invent one's own.
This section describes how to
execute a program from within another.
.NH 2
The ``System'' Function
.PP
The easiest way to execute a program from another
is to use
the standard library routine
.UL system .
.UL system
takes one argument, a command string exactly as typed
at the terminal
(except for the newline at the end)
and executes it.
For instance, to time-stamp the output of a program,
.P1
main()
{
	system("date");
	/* rest of processing */
}
.P2
If the command string has to be built from pieces,
the in-memory formatting capabilities of
.UL sprintf
may be useful.
.PP
Remember than
.UL getc
and
.UL putc
normally buffer their input;
terminal I/O will not be properly synchronized unless
this buffering is defeated.
For output, use 
.UL fflush ;
for input, see
.UL setbuf 
in the appendix.
.NH 2
Low-Level Process Creation \(em Execl and Execv
.PP
If you're not using the standard library,
or if you need finer control over what
happens,
you will have to construct calls to other programs
using the more primitive routines that the standard
library's
.UL system
routine is based on.
.PP
The most basic operation is to execute another program
.ul
without
.IT returning ,
by using the routine
.UL execl  .
To print the date as the last action of a running program,
use
.P1
execl("/bin/date", "date", NULL);
.P2
The first argument to
.UL execl
is the
.ul
file name
of the command; you have to know where it is found
in the file system.
The second argument is conventionally
the program name
(that is, the last component of the file name),
but this is seldom used except as a place-holder.
If the command takes arguments, they are strung out after
this;
the end of the list is marked by a 
.UL NULL
argument.
.PP
The
.UL execl
call
overlays the existing program with
the new one,
runs that, then exits.
There is
.ul
no
return to the original program.
.PP
More realistically,
a program might fall into two or more phases
that communicate only through temporary files.
Here it is natural to make the second pass
simply an
.UL execl
call from the first.
.PP
The one exception to the rule that the original program never gets control
back occurs when there is an error, for example if the file can't be found
or is not executable.
If you don't know where
.UL date
is located, say
.P1
execl("/bin/date", "date", NULL);
execl("/usr/bin/date", "date", NULL);
fprintf(stderr, "Someone stole 'date'\en");
.P2
.PP
A variant of
.UL execl
called
.UL execv
is useful when you don't know in advance how many arguments there are going to be.
The call is
.P1
execv(filename, argp);
.P2
where
.UL argp
is an array of pointers to the arguments;
the last pointer in the array must be 
.UL NULL
so
.UL execv
can tell where the list ends.
As with
.UL execl ,
.UL filename
is the file in which the program is found, and
.UL argp[0]
is the name of the program.
(This arrangement is identical to the
.UL argv
array for program arguments.)
.PP
Neither of these routines provides the niceties of normal command execution.
There is no automatic search of multiple directories \(em
you have to know precisely where the command is located.
Nor do you get the expansion of metacharacters like
.UL < ,
.UL > ,
.UL * ,
.UL ? ,
and
.UL []
in the argument list.
If you want these, use
.UL execl
to invoke the shell
.UL sh ,
which then does all the work.
Construct a string
.UL commandline
that contains the complete command as it would have been typed
at the terminal, then say
.P1
execl("/bin/sh", "sh", "-c", commandline, NULL);
.P2
The shell is assumed to be at a fixed place,
.UL /bin/sh .
Its argument
.UL -c
says to treat the next argument
as a whole command line, so it does just what you want.
The only problem is in constructing the right information
in
.UL commandline .
.NH 2
Control of Processes \(em Fork and Wait
.PP
So far what we've talked about isn't really all that useful by itself.
Now we will show how to regain control after running
a program with
.UL execl
or
.UL execv .
Since these routines simply overlay the new program on the old one,
to save the old one requires that it first be split into
two copies;
one of these can be overlaid, while the other waits for the new,
overlaying program to finish.
The splitting is done by a routine called
.UL fork :
.P1
proc_id = fork();
.P2
splits the program into two copies, both of which continue to run.
The only difference between the two is the value of
.UL proc_id ,
the ``process id.''
In one of these processes (the ``child''),
.UL proc_id
is zero.
In the other
(the ``parent''),
.UL proc_id
is non-zero; it is the process number of the child.
Thus the basic way to call, and return from,
another program is
.P1
if (fork() == 0)
	execl("/bin/sh", "sh", "-c", cmd, NULL);	/* in child */
.P2
And in fact, except for handling errors, this is sufficient.
The
.UL fork
makes two copies of the program.
In the child, the value returned by
.UL fork
is zero, so it calls
.UL execl
which does the
.UL command
and then dies.
In the parent,
.UL fork
returns non-zero
so it skips the
.UL execl.
(If there is any error,
.UL fork
returns
.UL -1 ).
.PP
More often, the parent wants to wait for the child to terminate
before continuing itself.
This can be done with
the function
.UL wait :
.P1
int status;

if (fork() == 0)
	execl(...);
wait(&status);
.P2
This still doesn't handle any abnormal conditions, such as a failure
of the
.UL execl
or
.UL fork ,
or the possibility that there might be more than one child running simultaneously.
(The
.UL wait
returns the
process id
of the terminated child, if you want to check it against the value
returned by
.UL fork .)
Finally, this fragment doesn't deal with any
funny behavior on the part of the child
(which is reported in
.UL status ).
Still, these three lines
are the heart of the standard library's
.UL system
routine,
which we'll show in a moment.
.PP
The
.UL  status 
returned by
.UL wait
encodes in its low-order eight bits
the system's idea of the child's termination status;
it is 0 for normal termination and non-zero to indicate
various kinds of problems.
The next higher eight bits are taken from the argument
of the call to
.UL exit
which caused a normal termination of the child process.
It is good coding practice
for all programs to return meaningful
status.
.PP
When a program is called by the shell,
the three file descriptors
0, 1, and 2 are set up pointing at the right files,
and all other possible file descriptors
are available for use.
When this program calls another one,
correct etiquette suggests making sure the same conditions
hold.
Neither
.UL fork
nor the
.UL exec
calls affects open files in any way.
If the parent is buffering output
that must come out before output from the child,
the parent must flush its buffers
before the
.UL execl .
Conversely,
if a caller buffers an input stream,
the called program will lose any information
that has been read by the caller.
.NH 2
Pipes
.PP
A
.ul
pipe
is an I/O channel intended for use
between two cooperating processes:
one process writes into the pipe,
while the other reads.
The system looks after buffering the data and synchronizing
the two processes.
Most pipes are created by the shell,
as in
.P1
ls | pr
.P2
which connects the standard output of
.UL ls
to the standard input of
.UL pr .
Sometimes, however, it is most convenient
for a process to set up its own plumbing;
in this section, we will illustrate how
the pipe connection is established and used.
.PP
The system call
.UL pipe
creates a pipe.
Since a pipe is used for both reading and writing,
two file descriptors are returned;
the actual usage is like this:
.P1
int	fd[2];

stat = pipe(fd);
if (stat == -1)
	/* there was an error ... */
.P2
.UL fd
is an array of two file descriptors, where
.UL fd[0]
is the read side of the pipe and
.UL fd[1] 
is for writing.
These may be used in
.UL read ,
.UL write
and
.UL close
calls just like any other file descriptors.
.PP
If a process reads a pipe which is empty,
it will wait until data arrives;
if a process writes into a pipe which
is too full, it will wait until the pipe empties somewhat.
If the write side of the pipe is closed,
a subsequent
.UL  read 
will encounter end of file.
.PP
To illustrate the use of pipes in a realistic setting,
let us write a function called
.UL popen(cmd,\ mode) ,
which creates a process
.UL cmd
(just as
.UL system 
does),
and returns a file descriptor that will either
read or write that process, according to 
.UL mode .
That is,
the call
.P1
fout = popen("pr", WRITE);
.P2
creates a process that executes
the
.UL pr
command;
subsequent
.UL write
calls using the file descriptor
.UL fout
will send their data to that process
through the pipe.
.PP
.UL popen
first creates the
the pipe with a
.UL pipe
system call;
it then
.UL fork s
to create two copies of itself.
The child decides whether it is supposed to read or write,
closes the other side of the pipe,
then calls the shell (via
.UL execl )
to run the desired process.
The parent likewise closes the end of the pipe it does not use.
These closes are necessary to make end-of-file tests work properly.
For example, if a child that intends to read
fails to close the write end of the pipe, it will never
see the end of the pipe file, just because there is one writer
potentially active.
.P1
#include <stdio.h>

#define	READ	0
#define	WRITE	1
#define	tst(a, b)	(mode == READ ? (b) : (a))
static	int	popen_pid;

popen(cmd, mode)
char	*cmd;
int	mode;
{
	int p[2];

	if (pipe(p) < 0)
		return(NULL);
	if ((popen_pid = fork()) == 0) {
		close(tst(p[WRITE], p[READ]));
		close(tst(0, 1));
		dup(tst(p[READ], p[WRITE]));
		close(tst(p[READ], p[WRITE]));
		execl("/bin/sh", "sh", "-c", cmd, 0);
		_exit(1);	/* disaster has occurred if we get here */
	}
	if (popen_pid == -1)
		return(NULL);
	close(tst(p[READ], p[WRITE]));
	return(tst(p[WRITE], p[READ]));
}
.P2
The sequence of
.UL close s
in the child
is a bit tricky.
Suppose
that the task is to create a child process that will read data from the parent.
Then the first
.UL close
closes the write side of the pipe,
leaving the read side open.
The lines
.P1
close(tst(0, 1));
dup(tst(p[READ], p[WRITE]));
.P2
are the conventional way to associate the pipe descriptor
with the standard input of the child.
The 
.UL close
closes file descriptor 0,
that is, the standard input.
.UL dup
is a system call that
returns a duplicate of an already open file descriptor.
File descriptors are assigned in increasing order
and the first available one is returned,
so
the effect of the
.UL dup
is to copy the file descriptor for the pipe (read side)
to file descriptor 0;
thus the read side of the pipe becomes the standard input.
(Yes, this is a bit tricky, but it's a standard idiom.)
Finally, the old read side of the pipe is closed.
.PP
A similar sequence of operations takes place
when the child process is supposed to write
from the parent instead of reading.
You may find it a useful exercise to step through that case.
.PP
The job is not quite done,
for we still need a function
.UL pclose
to close the pipe created by
.UL popen .
The main reason for using a separate function rather than
.UL close
is that it is desirable to wait for the termination of the child process.
First, the return value from
.UL pclose
indicates whether the process succeeded.
Equally important when a process creates several children
is that only a bounded number of unwaited-for children
can exist, even if some of them have terminated;
performing the
.UL wait
lays the child to rest.
Thus:
.P1
#include <signal.h>

pclose(fd)	/* close pipe fd */
int fd;
{
	register r, (*hstat)(), (*istat)(), (*qstat)();
	int	 status;
	extern int popen_pid;

	close(fd);
	istat = signal(SIGINT, SIG_IGN);
	qstat = signal(SIGQUIT, SIG_IGN);
	hstat = signal(SIGHUP, SIG_IGN);
	while ((r = wait(&status)) != popen_pid && r != -1);
	if (r == -1)
		status = -1;
	signal(SIGINT, istat);
	signal(SIGQUIT, qstat);
	signal(SIGHUP, hstat);
	return(status);
}
.P2
The calls to
.UL signal
make sure that no interrupts, etc.,
interfere with the waiting process;
this is the topic of the next section.
.PP
The routine as written has the limitation that only one pipe may
be open at once, because of the single shared variable
.UL popen_pid ;
it really should be an array indexed by file descriptor.
A
.UL popen
function, with slightly different arguments and return value is available
as part of the standard I/O library discussed below.
As currently written, it shares the same limitation.