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.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
.TH "PCLOSE" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" pclose
.SH NAME
pclose \- close a pipe stream to or from a process
.SH SYNOPSIS
.LP
\fB#include <stdio.h>
.br
.sp
int pclose(FILE *\fP\fIstream\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIpclose\fP() function shall close a stream that was opened by
\fIpopen\fP(), wait
for the command to terminate, and return the termination status of
the process that was running the command language interpreter.
However, if a call caused the termination status to be unavailable
to \fIpclose\fP(), then \fIpclose\fP() shall return -1 with
\fIerrno\fP set to [ECHILD] to report this situation. This can happen
if the application calls one of the following functions:
.IP " *" 3
\fIwait\fP()
.LP
.IP " *" 3
\fIwaitpid\fP() with a \fIpid\fP argument less than or equal to 0
or equal to the
process ID of the command line interpreter
.LP
.IP " *" 3
Any other function not defined in this volume of IEEE\ Std\ 1003.1-2001
that could do one of the above
.LP
.LP
In any case, \fIpclose\fP() shall not return before the child process
created by \fIpopen\fP() has terminated.
.LP
If the command language interpreter cannot be executed, the child
termination status returned by \fIpclose\fP() shall be as if
the command language interpreter terminated using \fIexit\fP(127)
or \fI_exit\fP(127).
.LP
The \fIpclose\fP() function shall not affect the termination status
of any child of the calling process other than the one
created by \fIpopen\fP() for the associated stream.
.LP
If the argument \fIstream\fP to \fIpclose\fP() is not a pointer to
a stream created by \fIpopen\fP(), the result of \fIpclose\fP() is
undefined.
.SH RETURN VALUE
.LP
Upon successful return, \fIpclose\fP() shall return the termination
status of the command language interpreter. Otherwise,
\fIpclose\fP() shall return -1 and set \fIerrno\fP to indicate the
error.
.SH ERRORS
.LP
The \fIpclose\fP() function shall fail if:
.TP 7
.B ECHILD
The status of the child process could not be obtained, as described
above.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
There is a requirement that \fIpclose\fP() not return before the child
process terminates. This is intended to disallow
implementations that return [EINTR] if a signal is received while
waiting. If \fIpclose\fP() returned before the child terminated,
there would be no way for the application to discover which child
used to be associated with the stream, and it could not do the
cleanup itself.
.LP
If the stream pointed to by \fIstream\fP was not created by \fIpopen\fP(),
historical
implementations of \fIpclose\fP() return -1 without setting \fIerrno\fP.
To avoid requiring \fIpclose\fP() to set \fIerrno\fP
in this case, IEEE\ Std\ 1003.1-2001 makes the behavior unspecified.
An application should not use \fIpclose\fP() to close
any stream that was not created by \fIpopen\fP().
.LP
Some historical implementations of \fIpclose\fP() either block or
ignore the signals SIGINT, SIGQUIT, and SIGHUP while waiting
for the child process to terminate. Since this behavior is not described
for the \fIpclose\fP() function in
IEEE\ Std\ 1003.1-2001, such implementations are not conforming. Also,
some historical implementations return [EINTR] if a
signal is received, even though the child process has not terminated.
Such implementations are also considered non-conforming.
.LP
Consider, for example, an application that uses:
.sp
.RS
.nf
\fBpopen("command", "r")
\fP
.fi
.RE
.LP
to start \fIcommand\fP, which is part of the same application. The
parent writes a prompt to its standard output (presumably
the terminal) and then reads from the \fIpopen\fP()ed stream. The
child reads the response
from the user, does some transformation on the response (pathname
expansion, perhaps) and writes the result to its standard output.
The parent process reads the result from the pipe, does something
with it, and prints another prompt. The cycle repeats. Assuming
that both processes do appropriate buffer flushing, this would be
expected to work.
.LP
To conform to IEEE\ Std\ 1003.1-2001, \fIpclose\fP() must use \fIwaitpid\fP(),
or some similar function, instead of \fIwait\fP().
.LP
The code sample below illustrates how the \fIpclose\fP() function
might be implemented on a system conforming to
IEEE\ Std\ 1003.1-2001.
.sp
.RS
.nf
\fBint pclose(FILE *stream)
{
int stat;
pid_t pid;
.sp
pid = <pid for process created for stream by popen()>
(void) fclose(stream);
while (waitpid(pid, &stat, 0) == -1) {
if (errno != EINTR){
stat = -1;
break;
}
}
return(stat);
}
\fP
.fi
.RE
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIfork\fP() , \fIpopen\fP() , \fIwaitpid\fP() ,
the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<stdio.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
|
